Building a Serverless API with AWS Lambda and API Gateway

Serverless architecture has become a preferred approach for modern application development due to its scalability, cost efficiency, and reduced operational overhead. By combining AWS Lambda and Amazon API Gateway, developers can build robust, scalable APIs without provisioning or managing servers.

This guide walks through the fundamentals of building a serverless API, the architecture involved, step-by-step setup, and best practices to ensure security, performance, and cost efficiency in 2025.

What Is Serverless and Why Use It?

Serverless computing enables developers to build and deploy applications without managing infrastructure. Instead, cloud providers handle provisioning, scaling, and maintenance automatically.

Key Benefits of Building Serverless APIs

• No server management or patching

• Automatic scaling based on request volume

• Pay only for usage

• Faster development and deployment cycles

• Seamless integration with AWS services

With Lambda handling backend logic and API Gateway providing a fully managed RESTful endpoint, serverless APIs are ideal for microservices, mobile and web backends, IoT, and event-driven systems.

AWS Lambda and API Gateway: How They Work Together

Architecture Flow:

1. Client sends a request to API Gateway

2. API Gateway triggers a Lambda function

3. Lambda executes business logic

4. Lambda returns response via API Gateway to the client

This loose coupling ensures flexibility, scalability, and modular design.

Step-by-Step Guide: Build Your First Serverless API

Prerequisites

• AWS account

• Basic knowledge of Python/Node.js

• IAM account with Lambda and API Gateway permissions

Step 1: Create an AWS Lambda Function

1. Go to AWS Console → Lambda → Create Function

2. Choose Author from scratch

3. Select runtime (Node.js, Python, or others)

4. Configure IAM execution role with permissions

5. Write your function code

Example Lambda Function (Node.js)

exports.handler = async (event) => {
  const response = {
    statusCode: 200,
    body: JSON.stringify({ message: "Hello from AWS Lambda API!" })
  };
  return response;
};

Click Deploy to save the function.

Step 2: Create an API in Amazon API Gateway

1. Open API Gateway → Create API

2. Select HTTP API (recommended for most use cases) or REST API

3. Add an integration and choose Lambda

4. Select the Lambda function you created

5. Configure the route (e.g., GET /hello)

Deploy the API and note the public URL provided.

Step 3: Test the API

You can test the API using a browser, Postman, or cURL:

curl https://your-api-id.execute-api.region.amazonaws.com/hello

A successful response returns the JSON message from Lambda.

Step 4: Add Additional API Routes

Add more Lambda functions or reuse one with routing logic.
For example, create routes such as:

• POST /users

• GET /users/{id}

• DELETE /users/{id}

API Gateway allows request validation, authentication, throttling, and transformations.

Step 5: Enable Security and Authorization

Security is a crucial part of production APIs.

Options for Securing Serverless APIs

• IAM Authorization: For internal AWS service access

• Amazon Cognito: For user authentication with JWT tokens

• API Keys: For basic client access control

• Lambda Authorizers: Custom token and request-based validation

Enable CORS if your API is accessed from browsers.

Step 6: Monitor and Optimize

Use AWS tools to track performance, errors, and costs.

Best Practices for Serverless APIs in 2025

1. Keep Lambda Functions Lightweight

Avoid large libraries or heavy dependencies to reduce cold starts.

2. Use Environment Variables

Store configuration securely and avoid hard-coding.

3. Implement Caching

Use:

• API Gateway Caching

• CloudFront for global delivery

This reduces latency and costs.

4. Adopt Infrastructure as Code (IaC)

Use AWS SAM, Serverless Framework, or Terraform for automated deployments.

5. Optimize Lambda Cost

• Use shorter timeouts

• Right-size memory settings

• Use asynchronous or event-driven patterns where possible

When to Use Serverless APIs

Serverless APIs are a great fit for:

• Microservices architectures

• Low to medium-traffic applications with unpredictable workloads

• Startups and MVPs needing low-cost development

• Applications requiring rapid scaling

• Event-driven systems integrating with AWS services

Not ideal for extremely latency-sensitive applications or long-running workloads.

Final Thoughts

Building a serverless API with AWS Lambda and API Gateway enables fast, scalable, and cost-efficient development with minimal infrastructure overhead. As serverless adoption grows in 2025, businesses benefit from automatic scaling, high availability, integrated security, and pay-as-you-go pricing.

By following the steps and best practices outlined in this guide, you can build a production-ready serverless API that is secure, reliable, and optimized for performance.