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<title data-rh="true">Service Level Indicator (SLI) - Alex Ewerlöf Notes</title>
<meta data-rh="true" name="theme-color" content="#001426"/><meta data-rh="true" property="og:type" content="article"/><meta data-rh="true" property="og:title" content="Service Level Indicators"/><meta data-rh="true" name="twitter:title" content="Service Level Indicators"/><meta data-rh="true" name="description" content="Unveiling the Essence of SLIs in System Reliability: This insightful article delves into the critical role of Service Level Indicators (SLIs) in measuring system reliability. It explores how SLIs, as percentages, reflect the &#x27;good&#x27; in a given period, differentiating them from other metrics like MAU. The piece simplifies the SLI formula, using real-world examples to illustrate its application in both event-based and time-based contexts. It also touches on common SLIs like availability, success rate, and latency, while highlighting what does not constitute a good SLI. This informative guide is essential for understanding the nuances of SLIs and their impact on system performance, paving the way for a future discussion on Service Level Objectives (SLOs)."/><meta data-rh="true" property="og:description" content="Introduction to SLI, examples, counterexamples and tips"/><meta data-rh="true" name="twitter:description" content="Introduction to SLI, examples, counterexamples and tips"/><meta data-rh="true" property="og:image" content="https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1200,h_675,c_fill,f_jpg,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png"/><meta data-rh="true" name="twitter:image" content="https://substackcdn.com/image/fetch/$s_!6svT!,f_auto,q_auto:best,fl_progressive:steep/https%3A%2F%2Falexewerlof.substack.com%2Fapi%2Fv1%2Fpost_preview%2F135805725%2Ftwitter.jpg%3Fversion%3D4"/><meta data-rh="true" name="twitter:card" content="summary_large_image"/><meta data-rh="true" property="article:modified_time" content="2024-05-04T18:43:38.483Z"/>
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It explores how SLIs, as percentages, reflect the 'good' in a given period, differentiating them from other metrics like MAU. The piece simplifies the SLI formula, using real-world examples to illustrate its application in both event-based and time-based contexts. It also touches on common SLIs like availability, success rate, and latency, while highlighting what does not constitute a good SLI. This informative guide is essential for understanding the nuances of SLIs and their impact on system performance, paving the way for a future discussion on Service Level Objectives (SLOs).","image":[{"@type":"ImageObject","url":"https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png"}],"datePublished":"2023-11-26T05:11:20+00:00","dateModified":"2023-11-26T05:11:20+00:00","isAccessibleForFree":false,"author":[{"@type":"Person","name":"Alex Ewerl\u00F6f","url":"https://substack.com/@alexewerlof","description":"Writes about technical leadership, growth mindset, and system reliability engineering. Senior Staff Engineer, MSc Systems Engineering from KTH, Stockholmer, dad, amateur artist. 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https://substackcdn.com/image/fetch/$s_!IRtW!,w_108,h_108,c_fill,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fbucketeer-e05bbc84-baa3-437e-9518-adb32be77984.s3.amazonaws.com%2Fpublic%2Fimages%2Fe2713990-da82-481b-b579-01a7aaa5b85b_560x560.jpeg 3x" width="36" height="36" draggable="false" class="img-OACg1c object-fit-cover-u4ReeV pencraft pc-reset"/></picture></div></div></a></div></div></div><div class="pencraft pc-display-flex pc-flexDirection-column pc-reset"><div class="pencraft pc-reset color-primary-zABazT line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><span style="min-width:0;" data-state="closed"><a href="https://substack.com/@alexewerlof" class="pencraft pc-reset decoration-hover-underline-ClDVRM reset-IxiVJZ">Alex Ewerlöf</a></span></div><div class="pencraft pc-display-flex pc-gap-4 pc-reset"><div class="pencraft pc-reset color-secondary-ls1g8s line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><time datetime="2023-11-26T05:11:20.311Z" title="2023-11-26T05:11:20.311Z">Nov 26, 2023</time></div><div class="pencraft pc-reset color-secondary-ls1g8s line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA">∙ Paid</div></div></div></div></div><div class="pencraft pc-display-flex pc-gap-16 pc-justifyContent-space-between pc-alignItems-center pc-reset flex-grow-rzmknG post-ufi"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><div class="like-button-container post-ufi-button style-button"><button tabindex="0" type="button" aria-label="Like (16)" aria-pressed="false" class="pencraft pc-reset pencraft post-ufi-button style-button has-label with-border"><svg aria-hidden="true" style="height:20px;width:20px;" width="20" height="20" viewBox="0 0 24 24" fill="#000000" stroke-width="2" stroke="#000" 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class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft post-ufi-button style-button has-label with-border"><div class="label">Share</div></button></div></div></div></div><div><div class="dt-post-body"><div class="available-content"><div dir="auto" class="body markup"><p><span>In </span><a href="https://blog.alexewerlof.com/p/why-bother-with-sli-and-slo">why bother with SLI and SLO</a><span> we discussed why service levels are important. This article focuses on SLI and a follow up will discuss </span><a href="https://blog.alexewerlof.com/p/sli">SLO</a><span>.</span></p><div class="pullquote"><p><span>A Service Level Indicator (SLI) is a reliability metric that shows the percentage of perceived </span><em>good</em><span> in a period of time and defines the service owners’ commitment towards service consumers.</span></p></div><p>Let’s unpack that:</p><ul><li><p><strong>SLI is a metric:</strong><span> it goes up and down. On its own SLI cannot tell if the system is reliable or not. We need the Service Level </span><em>Objective</em><span> (SLO) to know what we promised and where do we stand in relation to that promise.</span></p></li><li><p><strong>SLI is a reliability metric:</strong><span> there are many metrics that can indicate the behavior of a system or group of systems. SLI is particularly focused on the reliability aspect. For example: MAU (monthly active users) of a system may be a great product metric but it is not a reliability metric. Sure, the reliability can impact the MAU, but the two metrics have fundamentally different audiences and control mechanisms. SLI caters towards engineers who build a solution.</span></p></li><li><p><strong>SLI is concerned with perceived reliability:</strong><span> SLI is set at the boundaries of your service towards its consumers. For example, if you offer an API to your users, they may perceive its reliability by its latency, error rate, or other metrics, but they don’t care and shouldn’t be concerned with how your API is built behind the scenes. As far as they are concerned, your service is a black box to them and it’s not up to them how you design it. This gives freedom to the service owners to </span><a href="https://blog.alexewerlof.com/p/you-build-it-you-own-it">truly own all aspects of the system</a><span>.</span></p></li><li><p><strong>SLI ties to alerting:</strong><em><strong> </strong></em><span>SLI defines what we measure and </span><a href="https://blog.alexewerlof.com/p/slo">SLO</a><span> sets a target for it while </span><a href="https://blog.alexewerlof.com/p/alerting-on-slo">alerts</a><span> are ultimately set after the SLOs to communicate the service owners’ accountability towards the service consumers.</span></p></li><li><p><strong>SLI is a percentage:</strong><span> as we’ll see the formula for SLI indicates the percentage of good in a set of values that are in the scope of optimization (valid). Therefore, the value of the SLI is always from 0 to 100. Although technically any raw metric can be considered an SLI, Google (where these ideas originated from “generally recommend treating the SLI as the ratio of two numbers” (</span><a href="https://sre.google/workbook/implementing-slos/">reference</a><span>).