236 lines
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<h1 class="title" itemprop="name">Magic Numbers and second guessing SLOs - why is 96% better than 95%?</h1>
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<div class="date"><time datetime='Wed, Nov 13, 2019' data-updated="true" itemprop="datePublished">Wed, Nov 13, 2019</time>
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<div class="tags">
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<a href="https://www.unixdaemon.net/categories/sysadmin"> sysadmin </a>
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</div>
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<div class="entry-content" itemprop="articleBody"><p>I’ve had a half written draft of this post sitting in a folder for the
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last six months and I’ve not been able to shake the root cause so I’m
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going to publish it and see what the feedback teaches me. But first
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the heresy - Service Level Objectives make me uncomfortable.</p>
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<p>I have no issue with the idea that you need some form of measurement and
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tracking to ensure you’re maintaining an acceptable level of service but
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when reading posts on SLOs, or watching recorded conference sessions,
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the concept seems to imply some rigour and background process to
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determine the numbers to work towards that feels decoupled from
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any hard details and often comes across as either a guesstimate or just a
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Current Representation of Actual Percentages.</p>
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<p>Maybe it’s the industry I’m currently in and the unique position it
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affords. As a Government department there’s no explicit per transaction
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financial goals. Cheaper and quicker is better but if we go over a per
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page serving cost of say 5p per page no one loses their job or brand new
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Mercedes bonus pot. A few of my previous roles were incredibly
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financially focused and knew how much they should be making every minute
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broken down by day and hour usage trends. I can see how service level
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objectives work in that world and I’d be a lot more comfortable with the
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concepts adoption if every use had something like this behind it:</p>
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<p>Let’s assume we work for ${ecommerce}.com and we always exactly hit our
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SLO in terms of successful requests and never go above or below it.
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Hopefully it’ll be quite easy to pull together our opening position.</p>
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<ul>
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<li>Our SLO is 95%</li>
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<li>We get 100,000 visits a day</li>
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<li>That’s 95,000 successful visits</li>
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<li>We make a sale in 5% of visits</li>
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<li>Which means we make 4,750 sales in a day</li>
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<li>Our average sale is 100USD so we make 475,000 USD a day</li>
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</ul>
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<p>With this kind of detail I can see how SLOs can help inform the discussion:</p>
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<ul>
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<li>Our new SLO will be 97%</li>
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<li>That’s 97,000 successful visits.</li>
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<li>We still make a sale in 5% of visits</li>
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<li>Which means we make 4,850 sales in a day</li>
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<li>Our average sale is still 100USD but now we make 485,000 USD a day</li>
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</ul>
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<p>In this scenario if we raised our SLO, and our game to match it, we
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would make an extra 10,000 USD each day. These numbers make it easier to build
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a case and decide how much engineering time and financial outlay we are
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willing to commit to possibly making that extra money and it shows what
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happens if we backslide in availability</p>
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<p>Unfortunately very few of the SLO posts and talks I’ve seen give any kind
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of breakdown like this. Instead they seem to rely on either picking an
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acceptable amount of downtime and encoding that or taking the current
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uptime percentage and elevating it beyond happenstance into a guiding
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principle. I guess my question would be: “If you changed your SLO value
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up or down 1-5 percentage points, what does that mean? What happens?”</p>
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<p>Maybe I’m looking for more than Service Level Objectives are offering or
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maybe the early adopters are all finding their way through it and the
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pioneering posts are deliberately simplified either to encourage people to
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take a look or hide the complexity of actual numbers that have no
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context outside of their home organisation but considering all the
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cheer leading I’d expect there to be more than just a helpful baseline
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that people can have in the back of their mind. So, where does that
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leave me? Continually questioning the process while trying to write
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enough SLOs that I either develop Stockholm syndrome or it finally
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clicks and that little nagging voice in the back of my head stops saying
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its just reliability theatre.</p>
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