How do you debug a failing Lambda function?
Debugging AWS Lambda Function Failures
Debugging a failing AWS Lambda function involves identifying the root cause using logs, error messages, and tracing tools. AWS provides integrated services like CloudWatch and X-Ray to assist with monitoring, logging, and tracing, making it easier to locate and fix issues.
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Check CloudWatch Logs for error stack traces, console output, and custom logs.
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Look for metrics like Invocation, Error, Throttles, and Duration in CloudWatch Metrics.
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Use AWS X-Ray to trace requests and identify latencies or downstream errors.
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Review environment variables and function configuration (timeout, memory, IAM permissions).
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Manually invoke the function using test events in the AWS Console to reproduce the error.
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Enable active tracing and log retention policies for better observability.
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Examine Dead Letter Queues (DLQs) for failed asynchronous events.
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Amazon CloudWatch Logs
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Amazon CloudWatch Metrics
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AWS X-Ray
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Lambda Destinations (for async failures)
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AWS CLI and AWS SDKs (for testing and troubleshooting)
Scenario Questions
0-2 years experience
- 1You have a Lambda triggered by S3 uploads that keeps timing out. Walk me through the steps you’d take to figure out why.
- 2When you look at the CloudWatch logs you see a 'ResourceNotFoundException' for your Lambda. What do you check first?
- 3How would you use AWS X‑Ray to identify which part of your Lambda code is slow?
2-5 years experience
- 1A recent code change added a third‑party library and now the Lambda is failing with a 502 error from API Gateway. How would you isolate the cause?
- 2During a deployment the new Lambda version returns intermittent 504 Gateway Timeout errors under load. Explain your debugging approach.
- 3Your Lambda processes Kinesis records but you notice occasional 'ProcessingFailed' errors only when the batch size is high. What would you investigate?
5-8 years experience
- 1A Lambda that reads from a Kinesis stream is falling behind, causing data loss. How would you redesign or debug the pipeline to handle high throughput reliably?
- 2You need to add detailed X‑Ray tracing to a critical Lambda without adding noticeable latency. What trade‑offs do you consider and how would you implement it?
- 3Describe how you’d set up automated alerts, DLQs, and a rollback strategy for a production Lambda that starts failing after a new release.
8+ years experience
- 1Your company is migrating a monolith to many Lambdas, and failures are hard to trace across services. Propose a cross‑team observability and debugging framework.
- 2Multiple teams deploy Lambdas into the same AWS account, and noisy alerts are overwhelming on‑call. How would you design governance and tooling to improve debugging ownership?
- 3Discuss the long‑term maintenance implications of using inline code versus container images for Lambdas, especially regarding debugging and migration planning.
Follow-up Questions
- Which CloudWatch metric would you watch to spot throttling?
- How do you verify the Lambda has the right IAM role for the resources it accesses?
- What would you do if the bug only appears in production and not in your test environment?