</span></p></li><li><p><strong>SLI is positive:</strong><span> SLI is concerned with </span><em>good</em><span>, not bad. For example, instead of focusing on error rate, SLI is concerned with the success rate. For the opposite view, please see the </span><a href="https://blog.alexewerlof.com/p/error-budget">error budgets</a><span>.</span></p></li><li><p><strong>SLI is aggregated over a period of time:</strong><span> Unlike many real-time metrics like CPU usage, SLI aggregates the values over a period of time. This aggregation is a very powerful concept as it allows for failure (</span><a href="https://blog.alexewerlof.com/p/error-budget">error budget</a><span>) in a controlled manner. The time period attribute (also known as the </span><a href="https://blog.alexewerlof.com/p/compliance-period">compliance period</a><span>) is an attribute of the </span><a href="https://blog.alexewerlof.com/p/slo">SLO</a><span>.</span></p></li></ul><h2 class="header-anchor-post">Formula<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§formula" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/formula" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h2><p>The formula for SLI is very simple:</p><div data-component-name="Latex" class="pencraft pc-display-flex pc-paddingTop-4px pc-paddingBottom-4px pc-justifyContent-center pc-reset flex-grow-rzmknG latex-rendered"><div class="pencraft pc-display-flex pc-flexDirection-column pc-justifyContent-center pc-reset"><span style="display:block;">\(SLI = \frac {\text{good}} {\text{valid}} \times 100\)</span></div></div><ul><li><p><span>The formula is the same whether we’re counting events (</span><em>good events</em><span> / </span><em>valid events</em><span>) or time slots (</span><em>good time slots</em><span> / </span><em>valid time slots</em><span>).</span></p></li><li><p><em>Valid</em><span> is often simplified to total</span></p></li></ul><p>We have covered these in detail before:</p><div data-component-name="DigestPostEmbed" class="digestPostEmbed-flwiST"><a href="https://blog.alexewerlof.com/p/time-based-vs-event-based" rel="noopener" target="_blank"><div class="pencraft pc-display-flex pc-gap-16 pc-reset"><div style="width:70px;height:70px;" class="pencraft pc-reset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!_h6V!,w_140,h_140,c_fill,f_webp,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fd3be84d4-5a4b-4e52-8e7e-77744df9eaa7_1093x705.png"/><img src="https://substackcdn.com/image/fetch/$s_!_h6V!,w_140,h_140,c_fill,f_auto,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fd3be84d4-5a4b-4e52-8e7e-77744df9eaa7_1093x705.png" sizes="100vw" alt="Time based vs Event based SLIs" width="140" height="140" class="img-OACg1c smSquare-NGbPBa pencraft pc-reset"/></picture></div><div class="pencraft pc-display-flex pc-flexDirection-column pc-reset"><h4 class="pencraft pc-reset color-pub-primary-text-NyXPlw line-height-24-jnGwiv font-display-nhmvtD size-20-P_cSRT weight-bold-DmI9lw reset-IxiVJZ">Time based vs Event based SLIs</h4><div class="pencraft pc-display-flex pc-gap-4 pc-alignItems-center pc-reset"><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><a href="https://substack.com/profile/87732486-alex-ewerlof" class="inheritColor-WetTGJ">Alex Ewerlöf</a></div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T reset-IxiVJZ">·</div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA">August 7, 2023</div></div><div class="pencraft pc-display-flex pc-gap-16 pc-paddingTop-0 pc-paddingBottom-0 pc-alignItems-center pc-reset"><a href="https://blog.alexewerlof.com/p/time-based-vs-event-based" class="pencraft pc-reset align-center-y7ZD4w line-height-20-t4M0El font-text-qe4AeH size-13-hZTUKr weight-medium-fw81nC reset-IxiVJZ"><div class="pencraft pc-display-flex pc-gap-8 pc-alignItems-center pc-reset link-HREYZo"><span class="pencraft pc-reset color-accent-BVX_7M line-height-20-t4M0El font-text-qe4AeH size-14-MLPa7j weight-semibold-uqA4FV reset-IxiVJZ">Read full story</span><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-arrow-right"><path d="M5 12h14"></path><path d="m12 5 7 7-7 7"></path></svg></div></a></div></div></div></a></div><div data-component-name="DigestPostEmbed" class="digestPostEmbed-flwiST"><a href="https://blog.alexewerlof.com/p/sli-good" rel="noopener" target="_blank"><div class="pencraft pc-display-flex pc-gap-16 pc-reset"><div style="width:70px;height:70px;" class="pencraft pc-reset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!3jNH!,w_140,h_140,c_fill,f_webp,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F21343ed3-10ed-46b9-b590-7990ae90d5b9_1021x624.png"/><img src="https://substackcdn.com/image/fetch/$s_!3jNH!,w_140,h_140,c_fill,f_auto,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F21343ed3-10ed-46b9-b590-7990ae90d5b9_1021x624.png" sizes="100vw" alt="Definition of good in SLI" width="140" height="140" class="img-OACg1c smSquare-NGbPBa pencraft pc-reset"/></picture></div><div class="pencraft pc-display-flex pc-flexDirection-column pc-reset"><h4 class="pencraft pc-reset color-pub-primary-text-NyXPlw line-height-24-jnGwiv font-display-nhmvtD size-20-P_cSRT weight-bold-DmI9lw reset-IxiVJZ">Definition of good in SLI</h4><div class="pencraft pc-display-flex pc-gap-4 pc-alignItems-center pc-reset"><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><a href="https://substack.com/profile/87732486-alex-ewerlof" class="inheritColor-WetTGJ">Alex Ewerlöf</a></div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T reset-IxiVJZ">·</div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA">August 8, 2023</div></div><div class="pencraft pc-display-flex pc-gap-16 pc-paddingTop-0 pc-paddingBottom-0 pc-alignItems-center pc-reset"><a href="https://blog.alexewerlof.com/p/sli-good" class="pencraft pc-reset align-center-y7ZD4w line-height-20-t4M0El font-text-qe4AeH size-13-hZTUKr weight-medium-fw81nC reset-IxiVJZ"><div class="pencraft pc-display-flex pc-gap-8 pc-alignItems-center pc-reset link-HREYZo"><span class="pencraft pc-reset color-accent-BVX_7M line-height-20-t4M0El font-text-qe4AeH size-14-MLPa7j weight-semibold-uqA4FV reset-IxiVJZ">Read full story</span><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-arrow-right"><path d="M5 12h14"></path><path d="m12 5 7 7-7 7"></path></svg></div></a></div></div></div></a></div><div data-component-name="DigestPostEmbed" class="digestPostEmbed-flwiST"><a href="https://blog.alexewerlof.com/p/valid-vs-total" rel="noopener" target="_blank"><div class="pencraft pc-display-flex pc-gap-16 pc-reset"><div style="width:70px;height:70px;" class="pencraft pc-reset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!oNjS!,w_140,h_140,c_fill,f_webp,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F34dfb671-d82c-4097-87b3-05a4a639935a_758x609.png"/><img src="https://substackcdn.com/image/fetch/$s_!oNjS!,w_140,h_140,c_fill,f_auto,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F34dfb671-d82c-4097-87b3-05a4a639935a_758x609.png" sizes="100vw" alt="SLI: Valid vs Total" width="140" height="140" class="img-OACg1c smSquare-NGbPBa pencraft pc-reset"/></picture></div><div class="pencraft pc-display-flex pc-flexDirection-column pc-reset"><h4 class="pencraft pc-reset color-pub-primary-text-NyXPlw line-height-24-jnGwiv font-display-nhmvtD size-20-P_cSRT weight-bold-DmI9lw reset-IxiVJZ">SLI: Valid vs Total</h4><div class="pencraft pc-display-flex pc-gap-4 pc-alignItems-center pc-reset"><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><a href="https://substack.com/profile/87732486-alex-ewerlof" class="inheritColor-WetTGJ">Alex Ewerlöf</a></div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T reset-IxiVJZ">·</div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA">August 8, 2023</div></div><div class="pencraft pc-display-flex pc-gap-16 pc-paddingTop-0 pc-paddingBottom-0 pc-alignItems-center pc-reset"><a href="https://blog.alexewerlof.com/p/valid-vs-total" class="pencraft pc-reset align-center-y7ZD4w line-height-20-t4M0El font-text-qe4AeH size-13-hZTUKr weight-medium-fw81nC reset-IxiVJZ"><div class="pencraft pc-display-flex pc-gap-8 pc-alignItems-center pc-reset link-HREYZo"><span class="pencraft pc-reset color-accent-BVX_7M line-height-20-t4M0El font-text-qe4AeH size-14-MLPa7j weight-semibold-uqA4FV reset-IxiVJZ">Read full story</span><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-arrow-right"><path d="M5 12h14"></path><path d="m12 5 7 7-7 7"></path></svg></div></a></div></div></div></a></div><h2 class="header-anchor-post">Example: Event based<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§example-event-based" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/example-event-based" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h2><p>Let’s imagine that we have two systems. They can be a front-end and backend, or two microservices, or a backend and database, or something else. It can’t get simpler than that! One system depends on the other.