Your load balancer shows 5xx errors jumping from 0.1 percent to 12 percent of requests over ten minutes. There was no deploy. What do you do?
What they are really testing: Whether you can triage without the comfort of a recent deploy to blame, and whether you know the difference between the load balancer failing and the target failing.
A real interview question
Your load balancer shows 5xx errors jumping from 0.1 percent to 12 percent of requests over ten minutes. There was no deploy. What do you do?
What most people say
drag me
“I would restart the service, that usually clears 5xx errors, and then see if it comes back.”
Restarting is a coin flip that destroys the evidence you needed, it shows no model of where the error is generated, and it gives you nothing to say when the errors return twenty minutes later.
The follow-ups they ask next
The 5xx are all 502s. What does that tell you?
The target closed the connection or returned a malformed response. Look for OOM kills, keepalive mismatch between the ALB idle timeout and the app timeout, or crashing workers.
How do you decide whether to page the downstream team?
Page them once you have one piece of evidence, not a suspicion. Bring the latency graph and a request id, and say what you have already done to contain it.
What the interviewer is listening for
- Distinguishes load balancer 5xx from target 5xx
- Adds capacity as a containment move before root causing
- Considers non deploy changes like secret rotation or certificate expiry
- Contains a failing dependency instead of trying to fix someone else's service at 2am
What sinks the answer
- Restarts everything as the first and only move
- Treats all 5xx as one undifferentiated blob
- Never checks whether errors are concentrated on one host or one route
- Declares victory the moment the graph dips for 30 seconds
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.
“I have not owned an ALB in production, but the reasoning I would use is: add capacity to contain it, then work out whether the error is being generated by the load balancer or by my own app, because those lead to different places.”
Keep going with troubleshooting
Junior
You get paged at 2am. Checkout latency has gone from 200ms to 4 seconds and it started right after the 11pm deploy. Walk me through what you do.
Junior
It is 3am and you get paged: the primary application server is at 100% disk usage and the app is throwing write errors. You are the only one online. Walk me through exactly what you do.
Junior
You push a new version of a service and within two minutes the pods are all showing CrashLoopBackOff. The team is watching you share your screen. Walk me through exactly what you do.
Junior
A developer pings you: the checkout service cannot reach the payments service, the calls just hang and time out. You have never worked on either service. Walk me through what you do.
Junior
A developer messages you: 'My app suddenly gets Access Denied writing to the S3 bucket. Nothing changed.' It worked yesterday. Walk me through what you do.
Mid
One of your containers keeps getting OOM killed. It restarts, serves traffic for about twenty minutes, then dies again. Talk me through how you handle it.
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