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hover:bg-gray-100" href="/product/uptime-monitoring/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><circle cx="8" cy="8" r="6" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></circle><polyline points="8,3 8,8 11,11" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Uptime Monitoring</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Measure the availability of your digital footprint</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/heartbeat-monitoring/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 12L10.5 9.5L12 11L13.5 8.5L14.5 10.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 12L5.5 9.5L4 11L2.5 8.5L1.5 10.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 4L9.5 5.5L10.5 4.5L11.5 6.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Heartbeat Monitoring</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Catch cron jobs and backups that fail silently</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/synthetic-monitoring/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M2 8L6 4L10 8L14 4" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M2 12L6 8L10 12L14 8" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Synthetic Monitoring</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Simulate real user interactions across your stack</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/testing/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><rect x="2" y="2" width="12" height="12" rx="2.5" stroke="currentColor" stroke-linejoin="round"></rect><path d="M5 8.2 7 10.2 11 5.8" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Testing</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Catch issues before production with an AI-powered test reporter</span></a></div></div><div><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Communicate</p><div class="grid grid-cols-2 gap-x-5 gap-y-0.5 mt-1.5 w-[460px]"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/status-pages/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><rect x="2" y="3" width="12" height="10" rx="1" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></rect><path d="M2 6H14" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><circle cx="4" cy="4.5" r="0.5" fill="currentColor"></circle><circle cx="6" cy="4.5" r="0.5" fill="currentColor"></circle><circle cx="8" cy="4.5" r="0.5" fill="currentColor"></circle></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Status Pages</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Communicate app availability to your customers</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/alerting/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 1L15 14H1L8 1Z" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 6V9" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><circle cx="8" cy="12" r="0.5" fill="currentColor"></circle></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Alerts</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Contextual alerting to notify the team right away</span></a></div></div><div><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Resolve</p><div class="grid grid-cols-2 gap-x-5 gap-y-0.5 mt-1.5 w-[460px]"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/ai-analysis/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 2L14 5V11L8 14L2 11V5L8 2Z" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L14 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L2 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8V14" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">AI Root Cause Analysis</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Automated root cause analysis powered by AI agents</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/product/traces/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M2 8L6 4L10 8L14 4" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M2 12L6 8L10 12L14 8" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><circle cx="6" cy="4" r="1" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></circle><circle cx="10" cy="8" r="1" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></circle><circle cx="10" cy="12" r="1" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></circle></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Traces</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Powerful OTel tracing for deeper insights</span></a></div></div></div><div class="mt-5 pt-4 border-t border-gray-200"><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Getting Started</p><div class="flex flex-wrap items-center gap-1 mt-1.5 -ml-2.5"><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="https://app.checklyhq.com/signup/">Start for free</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/demo-request/">Book a demo</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/contact-sales/">Talk to Sales</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/product/monitoring-as-code/">Monitoring as Code</a></div></div></div></div></div></div><div class="relative flex items-center h-[var(--nav-height)]"><button class="nav-item" aria-expanded="false" aria-controls="nav-dropdown-solutions" aria-haspopup="true">Solutions<span class="dropdown-icon" aria-hidden="true"></span></button><div id="nav-dropdown-solutions" role="region" aria-label="Solutions" style="left:0" inert="" class="absolute top-full pt-2 z-30 origin-top transition duration-200 ease-out motion-reduce:transition-none motion-reduce:transform-none -translate-y-1 scale-95 opacity-0 pointer-events-none"><div class="nav-panel w-max max-w-[calc(100vw-2rem)] max-h-[calc(100vh-var(--nav-height)-1.5rem)] overflow-y-auto rounded-2xl border border-gray-200 bg-white text-checkly-dark-blue"><div class="p-6"><div class="flex gap-6"><div class="w-[248px]"><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Personas</p><div class="flex flex-col gap-y-0.5 mt-1.5"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/developers/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><polyline points="5.5,5 2.5,8 5.5,11" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><polyline points="10.5,5 13.5,8 10.5,11" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><path d="M9 3.5L7 12.5" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Developers</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Checks in TypeScript, in your repo, deployed with your app</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/sre/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 2.5L14 5.5L8 8.5L2 5.5L8 2.5Z" stroke="currentColor" stroke-linejoin="round"></path><polyline points="2,8.5 8,11.5 14,8.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><polyline points="2,11.5 8,14.5 14,11.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">SRE &amp; Platform</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">The synthetic layer for your observability stack</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/qa/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><polyline points="2,5 3.2,6.2 5.4,4" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><polyline points="2,11 3.2,12.2 5.4,10" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><path d="M7.5 5H14" stroke="currentColor" stroke-linecap="round"></path><path d="M7.5 11H14" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">QA Engineers</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Run your Playwright suite as production monitors</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/engineering-managers/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><circle cx="6" cy="5.5" r="2.25" stroke="currentColor"></circle><path d="M2 13.5c0-2.2 1.8-3.6 4-3.6s4 1.4 4 3.6" stroke="currentColor" stroke-linecap="round"></path><path d="M10.5 3.6a2.25 2.25 0 0 1 0 3.8" stroke="currentColor" stroke-linecap="round"></path><path d="M11.5 10.3c1.5.4 2.5 1.6 2.5 3.2" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Engineering Managers</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Give every team ownership of its own monitors</span></a></div></div><div class="w-[248px]"><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Use Cases</p><div class="flex flex-col gap-y-0.5 mt-1.5"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/use-cases/critical-user-flows/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><circle cx="3.25" cy="8" r="1.75" stroke="currentColor"></circle><circle cx="12.75" cy="4.25" r="1.75" stroke="currentColor"></circle><circle cx="12.75" cy="11.75" r="1.75" stroke="currentColor"></circle><path d="M4.9 7.4 11.1 4.9" stroke="currentColor" stroke-linecap="round"></path><path d="M4.9 8.6 11.1 11.1" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Critical user flows</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Watch login, checkout, and signup from real browsers</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/use-cases/api-monitoring/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M6.8 3.5C5.8 3.5 5.5 4.2 5.5 5.5C5.5 6.8 5.3 7.5 4.3 8C5.3 8.5 5.5 9.2 5.5 10.5C5.5 11.8 5.8 12.5 6.8 12.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M9.2 3.5C10.2 3.5 10.5 4.2 10.5 5.5C10.5 6.8 10.7 7.5 11.7 8C10.7 8.5 10.5 9.2 10.5 10.5C10.5 11.8 10.2 12.5 9.2 12.