Debugging questions.
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An API endpoint started returning 503s only at peak traffic in one of three regions. Walk me through how you investigate.A nightly batch job has been silently producing duplicate rows for the last three nights. Where do you look first?A user reports that their cart 'sometimes' empties between page loads. You can't reproduce it. Walk me through your approach.A service's p99 latency spiked from 80ms to 600ms after a deploy that 'changed nothing relevant.' What's your investigation path?A queue worker is processing messages but the queue depth keeps growing. Walk me through what you check.Your service is healthy on every dashboard, but customers report errors. How do you bridge the gap?A weekly report has the wrong totals — they're off by exactly the same percentage every week. What do you investigate?A test in CI fails about 1 in 20 runs and passes when re-run. Walk me through how you'd track it down.A search service is returning correct results, but they're in the wrong order. The ranking team says nothing's changed. What do you check?A function is consuming 4GB of memory in production but only 200MB in staging on identical inputs. How do you find what's different?A bug ticket says 'login is broken' with no other detail. The user can't be reached. What do you do?Performance metrics show a database is healthy, but every query taking it as a dependency is slow. What's most likely?Customers in one country report payment failures, but staging works fine and the same flow works for adjacent countries. Walk me through your investigation.A microservice's error rate spikes for exactly 90 seconds every six hours. Where do you start?A new release shows no error spike but customer NPS drops by 12 points. Where do you look?A customer reports that their data is wrong on the dashboard, but the underlying database shows the right values. Walk me through.A new error appears in logs with the text 'connection reset by peer' at low but rising frequency. What does that tell you and what do you check next?Reports come in that a feature 'feels slow' but no latency metric is showing degradation. Walk me through the possibilities.A bug only happens when two specific microservices are deployed at the same time. How do you debug it without rolling back?A long-running stream-processing job stops emitting after 4 hours, every time. The process is still alive. Where do you look?A revenue-affecting bug has been live for a week and no alert fired. What's your first move after the bleeding stops?An API client is failing on what looks like a CORS error in browser, but identical requests from server-side work fine. Walk me through.A scheduled task isn't running but the scheduler claims it is. What do you do to confirm and then locate the gap?A workflow fails for exactly one of your enterprise customers and passes for everyone else. Where do you start?An app's memory grows steadily over hours until OOM, on a hot path you've never touched. How do you find the leak?Your team owns a service whose owners just left the company. Production traffic is fine, but you can't make sense of the logs. What's your first week look like?A service that's been stable for a year started flaking. The last code change was two months ago. What's most likely?A bug shows up only when two specific feature flags are both on. How would you confirm and isolate it?A test passes locally and on your colleague's machine but fails on CI consistently. Where do you look?A change was rolled forward, broke production, and rolled back — but the symptoms didn't fully disappear. What does that tell you and what do you check?A form submission works fine in Chrome but fails silently in Safari. The network tab shows a 200 response. Walk me through how you'd debug this.Your app crashes with 'undefined is not a function' in production but works locally. What are the top three things you check and in what order?A button click sometimes triggers the action twice, but you've clicked it 50 times and can't reproduce it. How do you approach this?Walk me through how you'd use browser DevTools to figure out why an image isn't loading on a webpage.Your REST API returns data, but the mobile app shows 'No results found.' The API logs show successful 200 responses. What do you check?A CSV export feature worked yesterday but today generates files with scrambled column headers. No code was deployed. What's your debugging process?You have three possible causes for a bug: a code logic error, bad input data, or a misconfigured environment variable. How do you decide what to check first?A race condition happens only under load. You don't have access to a staging environment with production-level traffic. How would you reproduce it?Describe how you'd use console.log, breakpoints, and the call stack to debug a function that's returning the wrong value.Error logs show 'database timeout' but the database monitoring dashboard shows CPU and memory are normal. What else should you look at?An LLM-backed feature returns malformed JSON for about 2% of production requests, but you've never seen it fail in testing. How do you track down what's different and harden the feature?A daily job ran twice last Sunday and didn't run at all on Monday. The code hasn't changed in months. What's your leading hypothesis and how do you confirm it?Two users report briefly seeing someone else's account data after a release. You can't reproduce it. What classes of bug produce cross-user data leaks, and which do you rule out first?A distributed trace shows a request took 2 seconds end to end, but the spans only add up to 400ms. Where does the missing time hide, and how do you make it visible?A dependency outage ended 30 minutes ago and its error rate is back