Top 5 Troubleshooting Interview Questions
Browse 5 real-world technical and behavioral interview questions about Troubleshooting. Review scenarios, edge cases, and architectural best practices.
Your GC log shows a stop-the-world pause far longer than the collection work inside it. Where did the rest of the time go?
Most likely into reaching the safepoint. Every application thread has to arrive at a poll point before the operation can begin, and one late thread stalls all of them, so the pause is time-to-safepoint plus the actual work. Safepoint logging separates the two.
Your application cannot reach a service that should be up. Walk me through diagnosing it from the shell.
Work up the layers and let each command eliminate a class of cause: resolve the name, check the route, see whether anything is listening, then decide from the failure mode whether packets are being refused, silently dropped, or answered by something that never replies.
A customer has escalated hard, insisting your product is broken, but the fault is in their own integration. How do you run the investigation and deliver the finding?
When a customer insists a product bug is actually their integration issue, investigate from a shared request ID, prove the boundary failure with reproducible evidence, and deliver the finding without embarrassing the customer engineer.
df says the filesystem is 100% full but du only accounts for half of it. What is going on?
df and du disagree because df reads filesystem block allocation while du walks visible directory entries. Deleted-but-open files, inode exhaustion, hidden files under mounts and reserved blocks are the usual causes. Use this LINUX answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects filesystems to the point an interviewer is testing.
A service dies with OutOfMemoryError in production. Walk me through diagnosing it.
Read the message text first, because it names which region ran out. Then use the GC log to see whether the live set after each full collection is trending upward, which means a leak, or flat and close to the ceiling, which means the heap is simply too small. A heap dump tells you what is retaining the memory.