How to Build a Serverless API with FastAPI and AWS Lambda

In today's fast-paced software development landscape, building efficient and scalable applications is more crucial than ever. One popular approach for achieving this is through serverless architecture, which allows developers to focus on writing code without worrying about server management. In this article, we will explore how to build a serverless API using FastAPI and AWS Lambda, two powerful tools that streamline the development process.

What is FastAPI?

FastAPI is a modern, fast (high-performance) web framework for building APIs with Python 3.6+ based on standard Python type hints. It is designed to be easy to use and has a minimalistic approach, making it a fantastic choice for developers looking to create APIs quickly. FastAPI leverages async capabilities, which can significantly improve performance in I/O-bound applications.

Key Features of FastAPI

• Automatic interactive API documentation: FastAPI automatically generates documentation using OpenAPI and JSON Schema.
• Data validation: With built-in support for Pydantic, FastAPI ensures that the data coming into your API is validated and serialized.
• Asynchronous support: FastAPI is built for async programming, allowing for concurrent requests handling.

What is AWS Lambda?

AWS Lambda is a serverless compute service provided by Amazon Web Services that lets you run code without provisioning or managing servers. You only pay for the compute time you consume, making it an economical choice for many applications.

Benefits of AWS Lambda

• Cost-effective: Pay only for the compute time used.
• Scalability: Automatically scales your application in response to incoming requests.
• Integration: Works seamlessly with other AWS services, such as DynamoDB, S3, and API Gateway.

Use Cases for Serverless APIs

Building a serverless API with FastAPI and AWS Lambda can serve various applications:

• Microservices: Break down monolithic applications into smaller, manageable services.
• Data Processing: Process data from various sources and trigger actions based on events.
• Webhooks: Create endpoints to receive webhooks from third-party services.
• Mobile Backends: Serve as the backend for mobile applications with minimal infrastructure overhead.

Getting Started: Setting Up Your Environment

Before we dive into the code, let’s set up our environment. You will need:

1. Python 3.6+: Ensure you have Python installed on your system.
2. AWS Account: Sign up for an AWS account if you don't have one.
3. AWS CLI: Install and configure the AWS Command Line Interface (CLI).
4. Serverless Framework: Install the Serverless Framework for easier deployment.

To install the Serverless Framework, run:

npm install -g serverless

Step 1: Create a FastAPI Application

Let’s start by building a simple FastAPI application. Create a new directory for your project:

mkdir fastapi-lambda
cd fastapi-lambda

Next, install FastAPI and an ASGI server, such as uvicorn:

pip install fastapi uvicorn

Now, create a file named main.py and add the following code:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str = None):
    return {"item_id": item_id, "query": q}

To run your FastAPI app locally, use the command:

uvicorn main:app --reload

Visit http://127.0.0.1:8000 in your browser to see your API in action.

Step 2: Prepare for AWS Lambda Deployment

To deploy your FastAPI application on AWS Lambda, we will use the Mangum adapter, which allows ASGI applications to run on AWS Lambda.

Install the mangum package:

pip install mangum

Update your main.py file to include Mangum:

from fastapi import FastAPI
from mangum import Mangum

app = FastAPI()
handler = Mangum(app)

@app.get("/")
def read_root():
    return {"Hello": "World"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str = None):
    return {"item_id": item_id, "query": q}

Step 3: Configure Serverless Framework

Create a serverless.yml configuration file in your project directory:

service: fastapi-lambda

provider:
  name: aws
  runtime: python3.8

functions:
  app:
    handler: main.handler
    events:
      - http:
          path: /
          method: get
      - http:
          path: items/{item_id}
          method: get

Step 4: Deploy Your API

Now that everything is set up, you can deploy your API to AWS Lambda with the following command:

serverless deploy

After a successful deployment, you will receive an endpoint URL. Use this URL to access your FastAPI application hosted on AWS Lambda.

Step 5: Testing Your API

You can test your API using tools like Postman or cURL. Here’s a simple cURL command to fetch data:

curl https://your-api-endpoint.amazonaws.com/dev/

Troubleshooting Common Issues

• Cold Start: AWS Lambda functions may experience cold starts, which can increase response times. To mitigate this, keep your functions warm by scheduling a regular ping.
• Timeouts: Ensure your Lambda function's timeout settings are appropriate for the expected execution time of your API.

Conclusion

Building a serverless API with FastAPI and AWS Lambda is a powerful way to create scalable and efficient applications. With FastAPI’s speed and ease of use combined with AWS Lambda’s serverless capabilities, you can focus on delivering features and improving user experience. By following the steps outlined in this article, you will have a robust API ready to handle various workloads, all while minimizing infrastructure overhead. Happy coding!