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Outages & Dependency Failure

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Your own outage

• Verify there is a written first response for "the site is down", and that it starts with checking what changed rather than reading code.
• Verify you can tell the difference between the application being down, the database being unreachable, and DNS failing — from outside your own network.
• Verify you have an external check you trust, so you are not debugging a problem that only exists on your machine.
• Verify you can restart or redeploy the service without waiting for a build.
• Verify you can serve a static maintenance page without the application being up, and that you have tried.
• Verify a health check flapping cannot restart-loop the service into a worse state.
• Verify the runbook says who to tell and when, not only what to type.

The connection is gone

• Verify certificate expiry is monitored with enough lead time, including on any certificate renewed manually.
• Verify domain registration auto-renews, and that the card on file is not expired.
• Verify registrar and DNS accounts have recovery paths that do not depend on email at the affected domain.
• Verify you know how to fail DNS over, and how long the TTL means it will take.
• Verify a CDN or edge outage has a decided response — wait, bypass, or switch — rather than being discovered as a question.
• Verify you can bypass the CDN and serve from origin if the origin can take it, and know whether it can.
• Verify database connection exhaustion has a specific response, since it is the most common cause of a total outage that is not a deploy.

Someone else's outage

• Verify every critical third party is listed with what breaks if it disappears.
• Verify each one has a subscribed status page, so you find out from them rather than from a customer.
• Verify the failure mode of each dependency is decided: fail closed, fail open, queue, or degrade.
• Verify authentication provider downtime does not lock out every existing session as well as new logins.
• Verify payment provider downtime has a decided behaviour — queue the intent, or refuse cleanly with an honest message.
• Verify email or SMS provider downtime does not silently drop messages that should be retried.
• Verify a timeout and circuit breaker exists on every outbound call, so their outage is not automatically yours.
• Verify a slow dependency degrades rather than exhausting your workers — see scale/05-async-and-queues.md.
• Verify you know which dependency has no fallback at all, and that this is a recorded decision.

Capacity and load

• Verify there is a decided response to a traffic spike: scale, shed, queue, or turn off the expensive feature.
• Verify you can turn off an expensive feature without a deploy.
• Verify a spike caused by abuse is handled differently from a spike caused by success, and that you can tell them apart — see security/core/18-abuse-and-availability.md.
• Verify the autoscaling ceiling is high enough to survive a real spike and low enough not to bankrupt you.