</p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!7TGg!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!7TGg!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png" width="891" height="320" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:320,&quot;width&quot;:891,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:22837,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!7TGg!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><p>Let’s say that the response latency is critical for system A. The owners of system A want the owners of system B to be responsible for a good latency. More specifically, they want the latency to be below 2000ms.</p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!z86H!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!z86H!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!z86H!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png" width="891" height="320" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/aaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:320,&quot;width&quot;:891,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:26702,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!z86H!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!z86H!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><p>Here are some example latencies:</p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!ZsT6!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!ZsT6!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 424w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 848w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1272w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png" width="888" height="443" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:443,&quot;width&quot;:888,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:49539,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!ZsT6!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 424w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 848w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1272w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><p>The ones in red are above the 2000ms threshold. Drawing the same numbers on a diagram, we get this:</p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!o3NC!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!o3NC!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 424w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 848w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1272w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png" width="941" height="532" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:532,&quot;width&quot;:941,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:32557,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!o3NC!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 424w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 848w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1272w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><p>The formula for SLI is:</p><div data-component-name="Latex" class="pencraft pc-display-flex pc-paddingTop-4px pc-paddingBottom-4px pc-justifyContent-center pc-reset flex-grow-rzmknG latex-rendered"><div class="pencraft pc-display-flex pc-flexDirection-column pc-justifyContent-center pc-reset"><span style="display:block;">\(SLI = \frac {\text{requests with good response time}} {\text{total requests}} \times 100 = \frac {4} {8} \times 100 = 50\%\)</span></div></div><p>In this example, we have 8 response latencies where 4 of them are responded in good time (less than 2000ms). The SLI in this particular period is:</p><p>Let’s use some numbers that are more realistic:</p><ul><li><p>In a month, we get 1,000,300 requests</p></li><li><p>992,300 of them were responded in less than 2000ms</p></li></ul><div data-component-name="Latex" class="pencraft pc-display-flex pc-paddingTop-4px pc-paddingBottom-4px pc-justifyContent-center pc-reset flex-grow-rzmknG latex-rendered"><div class="pencraft pc-display-flex pc-flexDirection-column pc-justifyContent-center pc-reset"><span style="display:block;">\(SLI = \frac {\text{992300}} {\text{1000300}} \times 100 = 99.2\%\)</span></div></div><h2 class="header-anchor-post">Example: Time based<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§example-time-based" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/example-time-based" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h2><p>Let’s say that the reliability of system B is measured by uptime. Its endpoint is probed every minute and we get this data:</p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!LSe5!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!LSe5!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 424w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 848w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1272w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png" width="946" height="317" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:317,&quot;width&quot;:946,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:15831,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!LSe5!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 424w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 848w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1272w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><p>SLI is the number of bad probes (or bad minutes) divided by the total number of probes (or the minutes in the measurement window). In this example, 28 out of 36 probes succeeded:</p><div data-component-name="Latex" class="pencraft pc-display-flex pc-paddingTop-4px pc-paddingBottom-4px pc-justifyContent-center pc-reset flex-grow-rzmknG latex-rendered"><div class="pencraft pc-display-flex pc-flexDirection-column pc-justifyContent-center pc-reset"><span style="display:block;">\(SLI = \frac {\text{28}} {\text{36}} \times 100 = 77.7\%\)</span></div></div><p><span>Uptime is a common SLI. You may have seen this type of diagram for other products. For example </span><a href="https://status.brave.com/">status.brave.com</a><span> looks like this:</span></p><div class="captioned-image-container"><figure><a target="_blank" href="https://substackcdn.com/image/fetch/$s_!p3pB!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png" data-component-name="Image2ToDOM" class="image-link image2 is-viewable-img can-restack"><div class="image2-inset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!p3pB!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 424w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 848w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1272w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1456w" sizes="100vw"/><img src="https://substackcdn.com/image/fetch/$s_!p3pB!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png" width="798" height="418" data-attrs="{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:418,&quot;width&quot;:798,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:25347,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}" alt srcset="https://substackcdn.com/image/fetch/$s_!p3pB!