API und Geschäftslogik implementieren

In the previous lesson, we designed a serverless microservice. Now, it's time to bring that design to life!

This lesson focuses on developing the actual Lambda functions and configuring API Gateway to power the core business logic of your microservice.

API Gateway acts as the secure entry point for your serverless microservice. It receives HTTP requests and routes them to the correct backend service, typically an AWS Lambda function.

• It defines your API's endpoints (paths and methods).
• It handles request validation and routing.
• It integrates directly with Lambda functions.

Your Lambda functions contain the actual business logic of your microservice. Each function typically handles a specific API endpoint and operation (e.g., GET /products/{id}, POST /products).

When API Gateway triggers a Lambda function, it passes all request details in an event object.

When using Lambda Proxy Integration with API Gateway, the event object passed to your Lambda function contains comprehensive details about the incoming HTTP request.

• httpMethod: The HTTP method (GET, POST, PUT, DELETE).
• pathParameters: Any variables from the URL path.
• queryStringParameters: Parameters from the URL query string.
• body: The request body (as a JSON string).

Let's create a simple Lambda function to handle a GET request, like fetching a product by its ID. We'll extract the product_id from the pathParameters.

Try running this example:

Your Lambda function must return a specific JSON structure for API Gateway to correctly process the response and send it back to the client. This is crucial for consistent API behavior.

• statusCode: The HTTP status code (e.g., 200 for success, 400 for bad request).
• headers: A dictionary of HTTP headers (e.g., 'Content-Type': 'application/json').
• body: The actual response data, which must be a JSON string.

Now, let's look at handling a POST request, typically used for creating new resources. Here, we'll parse the request body to get the new product data.

Try running this example (imagine a POST request with {"name": "New Item", "price": 10.50} as body):

Good APIs provide clear error messages. In Lambda, you should catch potential issues and return appropriate HTTP status codes and detailed error bodies.

• Client Errors (4xx): Bad input, missing parameters.
• Server Errors (5xx): Unexpected issues in your Lambda function or downstream services.

Always return a consistent error structure for easier client-side handling.

After writing your Lambda functions, you need to configure API Gateway to trigger them. This involves:

1. Creating an API Gateway resource (e.g., /products).
2. Adding an HTTP method (e.g., GET, POST) to that resource.
3. Setting the integration type to Lambda Proxy.
4. Specifying the target Lambda function's ARN.

This creates the bridge between your API endpoint and your function's business logic.

Consider a Lambda function designed to update a user's profile. If the request body is empty, what HTTP status code should the Lambda function return to indicate a client-side error?

You've learned how to implement the core API and business logic for a serverless microservice! We covered:

• The roles of API Gateway and Lambda in a microservice.
• Extracting data from the Lambda event object for different HTTP methods.
• Constructing proper API Gateway-compatible responses.
• The importance of robust error handling.
• Briefly, how API Gateway is configured to trigger your Lambda functions.

You're now ready to build functional serverless API endpoints!