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">API &amp; backend monitoring</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Validate endpoints, chains, and auth on a schedule</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/use-cases/tests-to-monitors/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><rect x="2" y="2" width="12" height="12" rx="2.5" stroke="currentColor" stroke-linejoin="round"></rect><path d="M5 8.2 7 10.2 11 5.8" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Tests to production monitors</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Promote tagged specs from CI to scheduled checks</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/use-cases/consolidation/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M2 3.5h2.5c2 0 2.5 4.5 4.5 4.5H14" stroke="currentColor" stroke-linecap="round"></path><path d="M2 12.5h2.5c2 0 2.5-4.5 4.5-4.5" stroke="currentColor" stroke-linecap="round"></path><polyline points="11.75,5.75 14,8 11.75,10.25" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Observability consolidation</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Replace the synthetic module bolted onto your APM</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/use-cases/ai-generated-code/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 2L14 5V11L8 14L2 11V5L8 2Z" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L14 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L2 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8V14" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Reliability for AI-generated code</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Verify what your agents ship before your users do</span></a></div></div><div class="w-[248px]"><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Industries</p><div class="flex flex-col gap-y-0.5 mt-1.5"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/industries/ecommerce/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><polyline points="1.75,2.5 3.4,2.5 5,9.5 12.25,9.5 13.75,4.5 4,4.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><circle cx="5.75" cy="12.75" r="1" stroke="currentColor"></circle><circle cx="11.5" cy="12.75" r="1" stroke="currentColor"></circle></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">E-commerce</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Protect checkout and revenue paths around the clock</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/industries/financial-services/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><polyline points="1.75,6 8,2.5 14.25,6" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></polyline><path d="M3.75 6.75v5" stroke="currentColor" stroke-linecap="round"></path><path d="M8 6.75v5" stroke="currentColor" stroke-linecap="round"></path><path d="M12.25 6.75v5" stroke="currentColor" stroke-linecap="round"></path><path d="M2 13.5h12" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Financial services</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Prove uptime and latency against strict SLAs</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/solutions/industries/saas/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M4.6 12.5a2.85 2.85 0 0 1-.35-5.68 3.65 3.65 0 0 1 7.02-.6A2.95 2.95 0 0 1 11.6 12.5H4.6Z" stroke="currentColor" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">SaaS &amp; B2B software</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Keep every tenant&#x27;s core workflows verified</span></a></div></div></div><div class="mt-5 pt-4 border-t border-gray-200"><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Moving from another tool?</p><div class="flex flex-wrap items-center gap-1 mt-1.5 -ml-2.5"><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/datadog-alternative/">vs. Datadog</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/new-relic-alternative/">vs. New Relic</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/alternative-pingdom/">vs. Pingdom</a><a class="px-2.5 py-1 text-[13px] font-medium whitespace-nowrap rounded-md transition-colors duration-150 hover:bg-gray-100 hover:text-checkly-blue" href="/switch/catchpoint/">vs. Catchpoint</a></div></div></div></div></div></div><div class="relative flex items-center h-[var(--nav-height)]"><button class="nav-item" aria-expanded="false" aria-controls="nav-dropdown-developers" aria-haspopup="true">Developers<span class="dropdown-icon" aria-hidden="true"></span></button><div id="nav-dropdown-developers" role="region" aria-label="Developers" style="left:0" inert="" class="absolute top-full pt-2 z-30 origin-top transition duration-200 ease-out motion-reduce:transition-none motion-reduce:transform-none -translate-y-1 scale-95 opacity-0 pointer-events-none"><div class="nav-panel w-max max-w-[calc(100vw-2rem)] max-h-[calc(100vh-var(--nav-height)-1.5rem)] overflow-y-auto rounded-2xl border border-gray-200 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stroke="currentColor"></circle><path d="M4.5 11 3 13M11.5 11 13 13M6.5 11.5l-.5 2M9.5 11.5l.5 2" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Quickstart</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Set up your first check fast</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/docs/guides/overview/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><circle cx="4" cy="4" r="1.5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></circle><circle cx="8" cy="8" r="1.5" stroke="currentColor" 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hover:text-checkly-blue" href="/docs/ai/skills/"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><path d="M8 2L14 5V11L8 14L2 11V5L8 2Z" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L14 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8L2 5" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M8 8V14" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path></svg></span>Agent Skills</a><a class="flex gap-2 items-center px-2.5 py-1 rounded-md text-[13px] font-medium whitespace-nowrap transition-colors duration-150 group hover:bg-gray-100 hover:text-checkly-blue" href="/product/terraform-provider/"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><img alt="" loading="lazy" width="20" height="20" decoding="async" data-nimg="1" style="color:transparent;width:20px;height:20px" src="/nav-icons/terraform.svg"/></span>Terraform</a><a class="flex gap-2 items-center px-2.5 py-1 rounded-md text-[13px] font-medium whitespace-nowrap transition-colors duration-150 group hover:bg-gray-100 hover:text-checkly-blue" href="/product/pulumi-provider/"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><img alt="" loading="lazy" width="20" height="20" decoding="async" data-nimg="1" style="color:transparent;width:20px;height:20px" src="/nav-icons/pulumi.svg"/></span>Pulumi</a><a class="flex gap-2 items-center px-2.5 py-1 rounded-md text-[13px] font-medium whitespace-nowrap transition-colors duration-150 group hover:bg-gray-100 hover:text-checkly-blue" href="/docs/integrations/alerts/discord/"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg class="w-[18px] h-[18px]" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg"><circle cx="4" cy="4" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="10" cy="4" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="16" cy="4" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="4" cy="10" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="10" cy="10" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="16" cy="10" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="4" cy="16" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="10" cy="16" r="1.5" fill="#222" fill-opacity="0.8"></circle><circle cx="16" cy="16" r="1.5" fill="#222" fill-opacity="0.8"></circle></svg></span>All Integrations</a><a class="flex gap-2 items-center px-2.5 py-1 rounded-md text-[13px] font-medium whitespace-nowrap transition-colors duration-150 group hover:bg-gray-100 hover:text-checkly-blue" href="/changelog/"><span class="text-gray-500 transition-colors 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src="https://images.prismic.io/checklyhq/acVojZGXnQHGZA8__agentic-header.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=3840"/></div><span class="block mt-3 text-[15px] font-semibold leading-snug text-checkly-dark-blue text-pretty transition-colors duration-150 group-hover:text-checkly-blue">Checkly and the Agentic Software Layer</span><span class="inline-flex gap-1 items-center mt-2 text-[13px] font-medium text-checkly-blue">Read article<span aria-hidden="true" class="transition-transform duration-150 group-hover:translate-x-0.5">→</span></span></a></aside><div class="flex flex-col gap-4"><div><p class="text-[11px] font-semibold tracking-wider uppercase text-gray-500">Resources</p><div class="grid grid-cols-2 gap-x-5 gap-y-0.5 mt-1.5 w-[460px]"><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/blog/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 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h-[18px]"><path d="M8 3 14.5 6 8 9 1.5 6 8 3Z" stroke="currentColor" stroke-linejoin="round"></path><path d="M4.5 7.2V10c0 0 1.5 1.6 3.5 1.6S11.5 10 11.5 10V7.2" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round"></path><path d="M14.5 6v3" stroke="currentColor" stroke-linecap="round"></path></svg></span><span class="flex gap-2 items-center text-[13px] font-medium transition-colors duration-150 group-hover:text-checkly-blue">Learn</span></span><span class="block pl-[28px] text-xs text-gray-500 leading-snug text-pretty">Tips and best practices for learning Playwright and more</span></a><a class="flex flex-col gap-0.5 px-2.5 py-[3px] -mx-2.5 rounded-md transition-colors duration-150 group hover:bg-gray-100" href="/webinars/"><span class="flex gap-2.5 items-center"><span class="text-gray-500 transition-colors duration-150 shrink-0 group-hover:text-checkly-blue"><svg viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-[18px] h-[18px]"><rect x="1.5" 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href="/blog/">Blog</a><span class="flex items-center gap-x-2"><span aria-hidden="true" class="text-gray-600">/</span><a class="uppercase transition-colors duration-150 hover:text-white" href="/blog/tag/javascript/">Javascript</a></span><span class="flex items-center gap-x-2"><span aria-hidden="true" class="text-gray-600">/</span><a class="uppercase transition-colors duration-150 hover:text-white" href="/blog/tag/performance/">Performance</a></span></div><h1 class="text-3xl title md:text-5xl">How a tcpdump led us to a bug in Node&#x27;s IPv6 handling</h1><div class="flex flex-wrap items-center gap-3"><a class="inline-flex items-center gap-x-2.5 text-gray-300 transition-colors duration-150 hover:text-white" href="/blog/author/umut/"><img