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 424w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 848w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1272w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1456w" sizes="100vw" loading="lazy" class="sizing-normal"/></picture><div class="image-link-expand"><div class="pencraft pc-display-flex pc-gap-8 pc-reset"><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container restack-image"><svg aria-hidden="true" width="20" height="20" viewBox="0 0 20 20" fill="none" stroke-width="1.5" stroke="var(--color-fg-primary)" stroke-linecap="round" stroke-linejoin="round" xmlns="http://www.w3.org/2000/svg"><g><path d="M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882"></path></g></svg></button><button tabindex="0" type="button" class="pencraft pc-reset pencraft icon-container view-image"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-maximize2 lucide-maximize-2"><polyline points="15 3 21 3 21 9"></polyline><polyline points="9 21 3 21 3 15"></polyline><line x1="21" x2="14" y1="3" y2="10"></line><line x1="3" x2="10" y1="21" y2="14"></line></svg></button></div></div></div></a></figure></div><h2 class="header-anchor-post">Common SLIs<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§common-slis" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/common-slis" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h2><p>Here’s a list of the most common SLIs:</p><ul><li><p><strong>Availability</strong><span>: the fraction of the time that a service is reachable</span></p></li><li><p><strong>Success rate:</strong><span> The percentage of requests that are successful (the opposite of </span><strong>Error Rate</strong><span>, but in SLI formula we focus on Good/Valid)</span></p></li><li><p><strong>Latency</strong><span>: how long it takes to return a response to a request or do a computation</span></p><ul><li><p><span>Latency and response time can be different things: </span><a href="https://scalabledeveloper.com/posts/latency-versus-response-time/">Latency versus Response Time | Scalable Developer</a></p></li></ul></li><li><p><strong>Throughput</strong><span>: The number of requests that can be processed per unit of time. Typically measured in requests per second, messages consumed (e.g. useful for queue-based workers), queries processed (e.g. for Databases), volume of logs ingested (e.g. for data processing systems)</span></p></li><li><p><strong>Saturation:</strong><span> The percentage of resources that are being used. Note that this can easily be a vanity metric. If the SLI does not map to how reliability is perceived from the service consumers point of view, it’s not a good indicator. A counter-example is client applications. For example, if the CPU/Memory usage of a mobile or web app is too high, it may impact the UX and you may want to set an object to reduce those numbers. You can use time-based (e.g. number of minutes where the users pace CPU usage is below 60%) or event-based (e.g. amount of memory consumed per opened file).</span></p></li><li><p><strong>Durability</strong><span>: the likelihood that data will be retained over a long period of time (useful for </span><a href="https://sre.google/sre-book/data-integrity/">data integrity</a><span>)</span></p></li></ul><h1 class="header-anchor-post">What is not a good SLI?<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§what-is-not-a-good-sli" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/what-is-not-a-good-sli" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h1><p><span>SLI indicates how reliability is </span><strong>perceived from the service consumers’ perspective</strong><span>. In a future article we will discuss 🔴what makes a good SLI.</span></p><p>SLIs primarily concern engineers responsible for the technical solution, not how the product performs. 🔴SLI is not KPI.</p><p>Most metrics are an aggregation of multiple variables. An engineering team doesn’t control all those variables. Therefore, we have to dig a layer deeper to find the part of the metric that is controlled by a specific part of the organization that owns it and is expected to commit to a SLI.</p><div class="pullquote"><p>You should only be accountable for what you control. The reverse is also true: you should control what you’re held accountable for.</p></div><p><span>Example for good metrics that are </span><strong>bad</strong><span> SLIs:</span></p><ul><li><p>Number of items sold per month (engineers don’t control user demand or purchase power)</p></li><li><p>Team health metrics from tools like OfficeVibe: assuming the EM has the mandate and accountability to shape the team health, this is an organizational metric.</p></li><li><p>CPU/RAM/Net/Disk usage: SLI ties to SLO which in turn ties to Alert which ties to on-call. If it doesn't worth waking someone up in the middle of the night, it’s not a good SLI. It may be a great metric to do root cause analysis once the incident happens, but alone, it’s not something to optimize unless we have insight that this is in fact the metric that defines how consumers perceive reliability.</p></li><li><p>User Satisfaction Survey: this is a great metric but not something that is completely under the control of the engineering team in the scope of system reliability. User satisfaction may be impacted by things that are outside the control of the engineers responsible for the technical solution.</p></li><li><p>Conversion rate: again, this is a metric that is primarily controlled by marketing, UX and design and not all parameters that can affect the conversion is under control of the engineers.</p></li><li><p>Test coverage: does not measure system reliability as the users perceive it. A system with poor test coverage that has an uptime above expectation is good. Another system with 100% test coverage which suffers from poor latency (assuming that latency is the SLI), is not good.</p></li></ul><div data-component-name="DigestPostEmbed" class="digestPostEmbed-flwiST"><a href="https://blog.alexewerlof.com/p/responsible-for-control" rel="noopener" target="_blank"><div class="pencraft pc-display-flex pc-gap-16 pc-reset"><div style="width:70px;height:70px;" class="pencraft pc-reset"><picture><source type="image/webp" srcset="https://substackcdn.com/image/fetch/$s_!18Wx!,w_140,h_140,c_fill,f_webp,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F7855ff73-782f-4250-b6f4-f6bb636d37af_1067x738.png"/><img src="https://substackcdn.com/image/fetch/$s_!18Wx!,w_140,h_140,c_fill,f_auto,q_auto:good,fl_progressive:steep,g_auto/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F7855ff73-782f-4250-b6f4-f6bb636d37af_1067x738.png" sizes="100vw" alt="You should never be responsible for what you don't control" width="140" height="140" class="img-OACg1c smSquare-NGbPBa pencraft pc-reset"/></picture></div><div class="pencraft pc-display-flex pc-flexDirection-column pc-reset"><h4 class="pencraft pc-reset color-pub-primary-text-NyXPlw line-height-24-jnGwiv font-display-nhmvtD size-20-P_cSRT weight-bold-DmI9lw reset-IxiVJZ">You should never be responsible for what you don't control</h4><div class="pencraft pc-display-flex pc-gap-4 pc-alignItems-center pc-reset"><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA"><a href="https://substack.com/profile/87732486-alex-ewerlof" class="inheritColor-WetTGJ">Alex Ewerlöf</a></div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T reset-IxiVJZ">·</div><div class="pencraft pc-reset color-pub-secondary-text-hGQ02T line-height-20-t4M0El font-meta-MWBumP size-11-NuY2Zx weight-medium-fw81nC transform-uppercase-yKDgcq reset-IxiVJZ meta-EgzBVA">February 20, 2024</div></div><div class="pencraft pc-display-flex pc-gap-16 pc-paddingTop-0 pc-paddingBottom-0 pc-alignItems-center pc-reset"><a href="https://blog.alexewerlof.com/p/responsible-for-control" class="pencraft pc-reset align-center-y7ZD4w line-height-20-t4M0El font-text-qe4AeH size-13-hZTUKr weight-medium-fw81nC reset-IxiVJZ"><div class="pencraft pc-display-flex pc-gap-8 pc-alignItems-center pc-reset link-HREYZo"><span class="pencraft pc-reset color-accent-BVX_7M line-height-20-t4M0El font-text-qe4AeH size-14-MLPa7j weight-semibold-uqA4FV reset-IxiVJZ">Read full story</span><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-arrow-right"><path d="M5 12h14"></path><path d="m12 5 7 7-7 7"></path></svg></div></a></div></div></div></a></div><h1 class="header-anchor-post">Pro-Tips<div class="pencraft pc-display-flex pc-alignItems-center pc-position-absolute pc-reset header-anchor-parent"><div class="pencraft pc-display-contents pc-reset pubTheme-yiXxQA"><div id="§pro-tips" class="pencraft pc-reset header-anchor offset-top"></div><button tabindex="0" type="button" aria-label="Link" data-href="https://blog.alexewerlof.com/i/135805725/pro-tips" class="pencraft pc-reset pencraft iconButton-mq_Et5 iconButtonBase-dJGHgN buttonBase-GK1x3M buttonStyle-r7yGCK size_sm-G3LciD priority_secondary-S63h9o"><svg xmlns="http://www.w3.org/2000/svg" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-link"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"></path><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"></path></svg></button></div></div></h1><p><em><span>These posts take anything from a few hours to days to draft, research, edit, illustrate, and publish. 