Simple FastAPI web app to subscribe & receive from an SNS Topic

sns_fastapi.py

from fastapi import FastAPI, Request
from dotenv import load_dotenv
import uvicorn
import requests
import os

load_dotenv()

app = FastAPI()

# The SNS_TOPIC_ARN env var must be set 
allowed_topics = [
    os.environ['SNS_TOPIC_ARN']
]

# Main endpoint to receive SNS messages
@app.post("/sns/subscription")
async def index(request: Request):
    if 'x-amz-sns-message-type' not in request.headers:
        print('A regular HTTP request, will be ignored!')
        return {"message": "Request filtered out", "status": 403}
    
    body = await request.json()
    message_type = request.headers.get('x-amz-sns-message-type', '')
    topic_arn = body["TopicArn"]

    if topic_arn not in allowed_topics:
        return {"message": "Topic not allowed", "status": 403}
    
    # More checks can be added here, like checking the signature of the message
    # Anyhow, this is just a simple example

    if message_type == 'SubscriptionConfirmation':
        print('Subscription confirmation in progress...')
        subscribe_url = body['SubscribeURL']
        # Confirm the subscription by sending a GET request to the SubscribeURL
        response = requests.get(subscribe_url)
        if response.status_code == 200:
            return {"message": "Successfully subscribed to the topic", "status": 200}
        else:
            return {"message": "Failed to subscribe to the topic", "status": 500}
        
    sns_message = body.get('Message', '')
    print(f'SNS message received, length: {len(sns_message)} bytes')

    # ============================================
    # Your message processing logic goes here!
    # ============================================

    return {"message": "Message received", "status": 200}

# Health check endpoint for Front Door and other load balancing probes  
@app.get("/healthz")
async def index(request: Request):
    return {"status": "OK"}

if __name__ == '__main__':
    uvicorn.run('main:app', host='0.0.0.0', port=8000)