alt="" loading="lazy" width="800" height="800" decoding="async" data-nimg="1" class="object-cover w-9 h-9 rounded-full" style="color:transparent" sizes="36px" srcSet="https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=16 16w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=32 32w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=48 48w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=64 64w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=96 96w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=128 128w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=256 256w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=384 384w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=640 640w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=750 750w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=828 828w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1080 1080w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1200 1200w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1920 1920w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=2048 2048w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=3840 3840w" src="https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=3840"/><span class="text-sm font-medium">Umut Uzgur</span></a><a class="inline-flex items-center gap-x-2.5 text-gray-300 transition-colors duration-150 hover:text-white" href="/blog/author/nica-mellifera/"><img alt="" loading="lazy" width="1625" height="1648" decoding="async" data-nimg="1" class="object-cover w-9 h-9 rounded-full" style="color:transparent" sizes="36px" srcSet="https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=16 16w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=32 32w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=48 48w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=64 64w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=96 96w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=128 128w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=256 256w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=384 384w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=640 640w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=750 750w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=828 828w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1080 1080w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1200 1200w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1920 1920w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=2048 2048w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=3840 3840w" src="https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=3840"/><span class="text-sm font-medium">Nočnica Mellifera</span></a></div><div class="flex items-baseline gap-2 text-sm text-gray-400"><time dateTime="2024-09-05">September 5, 2024</time><span>· Updated<!-- --> <time dateTime="2024-10-24T06:51:07+0000">October 24, 2024</time></span></div></div></div></header><div style="background:linear-gradient(to bottom, #041734, #041734 35%, transparent 35%)"><div class="container"><img alt="" loading="lazy" width="1280" height="976" decoding="async" data-nimg="1" class="max-h-[605px] w-full max-w-5xl rounded-2xl border border-blue-300/20 object-cover shadow-xl shadow-checkly-deep-blue/10 opacity-0 animate-[fadeSlideIn_400ms_cubic-bezier(0.23,1,0.32,1)_forwards]" style="color:transparent" sizes="(max-width 1279px) 100vw, 1024px" srcSet="https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=640 640w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=750 750w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=828 828w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=1080 1080w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=1200 1200w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=1920 1920w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=2048 2048w, https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=3840 3840w" src="https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format%2Ccompress&amp;fit=max&amp;w=3840"/></div></div><div class="container flex items-start py-16 gap-x-28 max-lg:flex-col"><div class="max-w-[75ch] w-full"><article class="rich-text lg:text-lg"><p><em>tl;dr: Node by default doesn’t implement the Happy Eyeballs algorithm for web requests, and therefore if a server has a broken IPv6 address that responds first, it will usually fail.</em></p><p>This is the story of how an otherwise working server was completely unavailable for users in a geographic region, and how in one very specific way Node doesn’t handle requests as well as an Opera browser from 2011.</p><h2 id="how-we-got-here-the-checkly-coralogix-integration"><a href="#how-we-got-here-the-checkly-coralogix-integration">How we got here: the Checkly Coralogix integration</a></h2><p>By now you’ve heard of the <a target="_blank" href="https://www.checklyhq.com/blog/sending-checkly-traces-to-coralogix/" rel="noreferrer">Checkly Coralogix integration</a>, part of a broader strategy at Checkly of embracing OpenTelemetry and using Checkly to give you insight into backend services’ performance. Released a couple of months ago, this integration lets you set some configuration options in Checkly to send our synthetic monitoring (a fancy term for scheduled end-to-end tests) traces to Coralogix. Once the integration is enabled you’ll see connected traces linked from the Checkly interface:</p><p class="block-img"><img src="https://images.prismic.io/checklyhq/Zt9vzBoQrfVKl35w_ZhmyRjjCgu4jzy7e_Screenshot2024-03-27at3.43.45PM-1-.avif?auto=format,compress"/></p><p><em>A Checkly trace with a link to Coralogix</em></p><p>And you can find the detailed reports from your synthetic monitoring in the Coralogix dashboard. This feature, combined with our CLI and support for <a target="_blank" href="https://www.checklyhq.com/guides/monitoring-as-code/" rel="noreferrer">Monitoring as Code</a>, mean that your whole team can be involved in a shift left of monitoring, sharing monitoring results more broadly and making multiple teams aware of monitoring data on systems that were previously a black box.</p><section><img alt="A Coralogix trace with Checkly info" loading="lazy" width="3140" height="1956" decoding="async" data-nimg="1" class="my-2 border border-gray-300 border-solid rounded-lg cursor-pointer border-opacity-20" style="color:transparent" srcSet="https://images.prismic.io/checklyhq/Zt9vpxoQrfVKl35t_ZhmygTjCgu4jzy7f_Screenshot2024-03-27at3.49.22PM-1-.png?auto=format%2Ccompress&amp;fit=max&amp;w=3840 1x" src="https://images.prismic.io/checklyhq/Zt9vpxoQrfVKl35t_ZhmygTjCgu4jzy7f_Screenshot2024-03-27at3.49.22PM-1-.png?auto=format%2Ccompress&amp;fit=max&amp;w=3840"/><div class="cursor-pointer hidden"><div class="fixed inset-0 z-[99999] bg-black bg-opacity-50 transition-opacity" aria-hidden="true"></div><div class="fixed inset-0 z-[99999] p-6 flex items-center h-full max-w-5xl mx-auto"><img alt="A Coralogix trace with Checkly info" loading="lazy" width="3140" height="1956" decoding="async" data-nimg="1" class="w-full max-h-full overflow-hidden border border-gray-300 border-solid rounded-lg border-opacity-20" style="color:transparent" srcSet="https://images.prismic.io/checklyhq/Zt9vpxoQrfVKl35t_ZhmygTjCgu4jzy7f_Screenshot2024-03-27at3.49.22PM-1-.png?auto=format%2Ccompress&amp;fit=max&amp;w=3840 1x" src="https://images.prismic.io/checklyhq/Zt9vpxoQrfVKl35t_ZhmygTjCgu4jzy7f_Screenshot2024-03-27at3.49.22PM-1-.png?auto=format%2Ccompress&amp;fit=max&amp;w=3840"/></div></div></section><p><em>A backend trace kicked off with a Checkly request, including the information from the Checkly runner</em></p><h3 id="the-problem-it-worked-fine-for-us-but-not-in-us-east-1"><a href="#the-problem-it-worked-fine-for-us-but-not-in-us-east-1">The problem: it worked fine for us, but not in US-East-1</a></h3><p>When testing the integration feature, one of the steps was selecting the Coralogix server where Checkly should send data, based on its region. For us in EU-2 it was working great, but when the Coralogix VP of Product tried it, it wasn’t working at all. We quickly found the difference: he was using US-EAST-1 and that was failing consistently. What could the matter be?</p><p>As this transition happened around the time we shifted to ephemeral pods, we were tempted to blame architectural changes as the culprit, we couldn’t find anything suggestive of how the change would break this connection for one region. We felt confident that the different AWS regions didn’t have way different network architectures, and our runners were otherwise totally happy talking to different regions.</p><h3 id="getting-into-the-weeds-tcpdumps"><a href="#getting-into-the-weeds-tcpdumps">Getting into the weeds: tcpdumps</a></h3><p>While we were investigating the network issues, we added an internal feature to our Checkly runner that allowed us to look at a full tcpdump for an API request. It was after doing this that we saw something that surprised us: every request from our runner sent two DNS queries. That can’t be right… can it?</p><h2 id="it-can-t-be-dns"><a href="#it-can-t-be-dns">It can’t be DNS</a></h2><p>Okay, this feels like a story about as well-known as the origins of Spider-Man but, let’s do this one more time: weren’t we supposed to get rid of IPv4 around the time Tobey Maguire took off the spandex suit?</p><p class="block-img"><img src="https://images.prismic.io/checklyhq/Zt9wAhoQrfVKl353_ipv4SpiderMan-1-.png?auto=format,compress" alt="Tobey McGuire&#x27;s Spider-Man realizing he&#x27;s still assigning ipv4 addresses"/></p><p><em>I guess, just like Spider-Man, there are some stories we can’t stop repeating</em></p><p>Short version: we were supposed to switch everything over to IPv6, we haven’t. Progress is being made but many many things have DNS entries for both an IPv4 and IPv6 address.