Remember the last annoying AWS Lambda function problem you faced after deployment? Cloud debugging is tough with serverless solutions like AWS Lambda. Luckily, AWS SAM CLI lets you test and debug Lambda functions locally before deployment. This post explains how to build up your local environment, replicate API Gateway events, and debug Lambda functions using AWS SAM. 

 

Understanding AWS SAM CLI 

AWS SAM is an open-source platform for serverless application development and deployment. It simplifies dealing with AWS Lambda, API Gateway, DynamoDB, and other services and lets you execute serverless apps locally. Local programming needs SAM CLI for lambda function testing and debugging. 

SAM CLI's main benefit is PC cloud simulation. Replicating AWS services tests Lambda functions and API Gateway events without code deployment. Improved development timelines and fewer surprises while deploying updates. 

Setting Up Your Local Environment 

You need a few things before you can test Lambda functions locally using SAM CLI. Install the AWS SAM CLI. This installation needs Docker since SAM CLI imitates Lambda environments using Docker containers. After installing SAM CLI, run AWS configure in your terminal to setup AWS credentials. 

Design your first Lambda function using SAM. Make a simple Node.js "Hello World" Lambda function. Define your SAM application using an app.js Lambda function and template.yaml file in your project directory. Template.yaml defines your serverless app's Lambda, API Gateway, and resources. 

Locally test Lambda function. The sam local invoke command simulates AWS Lambda on your machine. Test API Gateway integration thereafter. 

Simulating API Gateway Events Locally 

HTTP requests activate AWS Lambda functions via API Gateway, a popular use case. Simulate API Gateway events using the SAM CLI to test locally. 

For local API Gateway operation, use sam local start-api. This command starts a local API Gateway-like HTTP server that triggers Lambda functions on HTTP requests. SAM CLI triggers Lambda functions when you submit HTTP requests to http://localhost:3000/your-api-endpoint. 

Add a basic API Gateway event to template.yaml and build an API endpoint to test. Use sam local start-api to request the endpoint. You may test the whole flow from API Gateway accepting HTTP requests to executing Lambda without cloud deployment. 

Debugging Lambda Locally 

Local Lambda function debugging is a valuable SAM CLI capability. You may walk through the code using breakpoints as in a local development environment. Using sam local invoke with --debug starts debugging. This lets you see your Lambda function execute. 

If you use VS Code, you can put breakpoints in Lambda function code and execute SAM with a debugger. This displays the function's execution in real time. You may check variables, code lines, and errors to find the problem. 

Additionally, SAM CLI interfaces with AWS CloudWatch Logs to display Lambda logs locally. This log provides error messages, log statements, and other diagnostic data about your function's execution to help you debug. 

Check for missing environment variables, API Gateway event formatting, and dependencies while debugging. Fix deployment problems locally before deploying to AWS to save time.  

Conclusion 

Debugging AWS Lambda functions locally with AWS SAM CLI transforms serverless development. By trying Lambda functions and modeling API Gateway events locally, you can save time and worry less about cloud release. You will be able to write, test, and fix Lambda functions faster if you follow this guide. If you are tired of the "publish and test" loop and want to improve the way you build serverless apps, try SAM CLI.