API Gateway Routing and Integration: A Beginner’s Guide

Previously in this course, we built the core logic of our application in Connecting Lambda to DynamoDB and handled data persistence in Project Integration: Saving App Data. While our backend is functional, it currently lacks an entry point accessible from the web.

In this lesson, we introduce API Gateway, the service that acts as the "front door" for our serverless applications. You will learn how to configure routing and integration to expose your Lambda functions as secure, scalable HTTP endpoints.

Understanding API Gateway Routing and Integration

In a serverless architecture, you rarely expose Lambda functions directly to the internet. Instead, you use an API Gateway. It acts as a managed proxy that handles traffic management, authorization, and—most importantly for this lesson—API routing.

Routing is the process of mapping a specific HTTP method (GET, POST, etc.) and a URL path (e.g., /users) to a specific backend resource, such as one of our Lambda functions.

The Mechanics of Integration

When a client sends a request to your API, the process follows this flow:

1. Request: A client (like a browser) hits the API Gateway URL.
2. Routing: The Gateway matches the path and method to a defined resource.
3. Integration: The Gateway triggers the associated Lambda function, passing the request details (headers, body, query params) as an event object.
4. Response: The Lambda function returns a JSON object, which the Gateway transforms back into an HTTP response for the client.

Configuring Your First API Resource

To get started, we will create an API resource and map it to the Lambda function we developed in our project.

1. Create a REST API

1. Navigate to the API Gateway console in your AWS account.
2. Select Create API and choose REST API (not the "Private" or "HTTP API" versions for this exercise).
3. Give it a name, such as my-web-app-api, and click Create API.

2. Define Resources and Methods

In API Gateway terms, a "Resource" represents a path segment.

• In the Actions menu, select Create Resource.
• Name it items.
• With the /items resource selected, select Create Method in the Actions menu.
• Choose POST from the dropdown and click the checkmark.
• For Integration type, select Lambda Function.
• Search for and select the Lambda function you created in our previous project lessons.

3. Deploying the API

An API is just a configuration until it is deployed to a "Stage."

• Select your API, click Actions, and choose Deploy API.
• Create a new stage named dev.
• Once deployed, AWS will provide you with an Invoke URL. This is the public endpoint for your application.

Testing Your Setup

With the deployment complete, you can test the routing directly from the console.

1. Click on the POST method you created under /items.
2. Click the Test button (the lightning bolt icon).
3. In the Request Body field, paste a sample JSON payload that your Lambda expects (e.g., {"name": "test-item"}).
4. Click Test. You should see the Lambda output in the Response Body on the right.

If you receive a 200 OK status, your integration is successfully routing traffic to your code.

Common Pitfalls to Avoid

• Missing Permissions: The most common error is a 500 Internal Server Error caused by API Gateway lacking permission to invoke your Lambda. Ensure your Lambda has a resource-based policy that allows lambda:InvokeFunction from the API Gateway service principal.
• Malformed Lambda Responses: API Gateway expects your Lambda function to return a specific JSON structure: { "statusCode": 200, "body": "..." }. If you return just a raw object, the API will fail to parse it.
• Hardcoding URLs: Avoid hardcoding your API Gateway URL in your frontend. Use environment variables or configuration files to inject the endpoint dynamically.

Frequently Asked Questions

Q: Should I use REST APIs or HTTP APIs? A: HTTP APIs are cheaper and faster, but REST APIs offer more granular control, such as API keys and usage plans, which we will cover in Securing APIs with API Keys.

Q: How do I handle path parameters like /items/{id}? A: In the API Gateway console, use the Create Resource action and type {id} as the name to create a parameterized path.

Recap

You have now moved from local code execution to a cloud-native architecture. By using API Gateway, you have decoupled your frontend interface from your backend logic, enabling a secure and standard way to communicate with your serverless functions.

Up next: Request Validation in API Gateway — we will learn how to enforce schema rules on incoming requests to ensure your Lambda functions never process bad data.