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It explores how SLIs, as percentages, reflect the 'good' in a given period, differentiating them from other metrics like MAU. The piece simplifies the SLI formula, using real-world examples to illustrate its application in both event-based and time-based contexts. It also touches on common SLIs like availability, success rate, and latency, while highlighting what does not constitute a good SLI. This informative guide is essential for understanding the nuances of SLIs and their impact on system performance, paving the way for a future discussion on Service Level Objectives (SLOs).\",\"search_engine_title\":\"Service Level Indicator (SLI)\",\"section_id\":67723,\"should_send_free_preview\":true,\"show_guest_bios\":true,\"slug\":\"sli\",\"social_title\":null,\"subtitle\":\"Introduction to SLI, examples, counterexamples and tips\",\"teaser_post_eligible\":true,\"title\":\"Service Level Indicators\",\"type\":\"newsletter\",\"video_upload_id\":null,\"write_comment_permissions\":\"only_paid\",\"meter_type\":\"none\",\"live_stream_id\":null,\"is_published\":true,\"restacks\":1,\"reactions\":{\"\u2764\":16},\"top_exclusions\":[],\"pins\":[],\"section_pins\":[],\"has_shareable_clips\":false,\"previous_post_slug\":\"multi-tiered-slos\",\"next_post_slug\":\"10x9\",\"cover_image\":\"https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png\",\"cover_image_is_square\":false,\"cover_image_is_explicit\":false,\"videoUpload\":null,\"podcastFields\":{\"post_id\":135805725,\"podcast_episode_number\":null,\"podcast_season_number\":null,\"podcast_episode_type\":null,\"should_syndicate_to_other_feed\":null,\"syndicate_to_section_id\":null,\"hide_from_feed\":false,\"free_podcast_url\":null,\"free_podcast_duration\":null,\"preview_contains_ad\":false,\"was_imported_self_serve_sync\":false,\"draft_free_podcast_url\":null,\"draft_free_podcast_duration\":null},\"podcastUpload\":null,\"podcastPreviewUpload\":null,\"voiceover_upload_id\":null,\"voiceoverUpload\":null,\"has_voiceover\":false,\"description\":\"Introduction to SLI, examples, counterexamples and tips\",\"body_html\":\"<p>In <a href=\\\"https://blog.alexewerlof.com/p/why-bother-with-sli-and-slo\\\">why bother with SLI and SLO</a> we discussed why service levels are important. This article focuses on SLI and a follow up will discuss <a href=\\\"https://blog.alexewerlof.com/p/sli\\\">SLO</a>.</p><div class=\\\"pullquote\\\"><p>A Service Level Indicator (SLI) is a reliability metric that shows the percentage of perceived <em>good</em> in a period of time and defines the service owners\u2019 commitment towards service consumers.</p></div><p>Let\u2019s unpack that:</p><ul><li><p><strong>SLI is a metric:</strong> it goes up and down. On its own SLI cannot tell if the system is reliable or not. We need the Service Level <em>Objective</em> (SLO) to know what we promised and where do we stand in relation to that promise.</p></li><li><p><strong>SLI is a reliability metric:</strong> there are many metrics that can indicate the behavior of a system or group of systems. SLI is particularly focused on the reliability aspect. For example: MAU (monthly active users) of a system may be a great product metric but it is not a reliability metric. Sure, the reliability can impact the MAU, but the two metrics have fundamentally different audiences and control mechanisms. SLI caters towards engineers who build a solution.</p></li><li><p><strong>SLI is concerned with perceived reliability:</strong> SLI is set at the boundaries of your service towards its consumers. For example, if you offer an API to your users, they may perceive its reliability by its latency, error rate, or other metrics, but they don\u2019t care and shouldn\u2019t be concerned with how your API is built behind the scenes. As far as they are concerned, your service is a black box to them and it\u2019s not up to them how you design it. This gives freedom to the service owners to <a href=\\\"https://blog.alexewerlof.com/p/you-build-it-you-own-it\\\">truly own all aspects of the system</a>.</p></li><li><p><strong>SLI ties to alerting:</strong><em><strong> </strong></em>SLI defines what we measure and <a href=\\\"https://blog.alexewerlof.com/p/slo\\\">SLO</a> sets a target for it while <a href=\\\"https://blog.alexewerlof.com/p/alerting-on-slo\\\">alerts</a> are ultimately set after the SLOs to communicate the service owners\u2019 accountability towards the service consumers.</p></li><li><p><strong>SLI is a percentage:</strong> as we\u2019ll see the formula for SLI indicates the percentage of good in a set of values that are in the scope of optimization (valid). Therefore, the value of the SLI is always from 0 to 100. Although technically any raw metric can be considered an SLI, Google (where these ideas originated from \u201Cgenerally recommend treating the SLI as the ratio of two numbers\u201D (<a href=\\\"https://sre.google/workbook/implementing-slos/\\\">reference</a>).</p></li><li><p><strong>SLI is positive:</strong> SLI is concerned with <em>good</em>, not bad. For example, instead of focusing on error rate, SLI is concerned with the success rate. For the opposite view, please see the <a href=\\\"https://blog.alexewerlof.com/p/error-budget\\\">error budgets</a>.</p></li><li><p><strong>SLI is aggregated over a period of time:</strong> Unlike many real-time metrics like CPU usage, SLI aggregates the values over a period of time. This aggregation is a very powerful concept as it allows for failure (<a href=\\\"https://blog.alexewerlof.com/p/error-budget\\\">error budget</a>) in a controlled manner. The time period attribute (also known as the <a href=\\\"https://blog.alexewerlof.com/p/compliance-period\\\">compliance period</a>) is an attribute of the <a href=\\\"https://blog.alexewerlof.com/p/slo\\\">SLO</a>.</p></li></ul><h2>Formula</h2><p>The formula for SLI is very simple:</p><div class=\\\"latex-rendered\\\" data-attrs=\\\"{&quot;persistentExpression&quot;:&quot;SLI = \\\\\\\\frac {\\\\\\\\text{good}} {\\\\\\\\text{valid}} \\\\\\\\times 100&quot;,&quot;id&quot;:&quot;CTJFWOKCBG&quot;}\\\" data-component-name=\\\"LatexBlockToDOM\\\"></div><ul><li><p>The formula is the same whether we\u2019re counting events (<em>good events</em> / <em>valid events</em>) or time slots (<em>good time slots</em> / <em>valid time slots</em>).</p></li><li><p><em>Valid</em> is often simplified to total</p></li></ul><p>We have covered these in detail before:</p><div class=\\\"digest-post-embed\\\" data-attrs=\\\"{&quot;nodeId&quot;:&quot;96aea14a-0134-4739-8cc2-a8091da24efd&quot;,&quot;caption&quot;:&quot;SLI is defined as the percentage of good divided by valid: There are two types of Service Level Indicators based on the definition of what good looks like: Time based: aims to maximize good time Event based: aims to maximize good events (e.g., requests that didn\u2019t lead to errors or got responded in a reasonable time)&quot;,&quot;cta&quot;:null,&quot;showBylines&quot;:true,&quot;showDescription&quot;:true,&quot;showImage&quot;:true,&quot;size&quot;:&quot;sm&quot;,&quot;isEditorNode&quot;:true,&quot;title&quot;:&quot;Time based vs Event based SLIs&quot;,&quot;publishedBylines&quot;:[{&quot;id&quot;:87732486,&quot;name&quot;:&quot;Alex Ewerl\u00F6f&quot;,&quot;bio&quot;:&quot;Senior Staff Engineer at Volvo. MSc Systems Engineering, KTH. Stockholmer, dad, amateur artist.