</p><p>Slightly longer: Think of the internet as a giant phone book. IPv4 is like those old landlines, while IPv6 is the newfangled smartphone. Now, when your computer wants to make a call (connect to a website), it has to decide which number to dial. IPv4 numbers are found under “A” in the phone book (DNS records), while IPv6 numbers are under “AAAA”—because, you know, four A’s are better than one.</p><p>Here’s where Node.js comes into play. It looks up both numbers separately and, depending on its version, it either sticks to the old reliable landline (IPv4) or grabs whichever number rings first. In Node.js 18 and earlier, it was all about that landline life, preferring IPv4. But starting from Node.js 18, it became a bit more open-minded with the “verbatim” setting, meaning it just grabs the first number that pops up, whether it’s IPv4 or IPv6.</p><p>However, there’s a catch. If you get an IPv6 number but your shiny smartphone (IPv6) has a broken signal, you’re stuck with a bad connection. Browsers, being the smarty-pants they are, foresaw this and implemented the “Happy Eyeballs” algorithm.</p><h3 id="“happy-eyeballs”-in-a-nutshell"><a href="#“happy-eyeballs”-in-a-nutshell">“Happy Eyeballs” in a nutshell</a></h3><p>I want to take one step back and summarize “Happy Eyeballs” since, if you’re implementing HTTP from scratch, you should know this too!</p><p>Imagine you&#x27;re at a buffet, and there are two lines—one for pizza and one for crab legs. You love both, so you jump into whichever line is moving faster. That’s the &quot;Happy Eyeballs&quot; algorithm for browsers. Instead of waiting around for one internet connection to get its act together, your browser checks out both IPv4 and IPv6 at the same time. The standard implementation will prefer ipv6 over ipv4 and if ipv4 responds first, it will wait up to 50ms, to see if the ipv6 will respond within that time.</p><p>The idea is to keep your eyes—and connections—on the prize, so you don’t end up staring at a loading screen longer than necessary. Notably, this algorithm should be fault-tolerant. If the faster of the two connections isn’t returned, the slower one will work. After all even if we get to the front of the buffet and they’re out of crab legs, we still want to eat, so pizza it is.</p><p>Happy eyeballs is more than a suggestion it’s <a target="_blank" href="https://www.rfc-editor.org/rfc/rfc8305.txt" rel="noreferrer">an internet standard</a>, so we generally expect it to happen everywhere, but the surprise was that this wasn’t happening by default on Node.</p><h2 id="…it-was-dns"><a href="#…it-was-dns">…it was DNS</a></h2><p>This particular failure state required specific conditions. The AAAA IPv6 entry had to be available but broken, and the IPv6 had to come back from DNS faster for the request to fail. In our case we were lucky that the problem, if the A records sometimes came back first it would cause an intermittent failure, and it’s hard to imagine how long that would have taken to diagnose!</p><h2 id="ipv6-on-node-pobody-s-nerfect"><a href="#ipv6-on-node-pobody-s-nerfect">IPv6 on Node: Pobody’s Nerfect</a></h2><p>There’s some <a target="_blank" href="https://github.com/nodejs/node/issues/48145" rel="noreferrer">discussion on GitHub</a> about why Node doesn’t implement this algorithm the way browsers do, and how changing Node’s behavior now can have unexpected effects. Even the current 20.1 release that does implement “Happy Eyeballs” has some unexpected effects.</p><p>An article I’m working on to for next week is on why Checkly is a Node shop. Any time anything doesn’t work on Node someone tells me I should have just used Go, Rust, or a Field-programmable Gate Array instead of the world’s most popular programming language. Is this problem singular to Node? Are Node’s built-in convenience functions a source of unexpected failures? Not really. Any modern language is going to have an HTTP implementation built in. Go has native support for everything from HTTP to composing HTML from templates, so it’s not a special vulnerability in Node that means tiny gaps exist between standard browser behavior and your server’s code execution.</p><h2 id="why-checkly-exists-observe-your-code-in-the-real-world"><a href="#why-checkly-exists-observe-your-code-in-the-real-world">Why Checkly exists: observe your code in the real world</a></h2><p>If we want to go broader, we’re also getting to the reason Checkly and Synthetic Monitoring exist at all: application code on a server isn’t a good imitation of web requests coming from a browser. Our Playwright-based Browser checks strive to accurately represent a user’s visual experience, and API checks can run from the same geographies and with the same retry logic as your real requests. To be certain that a service is working as expected, you have to observe it with real requests.</p><p>In the end it took nearly a week of investigation to find the problem, and five minutes to fix by <a target="_blank" href="https://nodejs.org/api/dns.html#dnssetdefaultresultorderorder" rel="noreferrer">configuring our requests.</a> The issue would have been easier to diagnose now, And if you’re curious why we <a target="_blank" href="https://feedback.checklyhq.com/changelog/announcing-ipv6-support" rel="noreferrer">recently announced support for IPv6 requests in our API checks</a>, we identified the need while investigating this issue!</p><h3 id="results-coralgix-&amp;-checkly-for-all-"><a href="#results-coralgix-&amp;-checkly-for-all-">Results: Coralgix &amp; Checkly for all!</a></h3><p>As a result of this process, we unblocked our extremely cool <a target="_blank" href="https://www.checklyhq.com/blog/sending-checkly-traces-to-coralogix/" rel="noreferrer">Checkly x Coralogix integration</a> for all users. That means more users of Checkly will be able to find insights like the one Jan found using <a target="_blank" href="https://www.checklyhq.com/blog/coralogix-and-opentelemetry-on-checkly/" rel="noreferrer">Coralogix and OpenTelemetry that saved thousands of hours of execution time with a tiny optimization</a>.</p></article><div class="flex flex-wrap gap-4 lg:-ml-2 mt-14"><a class="group inline-flex items-center p-2 pr-6 text-gray-500 transition-colors duration-150 rounded-full gap-x-3 hover:bg-blue-300/20" href="/blog/author/umut/"><img alt="" loading="lazy" width="800" height="800" decoding="async" data-nimg="1" class="object-cover rounded-full w-18 h-18 ring-2 ring-transparent transition-[box-shadow] duration-200 group-hover:ring-checkly-blue/30" style="color:transparent" sizes="48px" srcSet="https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=16 16w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=32 32w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=48 48w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=64 64w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=96 96w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=128 128w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=256 256w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=384 384w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=640 640w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=750 750w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=828 828w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1080 1080w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1200 1200w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=1920 1920w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=2048 2048w, https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=3840 3840w" src="https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress%2Cformat&amp;fit=max&amp;w=3840"/><div><p>Umut Uzgur</p><p class="mb-1 text-xs">Senior Engineering Team Lead</p><div class="flex gap-x-1"></div></div></a><a class="group inline-flex items-center p-2 pr-6 text-gray-500 transition-colors duration-150 rounded-full gap-x-3 hover:bg-blue-300/20" href="/blog/author/nica-mellifera/"><img alt="" loading="lazy" width="1625" height="1648" decoding="async" data-nimg="1" class="object-cover rounded-full w-18 h-18 ring-2 ring-transparent transition-[box-shadow] duration-200 group-hover:ring-checkly-blue/30" style="color:transparent" sizes="48px" srcSet="https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=16 16w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=32 32w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=48 48w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=64 64w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=96 96w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=128 128w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=256 256w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=384 384w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=640 640w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=750 750w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=828 828w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1080 1080w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1200 1200w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=1920 1920w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=2048 2048w, https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=3840 3840w" src="https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress%2Cformat&amp;fit=max&amp;w=3840"/><div><p>Nočnica Mellifera</p><p class="mb-1 text-xs">Developer Advocate</p><p class="max-w-xl mb-1 text-sm">Nočnica Mellifera (She/Her) was a developer for seven years before moving into developer relations. She specializes in containerized workloads, serverless, and public cloud engineering. Nočnica has long been an advocate for open standards, and has given talks and workshops on OpenTelemetry and Kubernetes architecture. She enjoys making music that friends have described as &quot;unlistenable.&quot; In her spare time she cares for her two children and hundreds of houseplants.