AWS Lambda timeout but function completes locally -- cold start and connections

Lambda function works locally but times out in AWS. Or it sporadically times out after working fine. Increasing timeout to 30s does not help.

Common causes: (1) Cold start: first invocation initializes runtime. Keep functions small, use provisioned concurrency for latency-sensitive paths. (2) VPC cold start: Lambda in VPC creates ENI on cold start (10-30s penalty). Use VPC only when needed. (3) Database connections: creating new DB connections per invocation is slow. Reuse connections outside the handler. (4) DNS resolution: VPC DNS can be slow. (5) Waiting for event loop: Node.js Lambda waits for empty event loop by default. Set context.callbackWaitsForEmptyEventLoop = false.

Lambda reuses execution environments across invocations. Resources initialized outside the handler persist. But cold starts create everything from scratch.