Test e debugging locali con SAM CLI

Deploying to AWS just to test a change is slow. The SAM CLI emulates Lambda and API Gateway locally using Docker, so you can iterate in seconds.

Local invocation needs:

• Docker running
• The SAM CLI installed
• A valid template.yaml

Run one function with a test event payload.

SAM can generate realistic event payloads for many AWS sources, saving you from writing them by hand.

Start a local HTTP server that mimics API Gateway, routing requests to your functions.

Supply env vars via a JSON file so local runs match what the function expects.

SAM watches your source: when you save a change, the next request uses the new code without restarting the local API, keeping the feedback loop tight.

Attach a debugger by passing a debug port. Your IDE connects to it so you can set breakpoints inside the running container.

Catch template errors before deploying with a quick validation command.

After a deploy you can tail CloudWatch logs from the terminal, filtering by function.

Local mode is great for logic, but it cannot perfectly mirror IAM permissions, real network latency, or managed services like DynamoDB. Always run an integration test in a real AWS stage before production.

You learned local development with SAM:

• sam local invoke runs a single function
• sam local start-api emulates API Gateway
• Generate events, supply env vars, and attach a debugger
• sam validate catches template errors early
• Local mode has limits — verify in a real stage before prod