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You can run and test AWS Lambda functions on a Windows machine without deploying after every code change by using the AWS Serverless Application Model Command Line Interface (AWS SAM CLI). For local container-based tests, install Docker and make sure it is running. Use sam local invoke for a single event, sam local start-api to test HTTP routes, or sam local start-lambda to exercise AWS CLI or SDK invocation patterns. These workflows run Lambda code locally, but calls your function makes to AWS services can still reach real resources.
Choose the local workflow that matches your test
AWS SAM CLI is AWS’s documented tool for local serverless development and testing. Its commands serve different invocation styles; choose the one that resembles how you need to exercise the function.
| Command | Use it for | How you invoke the function |
|---|---|---|
sam local invoke |
Testing one function with a specific event | Run a one-time invocation using an event file or JSON supplied through standard input. AWS SAM CLI: sam local invoke |
sam local start-api |
Testing an API route | Start a local HTTP server and send requests to its routes. AWS SAM CLI: sam local start-api |
sam local start-lambda |
Testing AWS CLI or SDK invocation patterns | Start a local Lambda-compatible endpoint for clients to call. AWS SAM CLI: sam local start-lambda |
For the broadest SAM CLI feature support, AWS recommends a native SAM template. SAM CLI can also work with CDK applications after they are synthesized to CloudFormation; Terraform support is limited and requires SAM template artifacts. See AWS SAM CLI overview for the documented infrastructure options.
Install the tools and prepare your Windows project
Follow AWS’s current Lambda local development guidance for installing AWS SAM CLI and setting up your editor. AWS describes Visual Studio Code, AWS Toolkit for Visual Studio Code, AWS credentials, and SAM CLI as components of its editor-based workflow. Docker is required for local container testing, though AWS describes it as optional for development more generally. Installation requirements can change, so use AWS’s instructions rather than relying on a fixed version or outdated setup steps.
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- Install AWS SAM CLI and Docker for Windows using the current official instructions.
- Start Docker Desktop, then confirm Docker is running before starting a local container test. SAM uses local containers to approximate Lambda execution environments. AWS’s Hello World tutorial also requires Docker for its optional local testing step.
- Open a terminal in the project root—the directory containing the SAM template. If that template defines more than one function, find the function’s logical ID; you will need it for a direct invocation.
- Configure AWS credentials if the function or your development workflow needs to access AWS. Keep local test settings separate from committed production configuration.
Invoke one function with a representative event
Create a JSON file containing an event that represents the input you want to test, such as an API Gateway request or an S3 event. The event shape matters: a local invocation is only useful to the extent that its input resembles what the function is designed to receive.
- Save the event as a file, for example
event.json, in or near the project directory. - From the project root, run
sam local invoke FunctionLogicalId -e event.json, replacingFunctionLogicalIdwith the function’s logical ID from the template. - If the template contains only one function, you can omit the logical ID:
sam local invoke -e event.json. - Review the function output and errors in the terminal. Adjust the event or code and invoke it again as needed.
The command also accepts event JSON from standard input, which is useful when another command generates the payload. AWS documents the project-directory requirement, event-file and stdin input, and function logical ID in its invoke command reference.
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Set local environment-variable overrides
For local runs, SAM supports environment-variable overrides in JSON or .env formats with invoke and start-api workflows. Use these to supply test-specific values without putting production secrets or settings into source control. Check the invoke reference for the supported format and command options.
Test an HTTP route or a Lambda-compatible endpoint
Use sam local start-api for HTTP routes
Run sam local start-api from the project directory to start a local HTTP server for routes defined in the SAM template. Send a request to the appropriate local route to exercise the handler through an HTTP interface rather than supplying a one-off event yourself. This is useful when you want to test the API request path and routing behavior together with the function. See AWS’s start-api guide.
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Use sam local start-lambda for AWS CLI or SDK patterns
Run sam local start-lambda to expose a local Lambda-compatible endpoint. Configure an AWS CLI or SDK client to call that endpoint when you want to exercise a client invocation pattern against your locally running function. This differs from start-api, which tests HTTP routes defined by the application. Consult AWS’s start-lambda guide and command reference for endpoint and command details.
Know what local execution does—and does not—emulate
SAM builds local function containers that use Lambda runtime environments, providing a closer approximation of runtime behavior than invoking ordinary code directly on Windows. But local execution does not automatically emulate the AWS services your function might call. Those calls can reach real AWS resources, incur charges, or change data—including production data if your credentials and configuration point there. AWS highlights this limitation in its Lambda testing guidance.
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- Use test resources and appropriately scoped credentials for local integration tests.
- Check environment variables, profiles, and resource identifiers before invoking code that accesses AWS.
- Keep a local test configuration separate from production values and avoid committing secrets.
- Use cloud-side testing when you need to verify behavior that depends on deployed AWS services or infrastructure; a local runtime test alone cannot establish that integration works as deployed.
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