How do you monitor and debug serverless functions?

Serverless monitoring and debugging uses a multi-layer approach: (1) CloudWatch Logs — Lambda automatically streams console output to CloudWatch Logs. Query with CloudWatch Log Insights using SQL-like syntax; (2) CloudWatch Metrics — built-in metrics: Invocations, Errors, Duration, Throttles, ConcurrentExecutions. Set alarms on error rate and duration; (3) AWS X-Ray — distributed tracing that shows the execution path across Lambda functions, DynamoDB, SQS, and external HTTP calls. Adds ~1-2% overhead; (4) Lambda Powertools (AWS) — open-source library adding structured logging (JSON), tracing, metrics emission, and idempotency utilities to Python, Java, Node.js, TypeScript, .NET functions; (5) Third-party APMs — Datadog Serverless, New Relic, Lumigo, Epsagon provide function-level visibility with less AWS-specific configuration; (6) Lambda test events — use the console's test event feature with real or mock event payloads; (7) AWS Lambda Power Tuning — open-source Step Functions state machine that finds the optimal memory/cost configuration for your function.