FastAPI on AWS: Choosing Between Lambda and Fargate

FastAPI has taken the Python world by storm, offering a modern, high-performance framework for building APIs. Its asynchronous nature and Pydantic data validation make it a joy to work with. When it comes to deploying a FastAPI application on AWS, two serverless options stand out: AWS Lambda and AWS Fargate.

Both are excellent choices, but they are designed for different use cases. Choosing the right one depends on the specific needs of your application. This guide breaks down the key differences to help you decide.

Option 1: AWS Lambda

AWS Lambda is a function-as-a-service (FaaS) platform. You upload your code, and Lambda runs it in response to events, such as an HTTP request from API Gateway. You don't manage any servers; you only pay for the execution time.

To run a FastAPI application on Lambda, you use a library like mangum, which acts as an adapter, translating API Gateway's event payload into a standard ASGI request that FastAPI can understand.

Pros of Using Lambda:

• Pay-per-request: If your API has low or unpredictable traffic, Lambda is incredibly cost-effective. You pay nothing when your API is idle.
• Infinite Scalability: Lambda scales automatically to handle virtually any amount of traffic, from one request per day to thousands per second.
• Simplicity: The deployment model is simple. You package your code and its dependencies, and Lambda handles the rest.

Cons of Using Lambda:

• Cold Starts: The first request to a new Lambda instance can have additional latency (a "cold start") as the runtime initializes. While this has improved significantly with features like SnapStart, it can still be a concern for highly latency-sensitive applications.
• Payload Size Limits: API Gateway has a 10 MB payload size limit, and Lambda's invocation payload limit is 6 MB for synchronous calls. This makes Lambda unsuitable for APIs that handle large file uploads or downloads.
• Execution Time Limit: Lambda functions have a maximum execution timeout of 15 minutes, making them unsuitable for very long-running background tasks.

Best for: APIs with spiky or infrequent traffic, microservices, and applications where cost-efficiency at low scale is a primary concern.

Option 2: AWS Fargate

AWS Fargate is a serverless compute engine for containers. You package your FastAPI application as a Docker container, and Fargate runs it for you without requiring you to manage the underlying EC2 instances. It's like running containers on demand.

Pros of Using Fargate:

• No Cold Starts: Your container is always running, so there are no cold starts. This provides consistent, low-latency performance, which is ideal for user-facing APIs.
• No Payload or Time Limits: Since you're running a standard container, you are not bound by the payload size or execution time limits of Lambda and API Gateway. You can handle large file uploads, streaming, and long-running background processes.
• Flexibility: You have full control over the container environment. You can run sidecar containers, use custom networking configurations, and have more control over the runtime.

Cons of Using Fargate:

• Higher Idle Cost: You pay for the CPU and memory allocated to your container for as long as it's running, even if it's not processing any requests. This makes Fargate more expensive than Lambda at low traffic levels.
• More Complex Deployment: Deploying to Fargate involves more moving parts: creating a Docker image, pushing it to ECR, defining a task definition, and configuring a service and a load balancer.
• Slower Scaling: While Fargate can scale automatically, it takes longer to launch a new container instance (typically a minute or two) compared to Lambda's sub-second scaling.

Best for: APIs with consistent traffic, applications that require low latency without cold starts, and services that need to handle large payloads or long-running tasks.

The Verdict: Which One Should You Choose?

Here’s a simple heuristic:

• Start with Lambda: For most new projects, especially internal services or APIs with unpredictable traffic, AWS Lambda is the best place to start. Its simplicity and pay-per-request model are hard to beat.

• Migrate to Fargate if Needed: If your application grows to have consistent traffic and cold starts become a noticeable issue, or if you run into payload or timeout limits, migrating to AWS Fargate is a logical next step. Because FastAPI is a standard web framework, moving from a Lambda deployment to a container-based deployment is relatively straightforward.

Ultimately, the choice between Lambda and Fargate is not a permanent one. Both are excellent serverless platforms, and the beauty of modern cloud architecture is the flexibility to choose the right tool for the job—and to change that tool as your application's needs evolve.