# Introduction to testing with sam local start-lambda
<a name="using-sam-cli-local-start-lambda"></a>

Use the Amazon SAM CLI subcommand `sam local start-lambda` to invoke your Lambda function through the Amazon CLI and SDKs. This command starts a local endpoint that emulates Lambda.
+ For an introduction to the Amazon SAM CLI, see [What is the Amazon SAM CLI?](what-is-sam-overview.md#what-is-sam-cli)
+ For a list of `sam local start-lambda` command options, see [sam local start-lambda](sam-cli-command-reference-sam-local-start-lambda.md).

**Note**  
`sam local start-lambda` supports durable functions with automatic checkpointing and replay capabilities. Durable functions work seamlessly with the local Lambda endpoint.

## Prerequisites
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To use `sam local start-lambda`, install the Amazon SAM CLI by completing the following:
+ [Amazon SAM prerequisites](prerequisites.md).
+ [Install the Amazon SAM CLI](install-sam-cli.md).

Before using `sam local start-lambda`, we recommend a basic understanding of the following:
+ [Configuring the Amazon SAM CLI](using-sam-cli-configure.md).
+ [Create your application in Amazon SAM](using-sam-cli-init.md).
+ [Introduction to building with Amazon SAM](using-sam-cli-build.md).
+ [Introduction to deploying with Amazon SAM](using-sam-cli-deploy.md).

## Using sam local start-lambda
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When you run `sam local start-lambda`, the Amazon SAM CLI assumes that your current working directory is your project’s root directory. The Amazon SAM CLI will first look for a `template.[yaml|yml]` file within a `.aws-sam` subfolder. If not found, the Amazon SAM CLI will look for a `template.[yaml|yml]` file within your current working directory.

**To use sam local start-lambda**

1. From the root directory of your project, run the following:

   ```
   $ sam local start-lambda <options>
   ```

1. The Amazon SAM CLI builds your Lambda functions in a local Docker container. It then outputs the local address to your HTTP server endpoint. The following is an example:

   ```
   $ sam local start-lambda
   Initializing the lambda functions containers.
   Local image is up-to-date
   Using local image: public.ecr.aws/lambda/python:3.9-rapid-x86_64.
   
   Mounting /Users/.../sam-app/hello_world as /var/task:ro,delegated, inside runtime container
   Containers Initialization is done.
   Starting the Local Lambda Service. You can now invoke your Lambda Functions defined in your template through the endpoint.
   2023-04-13 07:25:43 WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
    * Running on http://127.0.0.1:3001
   2023-04-13 07:25:43 Press CTRL+C to quit
   ```

1. Use the Amazon CLI or SDKs to invoke your Lambda function locally.

   The following is an example using the Amazon CLI:

   ```
   $ aws lambda invoke --function-name "HelloWorldFunction" --endpoint-url "http://127.0.0.1:3001" --no-verify-ssl out.txt
       
   StatusCode: 200
   (END)
   ```

   The following is an example using the Amazon SDK for Python:

   ```
   import boto3
   from botocore.config import Config
   from botocore import UNSIGNED
   
   lambda_client = boto3.client('lambda',
                                endpoint_url="http://127.0.0.1:3001",
                                use_ssl=False,
                                verify=False,
                                config=Config(signature_version=UNSIGNED,
                                              read_timeout=1,
                                              retries={'max_attempts': 0}
                                              )
                               )
   lambda_client.invoke(FunctionName="HelloWorldFunction")
   ```

## Options
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### Specify a template
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To specify a template for the Amazon SAM CLI to reference, use the `--template` option. The Amazon SAM CLI will load just that Amazon SAM template and the resources it points to. The following is an example:

```
$ sam local start-lambda --template myTemplate.yaml
```

For more information on Amazon SAM templates, see [Amazon SAM template anatomy](sam-specification-template-anatomy.md).

## Best practices
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If your application has a `.aws-sam` directory from running `sam build`, be sure to run `sam build` every time you update your function code. Then, run `sam local start-lambda` to locally test your updated function code.

Local testing is a great solution for quick development and testing before deploying to the cloud. However, local testing doesn’t validate everything, such as permissions between your resources in the cloud. As much as possible, test your applications in the cloud. We recommend [using `sam sync`](using-sam-cli-sync.md) to speed up your cloud testing workflows.

## Learn more
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For a list of all `sam local start-lambda` options, see [sam local start-lambda](sam-cli-command-reference-sam-local-start-lambda.md).