Fast Lambda

FastAPI in AWS Lambda

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1. Why Run FastAPI in Lambda?

• Serverless → Pay only for requests.
• Scalability → Auto-scales with demand.
• Low Ops Overhead → No servers to manage.
• Integrates Easily → With API Gateway, S3, DynamoDB, SNS/SQS.

⚠️ Lambda isn’t perfect for all use cases: it has cold starts and execution time limits (15 mins).

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2. Setup

Install Tools

pip install mangum fastapi uvicorn

• Mangum → ASGI adapter for AWS Lambda.

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3. FastAPI App with Lambda

main.py

from fastapi import FastAPI
from mangum import Mangum

app = FastAPI()

@app.get("/")
def root():
    return {"msg": "Hello from FastAPI on Lambda"}

@app.get("/items/{item_id}")
def get_item(item_id: int):
    return {"item": item_id, "detail": "Fetched from Lambda"}

# Adapter
handler = Mangum(app)

👉 handler is the AWS Lambda entrypoint.

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4. Deploy with AWS Lambda

Option A) AWS Console (Manual Upload)

1. Zip your project (main.py, dependencies in package/).

2. Upload zip to AWS Lambda.

3. Set handler as:

   main.handler

4. Connect via API Gateway.

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Option B) Serverless Framework

serverless.yml:

service: fastapi-lambda
provider:
  name: aws
  runtime: python3.11
  region: us-east-1
functions:
  api:
    handler: main.handler
    events:
      - httpApi:
          path: /
          method: get
      - httpApi:
          path: /items/{item_id}
          method: get

Deploy:

sls deploy

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Option C) AWS SAM CLI

template.yaml:

AWSTemplateFormatVersion: "2010-09-09"
Transform: AWS::Serverless-2016-10-31
Resources:
  FastApiFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: .
      Handler: main.handler
      Runtime: python3.11
      Events:
        ApiGateway:
          Type: Api
          Properties:
            Path: /{proxy+}
            Method: ANY

Deploy:

sam build && sam deploy --guided

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5. Folder Structure

project/
│── app/
│   ├── main.py        # FastAPI app + Mangum handler
│   ├── requirements.txt
│
│── template.yaml      # SAM template OR serverless.yml
│── package/           # Dependencies for Lambda layer

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6. Best Practices

• ✅ Use Mangum for ASGI support.

• ✅ Use Lambda Layers for dependencies → reduces package size.

• ✅ Minimize cold starts by:

• Keeping imports light.

• Using smaller dependencies.

  • ✅ Use API Gateway HTTP API (faster, cheaper than REST API).
  • ✅ For heavy workloads → use AWS Fargate / ECS instead of Lambda.
  • ✅ Log with CloudWatch.
  • ✅ Monitor with X-Ray (tracing).
  • ✅ Store secrets in AWS Secrets Manager.

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7. When to Use FastAPI on Lambda?

✅ Small to medium APIs with spiky workloads.

✅ Event-driven triggers (S3 uploads, SNS, SQS).

✅ Prototypes and low-cost APIs.

❌ Not great for high-throughput, always-on, low-latency services (use ECS/EKS).