Debug: AWS Lambda function timeout or cold start issues

AWS Lambda function times out, has slow cold starts, or runs out of memory.

Lambda debugging and optimization:

COLD START OPTIMIZATION:

1. Reduce package size
   - Use Lambda layers for large dependencies
   - Exclude dev dependencies and test files
   - Use tree-shaking (webpack/esbuild for Node.js)
   - Python: only include needed packages

2. Choose the right runtime
   - Node.js/Python: ~100-200ms cold start
   - Java/.NET: ~1-3s cold start (use SnapStart for Java)
   - Go/Rust: ~50-100ms cold start (compiled, no runtime)

3. Increase memory (also increases CPU!)
   - Lambda allocates CPU proportional to memory
   - 128MB = fraction of a vCPU
   - 1769MB = 1 full vCPU
   - More memory = faster execution (often cheaper overall)

4. Keep functions warm
   - Use provisioned concurrency for critical paths
   - EventBridge scheduled ping (every 5 min)

TIMEOUT DEBUGGING:

1. Check CloudWatch Logs
   - START/END/REPORT lines show actual duration
   - Look for: Init Duration (cold start time)

2. Common timeout causes:
   - Waiting for unreachable service (VPC NAT gateway needed?)
   - DNS resolution in VPC (use VPC endpoints)
   - Connection pool exhaustion (DB connections)
   - Large payload processing

3. VPC Lambda issues:
   - Lambda in VPC can't reach internet by default!
   - Need: NAT Gateway or VPC endpoints
   - For S3/DynamoDB: Use VPC endpoints (free, faster)

MEMORY DEBUGGING:

1. Check REPORT line in CloudWatch:
   REPORT Duration: 500ms Max Memory Used: 200MB
   - If Max Memory Used is close to configured memory -> OOM risk

2. Increase memory allocation
   - Start at 256MB, adjust based on Max Memory Used
   - Use AWS Lambda Power Tuning tool to find optimal

# Test locally
sam local invoke MyFunction -e event.json

# View recent logs
aws logs tail /aws/lambda/MyFunction --follow

# Check configuration
aws lambda get-function-configuration --function-name MyFunction

Lambda has unique constraints: cold starts, fixed timeout, memory-linked CPU. Understanding these constraints is essential for reliable serverless applications.

AWS Lambda serverless debugging