</p><div class="flex gap-x-1"><a href="https://twitter.com/Serverless_Mom" target="_blank" rel="noopener noreferrer" class="button w-8 h-8 p-0 rounded-full flex justify-center items-center transition-transform duration-100 active:!scale-[0.92]"><svg viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" class="w-4"><path d="M18.244 2.25h3.308l-7.227 8.26 8.502 11.24H16.17l-5.214-6.817L4.99 21.75H1.68l7.73-8.835L1.254 2.25H8.08l4.713 6.231 5.45-6.231Zm-1.161 17.52h1.833L7.084 4.126H5.117L17.083 19.77Z" fill="currentColor"></path></svg></a><a href="https://github.com/serverless-mom" target="_blank" rel="noopener noreferrer" class="button w-8 h-8 p-0 rounded-full flex justify-center items-center transition-transform duration-100 active:!scale-[0.92]"><svg fill="none" height="24" shape-rendering="geometricPrecision" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" viewBox="0 0 24 24" width="24" class="w-4"><path d="M18 13v6a2 2 0 01-2 2H5a2 2 0 01-2-2V8a2 2 0 012-2h6"></path><path d="M15 3h6v6"></path><path d="M10 14L21 3"></path></svg></a></div></div></a></div></div><aside class="sticky top-24 hidden lg:block shrink-0 w-[280px]"><nav aria-label="Table of contents"><p class="text-xs font-medium uppercase tracking-wide text-gray-400 mb-4">In This Article</p><ul class="flex flex-col gap-2.5"><li class="border-l-2 pl-3 transition-colors duration-150 border-transparent"><a class="text-sm leading-snug transition-colors duration-150 text-gray-500 hover:text-gray-900" href="#how-we-got-here-the-checkly-coralogix-integration">How we got here: the Checkly Coralogix integration</a></li><li class="border-l-2 pl-3 transition-colors duration-150 border-transparent"><a class="text-sm leading-snug transition-colors duration-150 text-gray-500 hover:text-gray-900" href="#it-can-t-be-dns">It can’t be DNS</a></li><li class="border-l-2 pl-3 transition-colors duration-150 border-transparent"><a class="text-sm leading-snug transition-colors duration-150 text-gray-500 hover:text-gray-900" href="#…it-was-dns">…it was DNS</a></li><li class="border-l-2 pl-3 transition-colors duration-150 border-transparent"><a class="text-sm leading-snug transition-colors duration-150 text-gray-500 hover:text-gray-900" href="#ipv6-on-node-pobody-s-nerfect">IPv6 on Node: Pobody’s Nerfect</a></li><li class="border-l-2 pl-3 transition-colors duration-150 border-transparent"><a class="text-sm leading-snug transition-colors duration-150 text-gray-500 hover:text-gray-900" href="#why-checkly-exists-observe-your-code-in-the-real-world">Why Checkly exists: observe your code in the real world</a></li></ul></nav><div class="mt-8"><p class="text text-xs">Share on social</p><div class="flex gap-1.5 mt-3"><a 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All rights reserved.</small></div></div></footer></div></div><script id="__NEXT_DATA__" type="application/json">{"props":{"pageProps":{"publish_date":"2024-09-05","title":[{"type":"heading1","text":"How a tcpdump led us to a bug in Node's IPv6 handling","spans":[],"direction":"ltr"}],"authors":[{"author":{"id":"ZD1n4BEAACEAmsKH","type":"person","tags":[],"lang":"en-us","slug":"umut-uzgur","first_publication_date":"2023-04-28T19:49:25+0000","last_publication_date":"2024-08-27T13:36:52+0000","uid":"umut","data":{"name":"Umut Uzgur","position":"Senior Engineering Team Lead","social_links":[{"label":null,"link":{"link_type":"Web","key":"cb7c9c2c-7897-40ae-a451-53502d4f3004","url":"https://www.linkedin.com/in/umutuzgur/","target":"_blank"}}],"image":{"dimensions":{"width":800,"height":800},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/afc7d131-2153-43f7-893e-6f6b21fce189_1619779697636.jpeg?auto=compress,format","id":"ZXn_1hEAACAALkpw","edit":{"x":0,"y":0,"zoom":1,"background":"#fff"}}},"link_type":"Document","key":"3a777b4b-de91-4265-9f5f-9591298445eb","isBroken":false}},{"author":{"id":"ZYI0ZxEAACYA6MCX","type":"person","tags":[],"lang":"en-us","slug":"nocnica-mellifera","first_publication_date":"2023-12-20T00:28:30+0000","last_publication_date":"2024-08-27T13:49:59+0000","uid":"nica-mellifera","data":{"name":"Nočnica Mellifera","position":"Developer Advocate","extended_bio":"Nočnica Mellifera (She/Her) was a developer for seven years before moving into developer relations. 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She enjoys making music that friends have described as \"unlistenable.\" In her spare time she cares for her two children and hundreds of houseplants.","social_links":[{"label":"Twitter","link":{"link_type":"Web","key":"24aa2e23-7a99-4372-81f5-6d42fad74dd0","url":"https://twitter.com/Serverless_Mom","target":"_blank"}},{"label":"GitHub","link":{"link_type":"Web","key":"8cb7c44a-a0cb-4b87-a0c4-5f5053c385b2","url":"https://github.com/serverless-mom","target":"_blank"}},{"label":null,"link":{"link_type":"Web","key":"fda3ff7e-0c0f-4d13-a5e4-2fcae626702f","url":"https://www.linkedin.com/in/serverlessmom/","target":"_blank"}}],"image":{"dimensions":{"width":1625,"height":1648},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZiEzG_Pdc1huKnJj_TrimmedAvatar-small.jpg?auto=compress,format","id":"ZiEzG_Pdc1huKnJj","edit":{"x":0,"y":0,"zoom":1,"background":"#fff"}}},"link_type":"Document","key":"7e9d242e-814d-4018-94b4-d6e362302a3c","isBroken":false}}],"tags":[{"tag":{"id":"ZD2NihEAACMAm3ML","type":"article_tag","tags":[],"lang":"en-us","slug":"javascript","first_publication_date":"2023-04-28T19:48:54+0000","last_publication_date":"2023-09-19T13:04:35+0000","uid":"javascript","data":{"name":[{"type":"heading1","text":"Javascript","spans":[]}]},"link_type":"Document","key":"e8ac7eb9-f4fa-4a92-9d0a-2a364ff8e146","isBroken":false}},{"tag":{"id":"ZD2NihEAACMAm3L7","type":"article_tag","tags":[],"lang":"en-us","slug":"performance","first_publication_date":"2023-04-28T19:48:54+0000","last_publication_date":"2023-09-19T12:59:43+0000","uid":"performance","data":{"name":[{"type":"heading1","text":"Performance","spans":[]}]},"link_type":"Document","key":"3132dada-d6a1-4baf-8ff1-3f54dc61f82f","isBroken":false}}],"image":{"dimensions":{"width":1280,"height":976},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/Zt9wqRoQrfVKl36H_Eyes_by_Lodovico_Mattioli.jpg?auto=format,compress","id":"Zt9wqRoQrfVKl36H","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},"slices":[{"primary":{"content":[{"type":"paragraph","text":"tl;dr: Node by default doesn’t implement the Happy Eyeballs algorithm for web requests, and therefore if a server has a broken IPv6 address that responds first, it will usually fail.","spans":[{"start":0,"end":182,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"This is the story of how an otherwise working server was completely unavailable for users in a geographic region, and how in one very specific way Node doesn’t handle requests as well as an Opera browser from 2011.","spans":[],"direction":"ltr"},{"type":"heading2","text":"How we got here: the Checkly Coralogix integration","spans":[],"direction":"ltr"},{"type":"paragraph","text":"By now you’ve heard of the Checkly Coralogix integration, part of a broader strategy at Checkly of embracing OpenTelemetry and using Checkly to give you insight into backend services’ performance. Released a couple of months ago, this integration lets you set some configuration options in Checkly to send our synthetic monitoring (a fancy term for scheduled end-to-end tests) traces to Coralogix. Once the integration is enabled you’ll see connected traces linked from the Checkly interface:","spans":[{"start":27,"end":56,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/sending-checkly-traces-to-coralogix/","target":"_blank"}}],"direction":"ltr"},{"type":"image","url":"https://images.prismic.io/checklyhq/Zt9vzBoQrfVKl35w_ZhmyRjjCgu4jzy7e_Screenshot2024-03-27at3.43.45PM-1-.avif?auto=format,compress","alt":null,"copyright":null,"dimensions":{"width":2000,"height":2000},"id":"Zt9vzBoQrfVKl35w","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},{"type":"paragraph","text":"A Checkly trace with a link to Coralogix","spans":[{"start":0,"end":40,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"And you can find the detailed reports from your synthetic monitoring in the Coralogix dashboard. This feature, combined with our CLI and support for Monitoring as Code, mean that your whole team can be involved in a shift left of monitoring, sharing monitoring results more broadly and making multiple teams aware of monitoring data on systems that were previously a black box.","spans":[{"start":149,"end":167,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/guides/monitoring-as-code/","target":"_blank"}}],"direction":"ltr"}]},"items":[],"variation":"default","version":"sktwi1xtmkfgx8626","id":"rich_text$0f4bd60f-6a59-4c36-80ed-f0d8d0d254ba","slice_type":"rich_text","slice_label":null},{"primary":{"image":{"dimensions":{"width":3140,"height":1956},"alt":"A Coralogix trace with Checkly info","copyright":null,"url":"https://images.prismic.io/checklyhq/Zt9vpxoQrfVKl35t_ZhmygTjCgu4jzy7f_Screenshot2024-03-27at3.49.22PM-1-.png?auto=format,compress","id":"Zt9vpxoQrfVKl35t","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"items":[],"variation":"default","version":"initial","id":"image_with_modal$c6e1ff41-149d-4ff6-8797-b31eedd5e329","slice_type":"image_with_modal","slice_label":null},{"primary":{"content":[{"type":"paragraph","text":"A backend trace kicked off with a Checkly request, including the information from the Checkly runner","spans":[{"start":0,"end":100,"type":"em"}],"direction":"ltr"},{"type":"heading3","text":"The problem: it worked fine for us, but not in US-East-1","spans":[],"direction":"ltr"},{"type":"paragraph","text":"When testing the integration feature, one of the steps was selecting the Coralogix server where Checkly should send data, based on its region. For us in EU-2 it was working great, but when the Coralogix VP of Product tried it, it wasn’t working at all. We quickly found the difference: he was using US-EAST-1 and that was failing consistently. What could the matter be?","spans":[],"direction":"ltr"},{"type":"paragraph","text":"As this transition happened around the time we shifted to ephemeral pods, we were tempted to blame architectural changes as the culprit, we couldn’t find anything suggestive of how the change would break this connection for one region. We felt confident