Is there any overhead when using ASGI based FastAPI for AWS Lambda?

Question

Can you please advise, if there are any issues with using of FastAPI Web Framework in AWS Lambda:

• What framework capabilities will be lost if using in AWS Lambda environment?
• Is there any overhead associated with ASGI technology or anything else, e.g. when we have to use additional libraries like Magnum?

Answer 1

We have been using the API gateway -> AWS Lambda -> Mangum -> FastApi combo in production for 3-4 months now and have had no issues.

The restrictions come from the fact that Lambda is a sync request/reply setup.

• Less benefit from async ( async inside the request but not between requests). AWS lambda will create a container/python process for each request so while your code is awaiting a call in a request it won't move on to the next request.
• No response streaming bodies.(use s3 for files etc)

The overhead is tiny, Mangum is super light ( see how little code is in: https://github.com/jordaneremieff/mangum/tree/main/mangum) its basically just restructuring the input dictionary.