Local Testing with SAM CLI

Deploying to test every change is slow. The SAM CLI emulates Lambda and API Gateway on your machine for a fast feedback loop.

Local emulation runs functions inside Docker containers that mirror the Lambda runtime, so you need Docker installed alongside the SAM CLI.

Use sam local invoke to run one function once with a test event, just like a single Lambda execution.

The event file is the JSON payload your handler receives. Keep a folder of sample events for different scenarios.

SAM can scaffold realistic events for common sources so you do not handcraft them.

sam local start-api stands up a local HTTP server that routes requests to your functions, mimicking API Gateway.

Once running, hit it with any HTTP client. The response comes from your real handler code.

Pass a JSON file of environment variable overrides so local runs point at test resources instead of production.

Start with -d to pause for a debugger to attach on a port, letting you set breakpoints in your function code.

Local mode cannot perfectly reproduce IAM, networking, or cold starts. Treat it as a fast first check, not a replacement for a cloud staging test.

Before deploying, run sam validate to catch template errors early.

Test your local-testing knowledge.

You learned to invoke functions, generate events, run a local API, inject env vars, and attach a debugger with the SAM CLI, plus its emulation limits.