to zero, but your service's latency is still elevated and recovering slowly. What mechanisms cause this lag, and what do you check?A change sailed through canary at 5% of traffic but broke when rolled to 100%. What categories of bug pass a canary and fail at full rollout, and how do you narrow down which one you have?Your coding assistant proposed a fix that makes the failing test pass, but the customer bug reports keep coming. What discipline keeps you from shipping fixes that cure the test instead of the bug?A p99 latency alert keeps firing, but when you search the logs you can't find a single request slower than the threshold. How can both be true, and how do you reconcile them?A bug appeared right after a deploy whose only diff is dependency version bumps — no application code changed. How do you isolate which dependency, and which change inside it, is responsible?Your feature works in development, but in production the app calls the wrong API URL. How do you find out what configuration the deployed app is actually running with?You fixed a bug and deployed, but a user on the call still sees the old behavior. What are the caching layers between your code and their screen, and in what order do you rule them out?You hit an exception with a 40-frame stack trace that's mostly framework code. How do you find the line that actually matters, and what do you read first?A page has gotten slower as the product grew, and the logs show hundreds of nearly identical database queries for a single page load. What's going on, and how do you confirm and fix it?At 14:00 UTC, every service in every region started failing authentication simultaneously. No deploys happened anywhere. What failure classes produce a synchronized global break, and how do you confirm the culprit fast?A cache hit rate dropped from 92% to 45% overnight with no code changes or traffic pattern shifts. Walk me through your debugging process.Users in a specific timezone report seeing stale data every morning between 6-7am, but logs show successful cache refreshes. What do you investigate?A feature flag rollout at 10% shows a 3x error rate compared to control, but only for Android users on version 12. How do you narrow down the root cause?Your observability dashboard shows CPU at 40%, memory at 60%, no errors logged, but the on-call page says the service is down. What's your first move?A microservice returns correct data when called directly via curl, but returns errors when called through the API gateway. Walk me through your debugging steps.Database replication lag is under 100ms, but users report seeing their own writes disappear for 5-10 seconds. What could cause this discrepancy?A flaky integration test passes locally and in CI 95% of the time, but fails 100% of the time when run in parallel with test suite B. How do you isolate the issue?Your distributed tracing shows every span completing successfully in under 50ms, but users report 10-second page loads. What's missing from your observability?A scheduled job ran successfully for 18 months, then suddenly started timing out. The data volume hasn't changed. What non-obvious factors do you check?Error logs show a third-party API returning 200 OK, but the response body is empty 2% of the time. The vendor insists their service is healthy. How do you prove what's happening?Memory usage grows steadily at 50MB/hour in production but stays flat in load testing with 10x the traffic. What could explain this?A POST endpoint returns 201 Created, the database shows the row inserted, but a GET immediately after returns 404. This happens randomly to 1 in 500 requests. Where do you start?Your alerting system fired 47 PagerDuty incidents last night for a service outage, but the service was actually healthy. How do you debug the monitoring itself?A GraphQL query returns in 80ms when requesting 5 fields but takes 12 seconds when requesting 6 fields. The 6th field is a simple string. What do you investigate?After a Kubernetes deployment, 3 out of 20 pods are serving traffic but showing different application versions in logs. How do you identify what went wrong?A financial reconciliation job reports totals that differ from the source system by $0.03 every single day. Manual spot checks show individual transactions are correct. What's your approach?WebSocket connections drop after exactly 60 seconds in production but stay open indefinitely in staging. Both environments use identical application code. What do you check?A user reports data corruption but you can't find the corrupted record in the database, backups, or audit logs. Walk me through how you verify their claim and investigate.Your CDN reports 99.9% cache hit rate, but origin server traffic hasn't decreased after enabling caching. What could explain this discrepancy?A race condition occurs only when deploying during business hours, never during off-hours deployments. The deployment process is identical. How do you reproduce and fix it?Your logging pipeline shows 1M events/minute ingested but only 400K/minute indexed in your search backend. There are no errors. What's happening to the missing events?An API consistently returns in 45ms for user A and 850ms for user B requesting identical data. Both are hitting the same server instance. What do you investigate?A blue-green deployment switches traffic successfully, but 0.1% of requests still route to the old environment for 6 hours afterward. How do you track down where they're coming from?Metrics show average response time is 120ms, but the histogram reveals a strange bimodal distribution with peaks at 50ms and 900ms. What patterns would you look for?A mobile app works perfectly on WiFi but fails to load data on cellular networks. The API logs show successful 200 responses for all requests. How do you debug this?