\\\\n\\\\nWrites about technical leadership, growth and software.\\\\n\\\\nRead more here: https://www.alexewerlof.com/who&quot;,&quot;photo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fbucketeer-e05bbc84-baa3-437e-9518-adb32be77984.s3.amazonaws.com%2Fpublic%2Fimages%2Fe2713990-da82-481b-b579-01a7aaa5b85b_560x560.jpeg&quot;,&quot;is_guest&quot;:false,&quot;bestseller_tier&quot;:null}],&quot;post_date&quot;:&quot;2023-08-07T21:31:48.931Z&quot;,&quot;cover_image&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fd3be84d4-5a4b-4e52-8e7e-77744df9eaa7_1093x705.png&quot;,&quot;cover_image_alt&quot;:null,&quot;canonical_url&quot;:&quot;https://blog.alexewerlof.com/p/time-based-vs-event-based&quot;,&quot;section_name&quot;:&quot;SRE&quot;,&quot;video_upload_id&quot;:null,&quot;id&quot;:135808888,&quot;type&quot;:&quot;newsletter&quot;,&quot;reaction_count&quot;:1,&quot;comment_count&quot;:0,&quot;publication_id&quot;:null,&quot;publication_name&quot;:&quot;Alex Ewerl\u00F6f Notes&quot;,&quot;publication_logo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F8c58cb07-9341-402b-bcdb-9fa767c2cdac_500x500.png&quot;,&quot;belowTheFold&quot;:true,&quot;youtube_url&quot;:null,&quot;show_links&quot;:null,&quot;feed_url&quot;:null}\\\"></div><div class=\\\"digest-post-embed\\\" data-attrs=\\\"{&quot;nodeId&quot;:&quot;588b46f7-3572-43f2-8f2e-9e3019ed4e22&quot;,&quot;caption&quot;:&quot;In a recent article, we discussed the service level indicator formula: Another article discussed the valid. This article talks about the definition of good. Depending on the type of SLI: Time-based SLI: good specifies a good time slot Event-based SLI:&quot;,&quot;cta&quot;:null,&quot;showBylines&quot;:true,&quot;showDescription&quot;:true,&quot;showImage&quot;:true,&quot;size&quot;:&quot;sm&quot;,&quot;isEditorNode&quot;:true,&quot;title&quot;:&quot;Definition of good in SLI&quot;,&quot;publishedBylines&quot;:[{&quot;id&quot;:87732486,&quot;name&quot;:&quot;Alex Ewerl\u00F6f&quot;,&quot;bio&quot;:&quot;Senior Staff Engineer at Volvo. MSc Systems Engineering, KTH. Stockholmer, dad, amateur artist.\\\\n\\\\nWrites about technical leadership, growth and software.\\\\n\\\\nRead more here: https://www.alexewerlof.com/who&quot;,&quot;photo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fbucketeer-e05bbc84-baa3-437e-9518-adb32be77984.s3.amazonaws.com%2Fpublic%2Fimages%2Fe2713990-da82-481b-b579-01a7aaa5b85b_560x560.jpeg&quot;,&quot;is_guest&quot;:false,&quot;bestseller_tier&quot;:null}],&quot;post_date&quot;:&quot;2023-08-08T21:44:47.211Z&quot;,&quot;cover_image&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F21343ed3-10ed-46b9-b590-7990ae90d5b9_1021x624.png&quot;,&quot;cover_image_alt&quot;:null,&quot;canonical_url&quot;:&quot;https://blog.alexewerlof.com/p/sli-good&quot;,&quot;section_name&quot;:&quot;SRE&quot;,&quot;video_upload_id&quot;:null,&quot;id&quot;:135845681,&quot;type&quot;:&quot;newsletter&quot;,&quot;reaction_count&quot;:0,&quot;comment_count&quot;:0,&quot;publication_id&quot;:null,&quot;publication_name&quot;:&quot;Alex Ewerl\u00F6f Notes&quot;,&quot;publication_logo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F8c58cb07-9341-402b-bcdb-9fa767c2cdac_500x500.png&quot;,&quot;belowTheFold&quot;:true,&quot;youtube_url&quot;:null,&quot;show_links&quot;:null,&quot;feed_url&quot;:null}\\\"></div><div class=\\\"digest-post-embed\\\" data-attrs=\\\"{&quot;nodeId&quot;:&quot;260a87c2-6cf9-4e59-b393-cb987687805f&quot;,&quot;caption&quot;:&quot;In simple terms, SLI is the percentage of good. More accurately however, it is the percentage of good (event or time) from a valid set (event or time): For simplicity, sometimes the word \\\\&quot;total\\\\&quot; is used instead of \\\\&quot;valid\\\\&quot; but there are differences between the two.&quot;,&quot;cta&quot;:null,&quot;showBylines&quot;:true,&quot;showDescription&quot;:true,&quot;showImage&quot;:true,&quot;size&quot;:&quot;sm&quot;,&quot;isEditorNode&quot;:true,&quot;title&quot;:&quot;SLI: Valid vs Total&quot;,&quot;publishedBylines&quot;:[{&quot;id&quot;:87732486,&quot;name&quot;:&quot;Alex Ewerl\u00F6f&quot;,&quot;bio&quot;:&quot;Senior Staff Engineer at Volvo. MSc Systems Engineering, KTH. Stockholmer, dad, amateur artist.\\\\n\\\\nWrites about technical leadership, growth and software.\\\\n\\\\nRead more here: https://www.alexewerlof.com/who&quot;,&quot;photo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fbucketeer-e05bbc84-baa3-437e-9518-adb32be77984.s3.amazonaws.com%2Fpublic%2Fimages%2Fe2713990-da82-481b-b579-01a7aaa5b85b_560x560.jpeg&quot;,&quot;is_guest&quot;:false,&quot;bestseller_tier&quot;:null}],&quot;post_date&quot;:&quot;2023-08-08T15:36:08.376Z&quot;,&quot;cover_image&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F34dfb671-d82c-4097-87b3-05a4a639935a_758x609.png&quot;,&quot;cover_image_alt&quot;:null,&quot;canonical_url&quot;:&quot;https://blog.alexewerlof.com/p/valid-vs-total&quot;,&quot;section_name&quot;:&quot;SRE&quot;,&quot;video_upload_id&quot;:null,&quot;id&quot;:135805327,&quot;type&quot;:&quot;newsletter&quot;,&quot;reaction_count&quot;:0,&quot;comment_count&quot;:0,&quot;publication_id&quot;:null,&quot;publication_name&quot;:&quot;Alex Ewerl\u00F6f Notes&quot;,&quot;publication_logo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F8c58cb07-9341-402b-bcdb-9fa767c2cdac_500x500.png&quot;,&quot;belowTheFold&quot;:true,&quot;youtube_url&quot;:null,&quot;show_links&quot;:null,&quot;feed_url&quot;:null}\\\"></div><h2>Example: Event based</h2><p>Let\u2019s imagine that we have two systems. They can be a front-end and backend, or two microservices, or a backend and database, or something else. It can\u2019t get simpler than that! One system depends on the other.</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!7TGg!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!7TGg!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png\\\" width=\\\"891\\\" height=\\\"320\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:320,&quot;width&quot;:891,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:22837,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!7TGg!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!7TGg!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F307d5e0f-7e33-45b4-b24d-ee899ae69923_891x320.png 1456w\\\" sizes=\\\"100vw\\\" loading=\\\"lazy\\\"></picture><div class=\\\"image-link-expand\\\"><div class=\\\"pencraft pc-display-flex pc-gap-8 pc-reset\\\"><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container restack-image\\\"><svg aria-hidden=\\\"true\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><p>Let\u2019s say that the response latency is critical for system A. The owners of system A want the owners of system B to be responsible for a good latency. More specifically, they want the latency to be below 2000ms.</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!z86H!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!z86H!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!z86H!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png\\\" width=\\\"891\\\" height=\\\"320\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/aaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:320,&quot;width&quot;:891,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:26702,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!z86H!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 424w, https://substackcdn.com/image/fetch/$s_!z86H!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 848w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1272w, https://substackcdn.com/image/fetch/$s_!z86H!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Faaa17e02-ef90-4f34-af44-3effff2e207c_891x320.png 1456w\\\" sizes=\\\"100vw\\\" loading=\\\"lazy\\\"></picture><div class=\\\"image-link-expand\\\"><div class=\\\"pencraft pc-display-flex pc-gap-8 pc-reset\\\"><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container restack-image\\\"><svg aria-hidden=\\\"true\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><p>Here are some example latencies:</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!ZsT6!