that the different AWS regions didn’t have way different network architectures, and our runners were otherwise totally happy talking to different regions.","spans":[],"direction":"ltr"},{"type":"heading3","text":"Getting into the weeds: tcpdumps","spans":[],"direction":"ltr"},{"type":"paragraph","text":"While we were investigating the network issues, we added an internal feature to our Checkly runner that allowed us to look at a full tcpdump for an API request. It was after doing this that we saw something that surprised us: every request from our runner sent two DNS queries. That can’t be right… can it?","spans":[],"direction":"ltr"},{"type":"heading2","text":"It can’t be DNS","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Okay, this feels like a story about as well-known as the origins of Spider-Man but, let’s do this one more time: weren’t we supposed to get rid of IPv4 around the time Tobey Maguire took off the spandex suit?","spans":[],"direction":"ltr"},{"type":"image","url":"https://images.prismic.io/checklyhq/Zt9wAhoQrfVKl353_ipv4SpiderMan-1-.png?auto=format,compress","alt":"Tobey McGuire's Spider-Man realizing he's still assigning ipv4 addresses","copyright":null,"dimensions":{"width":3186,"height":3333},"id":"Zt9wAhoQrfVKl353","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},{"type":"paragraph","text":"I guess, just like Spider-Man, there are some stories we can’t stop repeating","spans":[{"start":0,"end":77,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"Short version: we were supposed to switch everything over to IPv6, we haven’t. Progress is being made but many many things have DNS entries for both an IPv4 and IPv6 address.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Slightly longer: Think of the internet as a giant phone book. IPv4 is like those old landlines, while IPv6 is the newfangled smartphone. Now, when your computer wants to make a call (connect to a website), it has to decide which number to dial. IPv4 numbers are found under “A” in the phone book (DNS records), while IPv6 numbers are under “AAAA”—because, you know, four A’s are better than one.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Here’s where Node.js comes into play. It looks up both numbers separately and, depending on its version, it either sticks to the old reliable landline (IPv4) or grabs whichever number rings first. In Node.js 18 and earlier, it was all about that landline life, preferring IPv4. But starting from Node.js 18, it became a bit more open-minded with the “verbatim” setting, meaning it just grabs the first number that pops up, whether it’s IPv4 or IPv6.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"However, there’s a catch. If you get an IPv6 number but your shiny smartphone (IPv6) has a broken signal, you’re stuck with a bad connection. Browsers, being the smarty-pants they are, foresaw this and implemented the “Happy Eyeballs” algorithm.","spans":[],"direction":"ltr"},{"type":"heading3","text":"“Happy Eyeballs” in a nutshell","spans":[],"direction":"ltr"},{"type":"paragraph","text":"I want to take one step back and summarize “Happy Eyeballs” since, if you’re implementing HTTP from scratch, you should know this too!","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Imagine you're at a buffet, and there are two lines—one for pizza and one for crab legs. You love both, so you jump into whichever line is moving faster. That’s the \"Happy Eyeballs\" algorithm for browsers. Instead of waiting around for one internet connection to get its act together, your browser checks out both IPv4 and IPv6 at the same time. The standard implementation will prefer ipv6 over ipv4 and if ipv4 responds first, it will wait up to 50ms, to see if the ipv6 will respond within that time.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"The idea is to keep your eyes—and connections—on the prize, so you don’t end up staring at a loading screen longer than necessary. Notably, this algorithm should be fault-tolerant. If the faster of the two connections isn’t returned, the slower one will work. After all even if we get to the front of the buffet and they’re out of crab legs, we still want to eat, so pizza it is.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Happy eyeballs is more than a suggestion it’s an internet standard, so we generally expect it to happen everywhere, but the surprise was that this wasn’t happening by default on Node.","spans":[{"start":46,"end":66,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.rfc-editor.org/rfc/rfc8305.txt","target":"_blank"}}],"direction":"ltr"},{"type":"heading2","text":"…it was DNS","spans":[],"direction":"ltr"},{"type":"paragraph","text":"This particular failure state required specific conditions. The AAAA IPv6 entry had to be available but broken, and the IPv6 had to come back from DNS faster for the request to fail. In our case we were lucky that the problem, if the A records sometimes came back first it would cause an intermittent failure, and it’s hard to imagine how long that would have taken to diagnose!","spans":[],"direction":"ltr"},{"type":"heading2","text":"IPv6 on Node: Pobody’s Nerfect","spans":[],"direction":"ltr"},{"type":"paragraph","text":"There’s some discussion on GitHub about why Node doesn’t implement this algorithm the way browsers do, and how changing Node’s behavior now can have unexpected effects. Even the current 20.1 release that does implement “Happy Eyeballs” has some unexpected effects.","spans":[{"start":13,"end":33,"type":"hyperlink","data":{"link_type":"Web","url":"https://github.com/nodejs/node/issues/48145","target":"_blank"}}],"direction":"ltr"},{"type":"paragraph","text":"An article I’m working on to for next week is on why Checkly is a Node shop. Any time anything doesn’t work on Node someone tells me I should have just used Go, Rust, or a Field-programmable Gate Array instead of the world’s most popular programming language. Is this problem singular to Node? Are Node’s built-in convenience functions a source of unexpected failures? Not really. Any modern language is going to have an HTTP implementation built in. Go has native support for everything from HTTP to composing HTML from templates, so it’s not a special vulnerability in Node that means tiny gaps exist between standard browser behavior and your server’s code execution.","spans":[],"direction":"ltr"},{"type":"heading2","text":"Why Checkly exists: observe your code in the real world","spans":[],"direction":"ltr"},{"type":"paragraph","text":"If we want to go broader, we’re also getting to the reason Checkly and Synthetic Monitoring exist at all: application code on a server isn’t a good imitation of web requests coming from a browser. Our Playwright-based Browser checks strive to accurately represent a user’s visual experience, and API checks can run from the same geographies and with the same retry logic as your real requests. To be certain that a service is working as expected, you have to observe it with real requests.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"In the end it took nearly a week of investigation to find the problem, and five minutes to fix by configuring our requests. The issue would have been easier to diagnose now, And if you’re curious why we recently announced support for IPv6 requests in our API checks, we identified the need while investigating this issue!","spans":[{"start":98,"end":123,"type":"hyperlink","data":{"link_type":"Web","url":"https://nodejs.org/api/dns.html#dnssetdefaultresultorderorder","target":"_blank"}},{"start":203,"end":265,"type":"hyperlink","data":{"link_type":"Web","url":"https://feedback.checklyhq.com/changelog/announcing-ipv6-support","target":"_blank"}}],"direction":"ltr"},{"type":"heading3","text":"Results: Coralgix \u0026 Checkly for all!","spans":[],"direction":"ltr"},{"type":"paragraph","text":"As a result of this process, we unblocked our extremely cool Checkly x Coralogix integration for all users. That means more users of Checkly will be able to find insights like the one Jan found using Coralogix and OpenTelemetry that saved thousands of hours of execution time with a tiny optimization.","spans":[{"start":61,"end":92,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/sending-checkly-traces-to-coralogix/","target":"_blank"}},{"start":200,"end":300,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/coralogix-and-opentelemetry-on-checkly/","target":"_blank"}}],"direction":"ltr"}]},"items":[],"variation":"default","version":"sktwi1xtmkfgx8626","id":"rich_text$397db42e-f011-4ba0-ae5c-cb58ef085cbb","slice_type":"rich_text","slice_label":null}],"relatedArticles":[{"id":"ZlzcyBYAADMABjZ4","uid":"coralogix-and-opentelemetry-on-checkly","url":"/blog/coralogix-and-opentelemetry-on-checkly","type":"article","href":"https://checklyhq.cdn.prismic.io/api/v2/documents/search?ref=aqPosxYAAC4AP09K\u0026q=%5B%5B%3Ad+%3D+at%28document.id%2C+%22ZlzcyBYAADMABjZ4%22%29+%5D%5D","tags":[],"first_publication_date":"2024-06-03T21:02:38+0000","last_publication_date":"2025-05-23T21:46:26+0000","slugs":["coralogix-and-opentelemetry-on-checkly-optimizing-latency"],"linked_documents":[],"lang":"en-us","alternate_languages":[],"data":{"seo_title":[{"type":"heading1","text":"Coralogix and OpenTelemetry on Checkly: Optimizing Latency","spans":[],"direction":"ltr"}],"seo_description":[{"type":"paragraph","text":"Discover how OpenTelemetry tracing and Coralogix analysis resolve latency issues, proving observability's role in enhancing system performance and reliability.","spans":[],"direction":"ltr"}],"og_image":{"dimensions":{"width":1600,"height":900},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZmChOZm069VX1fV8_coralogixTraceCover.png?auto=format,compress","id":"ZmChOZm069VX1fV8","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},"title":[{"type":"heading1","text":"Saving Three Months of Latency with a Single Trace: Coralogix and OpenTelemetry on Checkly","spans":[],"direction":"ltr"}],"short_title":[{"type":"paragraph","text":"CX and OTel on