At midnight everything started failing with TLS errors. What happened and what do you do?
What they are really testing: Pattern recognition on one of the most common self-inflicted outages, plus whether your instinct after fixing it is to prevent the class rather than the instance.
A real interview question
At midnight everything started failing with TLS errors. What happened and what do you do?
What most people say
drag me
“I would restart all the services to see if that clears the TLS errors.”
Restarting cannot renew a certificate, so it wastes minutes during a total outage and adds churn. It also signals that the candidate did not recognise the strongest clue available, which was the exact-midnight timing.
The follow-ups they ask next
Renewal is automated but it expired anyway. What went wrong?
Usually the renewal ran and the new certificate was never loaded, because the process needs a reload it did not get, or the automation failed silently weeks ago with nobody alerting on its failure.
Only some clients fail while others work. What does that suggest?
An intermediate chain problem: some clients cache or bundle the intermediate and some do not. Serve the full chain, and check with a client that has no cached intermediates.
What the interviewer is listening for
- Reads the midnight timing as the key clue
- Checks all certificate layers, not just the edge
- Ends with automated renewal and 30-day alerting
What sinks the answer
- Restarts services
- Renews only the edge certificate
- Treats it as bad luck rather than a preventable class
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“Everything failing at [a hard time boundary] is [certificate expiry until proven otherwise]. I [inspect the served cert and read notAfter], then [renew and deploy or fail over]. I check [every layer: edge, internal mTLS, client certs, pinned chains]. Then [automated renewal plus alerting 30 days out], since [a human remembering is not a control].”
Keep going with troubleshooting
Junior
A developer says the build passes locally but fails in CI. How do you debug that?
Junior
A production server is failing writes. Disk is 100% full. Walk me through what you do.
Mid
A pod is in CrashLoopBackOff after a deploy. Walk me through your debugging, command by command.
Mid
The deploy reports success, pods are Running, but users get 502s. Where do you look?
Mid
A Kubernetes node goes NotReady and pods are stuck Terminating. What is happening and what do you do?
Mid
You get paged: the site is down. You have no other information. What are your first five minutes?
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