How to Test AWS Lambda Functions Locally

You can test AWS Lambda functions locally using the AWS SAM CLI, which simulates the Lambda runtime on your machine. This lets you run, debug, and invoke your Lambda code without deploying it to the cloud.

Syntax

To test a Lambda function locally with AWS SAM CLI, use the command sam local invoke. This runs your Lambda handler code in a local Docker container that mimics the AWS Lambda environment.

Basic syntax:

• sam local invoke [FunctionLogicalID] -e event.json - invokes the Lambda function with an event payload.
• sam local start-api - starts a local API Gateway to test Lambda functions triggered by HTTP requests.

bash

sam local invoke MyFunction -e event.json

Example

This example shows how to test a simple Lambda function locally using AWS SAM CLI. The Lambda function returns a greeting message.

plaintext

/* app.js */
exports.lambdaHandler = async (event) => {
    return {
        statusCode: 200,
        body: JSON.stringify({ message: "Hello from Lambda!" })
    };
};

/* template.yaml */
AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Resources:
  HelloFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: app.lambdaHandler
      Runtime: nodejs18.x

/* event.json */
{}

Example

bash

sam build
sam local invoke HelloFunction -e event.json

Output

{
    "statusCode": 200,
    "body": "{\"message\":\"Hello from Lambda!\"}"
}

Common Pitfalls

Common mistakes when testing Lambda locally include:

• Not having Docker installed or running, since sam local uses Docker containers.
• Using an event file that does not match the expected input format of your Lambda function.
• Forgetting to build the project with sam build before invoking.
• Not setting environment variables locally that your Lambda depends on.

Always check Docker is running and your event JSON matches your function's input.

bash

/* Wrong: invoking without Docker running */
sam local invoke HelloFunction -e event.json

/* Right: start Docker first, then build and invoke */
docker start
sam build
sam local invoke HelloFunction -e event.json

Quick Reference

Tips for testing Lambda locally:

• Install Docker and AWS SAM CLI before testing.
• Use sam build to prepare your code.
• Use sam local invoke with an event file to simulate Lambda invocation.
• Use sam local start-api to test HTTP-triggered Lambdas.
• Check logs and errors in the terminal for debugging.

Key Takeaways

Use AWS SAM CLI with Docker to run Lambda functions locally.

Always build your Lambda project with 'sam build' before invoking locally.

Provide a proper event JSON file to simulate Lambda input.

Ensure Docker is installed and running before local testing.

Use 'sam local start-api' to test Lambda functions triggered by HTTP requests.