Checkly","spans":[],"direction":"ltr"}],"publish_date":"2024-05-31","image":{"dimensions":{"width":1600,"height":900},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZmChOZm069VX1fV8_coralogixTraceCover.png?auto=format,compress","id":"ZmChOZm069VX1fV8","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},"tags":[{"tag":{"id":"ZZhazBEAACIApdiz","type":"article_tag","tags":[],"lang":"en-us","slug":"open-source","first_publication_date":"2024-01-05T19:38:56+0000","last_publication_date":"2024-01-05T19:38:56+0000","uid":"open-source","link_type":"Document","key":"0483eac4-bd1b-46ff-a116-79bad5611ad9","isBroken":false}},{"tag":{"id":"ZmBx5BMAACkAjMtK","type":"article_tag","tags":[],"lang":"en-us","slug":"opentelemetry","first_publication_date":"2024-06-05T14:10:47+0000","last_publication_date":"2024-06-05T14:10:47+0000","uid":"opentelemetry","link_type":"Document","key":"5e040029-925a-4d7b-b674-4f5d3d3c41dd","isBroken":false}}],"authors":[{"author":{"id":"ZD1n4BEAACEAmsKL","type":"person","tags":[],"lang":"en-us","slug":"jan-grzesik","first_publication_date":"2023-04-28T19:49:25+0000","last_publication_date":"2024-08-27T13:47:00+0000","uid":"jan","link_type":"Document","key":"a1b9d183-efd1-4873-b035-2248de477247","isBroken":false}}],"related_articles":[{"article":{"id":"ZhmxshIAAIAlu_Hc","type":"article","tags":[],"lang":"en-us","slug":"checkly-adds-synthetic-monitoring-integration-with-coralogix","first_publication_date":"2024-04-17T08:36:55+0000","last_publication_date":"2024-11-22T08:28:12+0000","uid":"sending-checkly-traces-to-coralogix","url":"/blog/sending-checkly-traces-to-coralogix","link_type":"Document","key":"cb6b0a1d-f183-4b48-8e3d-0c64bda73e4b","isBroken":false}},{"article":{"id":"Zdk3txIAACUzGql9","type":"article","tags":[],"lang":"en-us","slug":"checkly--coralogix-webinar-master-the-future-of-observability","first_publication_date":"2024-02-24T01:46:25+0000","last_publication_date":"2025-01-16T08:33:17+0000","uid":"checkly-and-coralogix-webinar","url":"/blog/checkly-and-coralogix-webinar","link_type":"Document","key":"3106fcd8-200e-4a38-a224-a6d29ca168c3","isBroken":false}}],"slices":[{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"Edit: after publishing this article, a user on Hacker News asked if we could explore the relative savings rather than the absolute. My response from that thread:","spans":[{"start":0,"end":161,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"Based on last week's data, our change reduced the task time by 40ms from an average of 3440ms, and this task runs 11 million times daily. This translates to a saving of about 1%.\n\nTo put it in perspective, thanks to OTEL and Coralogix, we identified and fixed the issue in about one day of work. Small efficiencies can indeed make a significant impact in large-scale systems.\n\n------\n\nWhat’s the point of observability? Surely if you write good code, maintain it, handle tech debt, and administer its resources correctly, it’ll run great? Why would you need to keep a close eye on services that have already been tested and are working great? In this article I want to show how continuous monitoring of your systems closely, with tools like Checkly and Coralogix, can find problems that would have been impossible to predict or pre-optimize. More generally, we’ll see how tiny fixes identified by observability can have big impacts.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"We monitored our Node service with OpenTelemetry and sent the data to a Coralogix dashboard, and with a little digging found an issue that was causing over a month of latency over all our tasks. A huge win with a small time investment, and a strong argument for observability tools.","spans":[],"direction":"ltr"},{"type":"heading2","text":"Part 1 — Adding OpenTelemetry monitoring for a slow process","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Our story begins with a mystery: a process that was taking longer than expected. The process involved was ‘housekeeping time:’ the time the Checkly service spent after a check has completed. In this time we write the trace to our datastore, send an event with the status, etc. All the stuff that needs to happen for the Checkly service to display data from the check, and notify the user if something is wrong.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"We decided to use OpenTelemetry and to identify the root of the problem in our back end code. Coralogix supports OpenTelemetry to get telemetry data (traces, logs, and metrics) from our application as requests travel through its many services and infrastructure.","spans":[{"start":158,"end":162,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/learn/opentelemetry/logs/","target":"_blank"}},{"start":168,"end":175,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/opentelemetry-metrics/","target":"_blank"}}],"direction":"ltr"},{"type":"heading3","text":"Taming OpenTelemetry Node Traces","spans":[],"direction":"ltr"},{"type":"paragraph","text":"The OpenTelemetry project contains multiple language projects at varying levels of maturity, but all those listed on the main language project page are used by large teams in production. Our initial experience with the Node OpenTelemetry project out of the box was that traces were extremely noisy.","spans":[{"start":282,"end":291,"type":"em"}],"direction":"ltr"}]},"id":"rich_text$8020e137-094b-4b63-be77-519b8da57d70","slice_type":"rich_text","slice_label":null},{"variation":"default","version":"initial","items":[],"primary":{"image":{"dimensions":{"width":4066,"height":2460},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZlzcpKWtHYXtUALp_Screenshot2024-02-27at6.48.04AM-1-.png?auto=format,compress","id":"ZlzcpKWtHYXtUALp","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"id":"image_with_modal$27cb9f90-8c1e-491e-9c16-25e2cbf44164","slice_type":"image_with_modal","slice_label":null},{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"One of our early traces with many MANY time spans in microseconds","spans":[{"start":0,"end":65,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"Consulting with some smart people on the CNCF Slack, we found a few existing issues with the OpenTelemetry Node instrumentation. It became clear the issue was capturing NodeJS’s file system, creating a span each time a module was required. By modifying our configuration we disabled file system instrumentation and started getting much more interesting trace data.","spans":[{"start":169,"end":189,"type":"hyperlink","data":{"link_type":"Web","url":"https://github.com/open-telemetry/opentelemetry-js-contrib/issues/1344","target":"_blank"}},{"start":283,"end":310,"type":"hyperlink","data":{"link_type":"Web","url":"https://github.com/open-telemetry/opentelemetry-js-contrib/tree/main/plugins/node/instrumentation-fs","target":"_self"}}],"direction":"ltr"}]},"id":"rich_text$96c8d0c4-141d-46b3-992c-dadc4e2c32cd","slice_type":"rich_text","slice_label":null},{"variation":"default","version":"initial","items":[],"primary":{"image":{"dimensions":{"width":4018,"height":2438},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZlzcvqWtHYXtUALq_Screenshot2024-02-27at6.49.36AM-1-.png?auto=format,compress","id":"ZlzcvqWtHYXtUALq","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"id":"image_with_modal$d1297572-ec7b-4f79-bde0-7b35324e4542","slice_type":"image_with_modal","slice_label":null},{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"With many types of spans and millisecond-length measurements, this is a trace worth looking at.","spans":[{"start":0,"end":95,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"There are many ways to filter your OpenTelemetry data, in fact if you’re curious about this topic you might check out Nica’s recent talk at KCD New York: “Controlling Data Overhead from the OpenTelemetry Collector.”","spans":[{"start":140,"end":215,"type":"hyperlink","data":{"link_type":"Web","url":"https://community.cncf.io/events/details/cncf-kcd-new-york-presents-kcd-new-york-2024/","target":"_blank"}}],"direction":"ltr"},{"type":"heading2","text":"Part 2 Finding the red flags 🚩 — a mysterious re-authentication","spans":[],"direction":"ltr"},{"type":"paragraph","text":"One of the wonderful things about doing Observability right is the role of human operators in the highest level of investigation. Automatic scanning is useful for so much, but a human being scrolling through a trace can notice things right away that just don’t look right.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Notably, if you want to make sure that your own checks’ traces are more human-readable, consider adding steps to your Playwright code to make sure others can see the purpose of check components.","spans":[{"start":97,"end":133,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/improve-your-playwright-documentation-with-steps/","target":"_blank"}}],"direction":"ltr"},{"type":"paragraph","text":"In the case of our traces, we noticed that a request for security credentials was happening more than once. The requests were spaced out enough that they didn’t show up in the same page of spans:","spans":[],"direction":"ltr"}]},"id":"rich_text$8af05d0e-a5cd-4daf-83dc-d73349471b52","slice_type":"rich_text","slice_label":null},{"variation":"default","version":"initial","items":[],"primary":{"image":{"dimensions":{"width":3014,"height":682},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZlzcOKWtHYXtUALk_Screenshot2024-02-27at6.51.00AM.png?auto=format,compress","id":"ZlzcOKWtHYXtUALk","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"id":"image_with_modal$435d90c0-b13f-46c2-a875-7747d42ed418","slice_type":"image_with_modal","slice_label":null},{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"When you’re fetching a security token, once is funny, twice is mean.","spans":[{"start":0,"end":68,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"Knowing that the issue was multiple requests to AWS authentication, a little searching brought us to the exact issue in the AWS SDK.