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!ZsT6!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 424w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 848w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1272w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png\\\" width=\\\"888\\\" height=\\\"443\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:443,&quot;width&quot;:888,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:49539,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!ZsT6!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 424w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 848w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1272w, https://substackcdn.com/image/fetch/$s_!ZsT6!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1afabd90-5b4d-42b2-acf7-d149c162b55a_888x443.png 1456w\\\" sizes=\\\"100vw\\\" loading=\\\"lazy\\\"></picture><div class=\\\"image-link-expand\\\"><div class=\\\"pencraft pc-display-flex pc-gap-8 pc-reset\\\"><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container restack-image\\\"><svg aria-hidden=\\\"true\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><p>The ones in red are above the 2000ms threshold. Drawing the same numbers on a diagram, we get this:</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!o3NC!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!o3NC!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 424w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 848w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1272w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png\\\" width=\\\"941\\\" height=\\\"532\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:532,&quot;width&quot;:941,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:32557,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!o3NC!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 424w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 848w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1272w, https://substackcdn.com/image/fetch/$s_!o3NC!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F26ebd0a9-0155-4eaa-a7e3-53932d3c46b4_941x532.png 1456w\\\" sizes=\\\"100vw\\\" loading=\\\"lazy\\\"></picture><div class=\\\"image-link-expand\\\"><div class=\\\"pencraft pc-display-flex pc-gap-8 pc-reset\\\"><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container restack-image\\\"><svg aria-hidden=\\\"true\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><p>The formula for SLI is:</p><div class=\\\"latex-rendered\\\" data-attrs=\\\"{&quot;persistentExpression&quot;:&quot;SLI = \\\\\\\\frac {\\\\\\\\text{requests with good response time}} {\\\\\\\\text{total requests}} \\\\\\\\times 100 = \\\\\\\\frac {4} {8} \\\\\\\\times 100 = 50\\\\\\\\%&quot;,&quot;id&quot;:&quot;UYZEGJMQEO&quot;}\\\" data-component-name=\\\"LatexBlockToDOM\\\"></div><p>In this example, we have 8 response latencies where 4 of them are responded in good time (less than 2000ms). The SLI in this particular period is:</p><p>Let\u2019s use some numbers that are more realistic:</p><ul><li><p>In a month, we get 1,000,300 requests</p></li><li><p>992,300 of them were responded in less than 2000ms</p></li></ul><div class=\\\"latex-rendered\\\" data-attrs=\\\"{&quot;persistentExpression&quot;:&quot;SLI = \\\\\\\\frac {\\\\\\\\text{992300}} {\\\\\\\\text{1000300}} \\\\\\\\times 100 = 99.2\\\\\\\\%&quot;,&quot;id&quot;:&quot;XMANTDCDYT&quot;}\\\" data-component-name=\\\"LatexBlockToDOM\\\"></div><h2>Example: Time based</h2><p>Let\u2019s say that the reliability of system B is measured by uptime. Its endpoint is probed every minute and we get this data:</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!LSe5!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!LSe5!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 424w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 848w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1272w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png\\\" width=\\\"946\\\" height=\\\"317\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:317,&quot;width&quot;:946,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:15831,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!LSe5!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 424w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_848,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 848w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1272,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1272w, https://substackcdn.com/image/fetch/$s_!LSe5!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F6cbe4582-fd1c-4a73-9af8-135550b67cc7_946x317.png 1456w\\\" sizes=\\\"100vw\\\" loading=\\\"lazy\\\"></picture><div class=\\\"image-link-expand\\\"><div class=\\\"pencraft pc-display-flex pc-gap-8 pc-reset\\\"><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container restack-image\\\"><svg aria-hidden=\\\"true\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><p>SLI is the number of bad probes (or bad minutes) divided by the total number of probes (or the minutes in the measurement window). In this example, 28 out of 36 probes succeeded:</p><div class=\\\"latex-rendered\\\" data-attrs=\\\"{&quot;persistentExpression&quot;:&quot;SLI = \\\\\\\\frac {\\\\\\\\text{28}} {\\\\\\\\text{36}} \\\\\\\\times 100 = 77.7\\\\\\\\%&quot;,&quot;id&quot;:&quot;VWOYRCCCVA&quot;}\\\" data-component-name=\\\"LatexBlockToDOM\\\"></div><p>Uptime is a common SLI. You may have seen this type of diagram for other products. For example <a href=\\\"https://status.brave.com/\\\">status.brave.com</a> looks like this:</p><div class=\\\"captioned-image-container\\\"><figure><a class=\\\"image-link image2 is-viewable-img\\\" target=\\\"_blank\\\" href=\\\"https://substackcdn.com/image/fetch/$s_!p3pB!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png\\\" data-component-name=\\\"Image2ToDOM\\\"><div class=\\\"image2-inset\\\"><picture><source type=\\\"image/webp\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!p3pB!,w_424,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 424w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_848,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 848w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1272,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1272w, https://substackcdn.com/image/fetch/$s_!p3pB!,w_1456,c_limit,f_webp,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 1456w\\\" sizes=\\\"100vw\\\"><img src=\\\"https://substackcdn.com/image/fetch/$s_!p3pB!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png\\\" width=\\\"798\\\" height=\\\"418\\\" data-attrs=\\\"{&quot;src&quot;:&quot;https://substack-post-media.s3.amazonaws.com/public/images/75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png&quot;,&quot;srcNoWatermark&quot;:null,&quot;fullscreen&quot;:null,&quot;imageSize&quot;:null,&quot;height&quot;:418,&quot;width&quot;:798,&quot;resizeWidth&quot;:null,&quot;bytes&quot;:25347,&quot;alt&quot;:null,&quot;title&quot;:null,&quot;type&quot;:&quot;image/png&quot;,&quot;href&quot;:null,&quot;belowTheFold&quot;:true,&quot;topImage&quot;:false,&quot;internalRedirect&quot;:null,&quot;isProcessing&quot;:false,&quot;align&quot;:null,&quot;offset&quot;:false}\\\" class=\\\"sizing-normal\\\" alt=\\\"\\\" srcset=\\\"https://substackcdn.com/image/fetch/$s_!p3pB!,w_424,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F75ecb03c-9658-42ee-9af5-ecd928fb76e3_798x418.png 424w, 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height=\\\"20\\\" viewBox=\\\"0 0 20 20\\\" fill=\\\"none\\\" stroke-width=\\\"1.5\\\" stroke=\\\"var(--color-fg-primary)\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" xmlns=\\\"http://www.w3.org/2000/svg\\\"><g><path d=\\\"M2.53001 7.81595C3.49179 4.73911 6.43281 2.5 9.91173 2.5C13.1684 2.5 15.9537 4.46214 17.0852 7.23684L17.6179 8.67647M17.6179 8.67647L18.5002 4.26471M17.6179 8.67647L13.6473 6.91176M17.4995 12.1841C16.5378 15.2609 13.5967 17.5 10.1178 17.5C6.86118 17.5 4.07589 15.5379 2.94432 12.7632L2.41165 11.3235M2.41165 11.3235L1.5293 