\n","spans":[{"start":105,"end":131,"type":"hyperlink","data":{"link_type":"Web","url":"https://github.com/aws/aws-sdk-js-v3/issues/4612","target":"_blank"}}],"direction":"ltr"},{"type":"image","url":"https://images.prismic.io/checklyhq/ZlzcTqWtHYXtUALm_noMemory.png?auto=format,compress","alt":null,"copyright":null,"dimensions":{"width":1256,"height":1018},"id":"ZlzcTqWtHYXtUALm","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}},{"type":"paragraph","text":"Charmingly, Yurii Siedin had shown clear evidence of the problem using his own tracing tool:","spans":[{"start":12,"end":24,"type":"hyperlink","data":{"link_type":"Web","url":"https://github.com/1nstinct","target":"_blank"}}],"direction":"ltr"}]},"id":"rich_text$1fc295f6-4a35-455d-8b10-565b39e496a0","slice_type":"rich_text","slice_label":null},{"variation":"default","version":"initial","items":[],"primary":{"image":{"dimensions":{"width":1662,"height":1562},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/ZlzcZaWtHYXtUALn_Untitled-6-.png?auto=format,compress","id":"ZlzcZaWtHYXtUALn","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"id":"image_with_modal$388f32be-a960-4ed8-be85-77792e273679","slice_type":"image_with_modal","slice_label":null},{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"Sometimes a trace is just the easiest way to show a problem.","spans":[{"start":0,"end":60,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"With the fix recommended in the GitHub issue, saving the results of our authentication call locally and obviating repeated requests, the issue disappeared from our traces.","spans":[],"direction":"ltr"},{"type":"heading2","text":"Part 3 — Little changes, big benefits","spans":[],"direction":"ltr"},{"type":"paragraph","text":"A little bump can mean big things. In this case, the merge of our fix resulted in a measured drop in the time of housekeeping tasks.","spans":[],"direction":"ltr"}]},"id":"rich_text$a6db3132-063a-42ed-9e18-88ba9ab231a6","slice_type":"rich_text","slice_label":null},{"variation":"default","version":"initial","items":[],"primary":{"image":{"dimensions":{"width":4017,"height":1852},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/Zlzb5aWtHYXtUALj_DropInExecutionTime-1-.png?auto=format,compress","id":"Zlzb5aWtHYXtUALj","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"id":"image_with_modal$77d55d31-fa25-4bdd-8df4-4312d2985680","slice_type":"image_with_modal","slice_label":null},{"variation":"default","version":"sktwi1xtmkfgx8626","items":[],"primary":{"content":[{"type":"paragraph","text":"A drop in 40 milliseconds of average execution time of the ‘housekeeping’ tasks run after every check.","spans":[{"start":0,"end":102,"type":"em"}],"direction":"ltr"},{"type":"paragraph","text":"It may not seem like much to reduce average times by 40 milliseconds, but these tasks ran after every single check. Recall that even a single page check run every 2 minutes, is over 20,000 checks per month. Given that Checkly runs hundreds of millions of checks per month, these 40 millisecond savings added up quickly.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"40 ms * 250 million checks / 3,600,000 milliseconds per hour = 2,777 hours of execution time","spans":[],"direction":"ltr"},{"type":"paragraph","text":"That means that in a single month of checks, we saved over 115 days of execution time! The result is slightly faster performance for every user, and a big impact on our infrastructure costs as we pay for less compute time to complete the same tasks.","spans":[],"direction":"ltr"},{"type":"heading2","text":"Conclusions — Monitor everything you can, also OpenTelemetry Rules","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Observability is about reducing the unknown unknowns — the unwanted surprises. This isn't about catching every possible error before it happens—that's an unrealistic goal. It's about having the visibility to understand and improve our systems in ways we hadn't anticipated. Part of our vision at Checkly is that with tools like the Checkly CLI we can empower every engineer to have a hand in their monitoring tools. This ‘Monitoring as Code’ (MaC) workflow means that you can reduce the number of surprises by letting experienced engineers help monitor the systems they know best.","spans":[],"direction":"ltr"},{"type":"heading3","text":"Why Observability Matters","spans":[],"direction":"ltr"},{"type":"paragraph","text":"The narrative here underscores a fundamental truth: no matter how thorough your testing or how clean your code, unforeseen issues will arise. SDKs will behave differently under varied conditions, and what works in a controlled test environment doesn’t exactly equal real-world operations. Some issues happen only in server environments (not locally), and OTEL is a great tool to introspect what is really happening inside a deployed and running application. Observability tools like OpenTelemetry provide the lens through which we can observe these conditions, understand their impact, and respond effectively.","spans":[{"start":458,"end":496,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/opentelemetry-observability/","target":"_self"}}],"direction":"ltr"},{"type":"paragraph","text":"Open standards matter. It’s why we at Checkly support the open-source Playwright library for writing all our site checks, allowing sophisticated site interactions like visual regression testing and waiting for elements automatically.","spans":[{"start":168,"end":193,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/visual-regression--snapshot-testing-on-checkly-is/","target":"_blank"}},{"start":198,"end":218,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/playwright-auto-waiting/","target":"_blank"}}],"direction":"ltr"},{"type":"heading3","text":"Coralogix for OpenTelemetry","spans":[],"direction":"ltr"},{"type":"paragraph","text":"Just as critical as capturing trace data about your most important processes, is having an easy way to view and share that data across your organization. At Checkly, we know our synthetics monitoring data wouldn’t be half so useful without an intuitive data to browse what we’ve measured.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"For OpenTelemetry data on your backend services, Coralogix is a great place to send that data and analyze what you find.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"This issue was easily observable using the Coralogix dashboard to sort and filter our OpenTelemetry data. Combine your telemetry data and Coralogix to supercharge your system’s observability.","spans":[{"start":119,"end":147,"type":"hyperlink","data":{"link_type":"Web","url":"https://coralogix.com/blog/configure-otel-demo-send-telemetry-data-coralogix/","target":"_blank"}}],"direction":"ltr"},{"type":"heading3","text":"The Impact of Small Fixes","spans":[],"direction":"ltr"},{"type":"paragraph","text":"The resolution of our issue—a seemingly minor optimization—had a profound impact on operational efficiency and cost, saving thousands of hours of execution time. Sometimes, it's about looking through a trace for what doesn’t ‘fit’ and finding out if others have experienced the same thing.","spans":[],"direction":"ltr"},{"type":"heading3","text":"Moving Forward: Checkly x Coralogix","spans":[],"direction":"ltr"},{"type":"paragraph","text":"OpenTelemetry promises big things for the storing and sharing of monitoring data. It was with OpenTelemetry standards for sending traces that the Checkly team implemented our Coralogix integration to send our synthetic user monitoring traces to your Coralogix dashboard. Check out our recent announcement on the new integration.","spans":[{"start":285,"end":327,"type":"hyperlink","data":{"link_type":"Web","url":"https://www.checklyhq.com/blog/sending-checkly-traces-to-coralogix/","target":"_blank"}}],"direction":"ltr"},{"type":"heading3","text":"Final Thoughts","spans":[],"direction":"ltr"},{"type":"paragraph","text":"The insights gained from a well-monitored system can guide optimizations, inform architectural decisions, and ultimately, lead to more resilient and efficient applications. That’s why we support the Monitoring as Code workflow, where tools like the Checkly CLI can help you shift monitoring to be part of every developers’ workflow, running and deploying tests right from the command line.","spans":[],"direction":"ltr"},{"type":"paragraph","text":"In conclusion, monitor everything you can. The insights you'll gain are invaluable. And yes, OpenTelemetry rules.","spans":[],"direction":"ltr"}]},"id":"rich_text$657fe1a9-83e4-43fc-a861-eb13ef0798d7","slice_type":"rich_text","slice_label":null}]}}],"last_publication_date":"2024-10-24T06:51:07+0000","seo":{"title":"How a TCPDump Revealed a Node.js IPv6 Bug","description":"Learn how a TCPDump uncovered a bug in Node.js IPv6 handling, highlighting limitations and lessons from Checkly’s Coralogix integration in monitoring services.","image":{"dimensions":{"width":1600,"height":900},"alt":null,"copyright":null,"url":"https://images.prismic.io/checklyhq/Zt9wdxoQrfVKl36A_ipv6ArticleBlogCover2.png?auto=format,compress","id":"Zt9wdxoQrfVKl36A","edit":{"x":0,"y":0,"zoom":1,"background":"transparent"}}},"canonical":"https://www.checklyhq.com/blog/how-a-tcpdump-led-us-to-a-bug-in-nodes-ipv6-handli/","uid":"how-a-tcpdump-led-us-to-a-bug-in-nodes-ipv6-handli","breadcrumbs_title":[{"type":"paragraph","text":"Node IPv6","spans":[],"direction":"ltr"}],"authorDetails":[{"name":"Umut Uzgur","url":"https://www.checklyhq.com/blog/author/umut/","sameAs":["https://www.linkedin.com/in/umutuzgur/"]},{"name":"Nočnica Mellifera","url":"https://www.checklyhq.com/blog/author/nica-mellifera/","sameAs":["https://twitter.com/Serverless_Mom","https://github.com/serverless-mom","https://www.linkedin.com/in/serverlessmom/"]}]},"__N_SSG":true},"page":"/blog/[uid]","query":{"uid":"how-a-tcpdump-led-us-to-a-bug-in-nodes-ipv6-handli"},"buildId":"mXzTdbXoRITmWIy8mZCIh","isFallback":false,"dynamicIds":[7887,91706],"gsp":true,"scriptLoader":[]}</script></body></html>