15.7353M2.41165 11.3235L6.38224 13.0882\\\"></path></g></svg></button><button tabindex=\\\"0\\\" type=\\\"button\\\" class=\\\"pencraft pc-reset pencraft icon-container view-image\\\"><svg xmlns=\\\"http://www.w3.org/2000/svg\\\" width=\\\"20\\\" height=\\\"20\\\" viewBox=\\\"0 0 24 24\\\" fill=\\\"none\\\" stroke=\\\"currentColor\\\" stroke-width=\\\"2\\\" stroke-linecap=\\\"round\\\" stroke-linejoin=\\\"round\\\" class=\\\"lucide lucide-maximize2 lucide-maximize-2\\\"><polyline points=\\\"15 3 21 3 21 9\\\"></polyline><polyline points=\\\"9 21 3 21 3 15\\\"></polyline><line x1=\\\"21\\\" x2=\\\"14\\\" y1=\\\"3\\\" y2=\\\"10\\\"></line><line x1=\\\"3\\\" x2=\\\"10\\\" y1=\\\"21\\\" y2=\\\"14\\\"></line></svg></button></div></div></div></a></figure></div><h2>Common SLIs</h2><p>Here\u2019s a list of the most common SLIs:</p><ul><li><p><strong>Availability</strong>: the fraction of the time that a service is reachable</p></li><li><p><strong>Success rate:</strong> The percentage of requests that are successful (the opposite of <strong>Error Rate</strong>, but in SLI formula we focus on Good/Valid)</p></li><li><p><strong>Latency</strong>: how long it takes to return a response to a request or do a computation</p><ul><li><p>Latency and response time can be different things: <a href=\\\"https://scalabledeveloper.com/posts/latency-versus-response-time/\\\">Latency versus Response Time | Scalable Developer</a></p></li></ul></li><li><p><strong>Throughput</strong>: The number of requests that can be processed per unit of time. Typically measured in requests per second, messages consumed (e.g. useful for queue-based workers), queries processed (e.g. for Databases), volume of logs ingested (e.g. for data processing systems)</p></li><li><p><strong>Saturation:</strong> The percentage of resources that are being used. Note that this can easily be a vanity metric. If the SLI does not map to how reliability is perceived from the service consumers point of view, it\u2019s not a good indicator. A counter-example is client applications. For example, if the CPU/Memory usage of a mobile or web app is too high, it may impact the UX and you may want to set an object to reduce those numbers. You can use time-based (e.g. number of minutes where the users pace CPU usage is below 60%) or event-based (e.g. amount of memory consumed per opened file).</p></li><li><p><strong>Durability</strong>: the likelihood that data will be retained over a long period of time (useful for <a href=\\\"https://sre.google/sre-book/data-integrity/\\\">data integrity</a>)</p></li></ul><h1>What is not a good SLI?</h1><p>SLI indicates how reliability is <strong>perceived from the service consumers\u2019 perspective</strong>. In a future article we will discuss \uD83D\uDD34what makes a good SLI.</p><p>SLIs primarily concern engineers responsible for the technical solution, not how the product performs. \uD83D\uDD34SLI is not KPI.</p><p>Most metrics are an aggregation of multiple variables. An engineering team doesn\u2019t control all those variables. Therefore, we have to dig a layer deeper to find the part of the metric that is controlled by a specific part of the organization that owns it and is expected to commit to a SLI.</p><div class=\\\"pullquote\\\"><p>You should only be accountable for what you control. The reverse is also true: you should control what you\u2019re held accountable for.</p></div><p>Example for good metrics that are <strong>bad</strong> SLIs:</p><ul><li><p>Number of items sold per month (engineers don\u2019t control user demand or purchase power)</p></li><li><p>Team health metrics from tools like OfficeVibe: assuming the EM has the mandate and accountability to shape the team health, this is an organizational metric.</p></li><li><p>CPU/RAM/Net/Disk usage: SLI ties to SLO which in turn ties to Alert which ties to on-call. If it doesn't worth waking someone up in the middle of the night, it\u2019s not a good SLI. It may be a great metric to do root cause analysis once the incident happens, but alone, it\u2019s not something to optimize unless we have insight that this is in fact the metric that defines how consumers perceive reliability.</p></li><li><p>User Satisfaction Survey: this is a great metric but not something that is completely under the control of the engineering team in the scope of system reliability. User satisfaction may be impacted by things that are outside the control of the engineers responsible for the technical solution.</p></li><li><p>Conversion rate: again, this is a metric that is primarily controlled by marketing, UX and design and not all parameters that can affect the conversion is under control of the engineers.</p></li><li><p>Test coverage: does not measure system reliability as the users perceive it. A system with poor test coverage that has an uptime above expectation is good. Another system with 100% test coverage which suffers from poor latency (assuming that latency is the SLI), is not good.</p></li></ul><div class=\\\"digest-post-embed\\\" data-attrs=\\\"{&quot;nodeId&quot;:&quot;e9c6d70c-8529-48cf-9f5f-8abbdf5b6f24&quot;,&quot;caption&quot;:&quot;One of my favorite sentences to say during Service Level Workshops is: You should never be responsible for what you don't control That is because most metrics usually aggregate a bunch of variables, not all of which are in your control. It is unfair and unrealistic to hold someone responsible for what they can\u2019t control.&quot;,&quot;cta&quot;:null,&quot;showBylines&quot;:true,&quot;showDescription&quot;:true,&quot;showImage&quot;:true,&quot;size&quot;:&quot;sm&quot;,&quot;isEditorNode&quot;:true,&quot;title&quot;:&quot;You should never be responsible for what you don't control&quot;,&quot;publishedBylines&quot;:[{&quot;id&quot;:87732486,&quot;name&quot;:&quot;Alex Ewerl\u00F6f&quot;,&quot;bio&quot;:&quot;Senior Staff Engineer at Volvo. MSc Systems Engineering, KTH. Stockholmer, dad, amateur artist.\\\\n\\\\nWrites about technical leadership, growth and software.\\\\n\\\\nRead more here: https://www.alexewerlof.com/who&quot;,&quot;photo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fbucketeer-e05bbc84-baa3-437e-9518-adb32be77984.s3.amazonaws.com%2Fpublic%2Fimages%2Fe2713990-da82-481b-b579-01a7aaa5b85b_560x560.jpeg&quot;,&quot;is_guest&quot;:false,&quot;bestseller_tier&quot;:null}],&quot;post_date&quot;:&quot;2024-02-20T18:54:47.854Z&quot;,&quot;cover_image&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F7855ff73-782f-4250-b6f4-f6bb636d37af_1067x738.png&quot;,&quot;cover_image_alt&quot;:null,&quot;canonical_url&quot;:&quot;https://blog.alexewerlof.com/p/responsible-for-control&quot;,&quot;section_name&quot;:&quot;SRE&quot;,&quot;video_upload_id&quot;:null,&quot;id&quot;:141855236,&quot;type&quot;:&quot;newsletter&quot;,&quot;reaction_count&quot;:13,&quot;comment_count&quot;:0,&quot;publication_id&quot;:null,&quot;publication_name&quot;:&quot;Alex Ewerl\u00F6f Notes&quot;,&quot;publication_logo_url&quot;:&quot;https://substackcdn.com/image/fetch/f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F8c58cb07-9341-402b-bcdb-9fa767c2cdac_500x500.png&quot;,&quot;belowTheFold&quot;:true,&quot;youtube_url&quot;:null,&quot;show_links&quot;:null,&quot;feed_url&quot;:null}\\\"></div><h1>Pro-Tips</h1><p><em>These posts take anything from a few hours to days to draft, research, edit, illustrate, and publish. The main chunk of the article is available for free. \uD83D\uDD13 For those who spare a few bucks, the \u201Cpro tips\u201D unlocked as a token of appreciation (Right now, you can get 20% off via <a href=\\\"https://blog.alexewerlof.com/protipsdiscount\\\">this link</a>). And for those who choose not to subscribe, it is fine too. I appreciate your time reading this far and hopefully you share it withing your circles to inspire others. \uD83D\uDE4FYou can also <a href=\\\"https://www.linkedin.com/in/alexewerlof/\\\">follow me on LinkedIn</a> where I share useful tips about technical leadership and growth mindset.</em></p>\",\"has_dynamic_content\":false,\"truncated_body_text\":\"In why bother with SLI and SLO we discussed why service levels are important. 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