Summary

• Mangum supports events from AWS API Gateway and AWS ALB.
• Mangum has a api_gateway_base_path argument to strip the prefix of the http path before passing to the underlying app
• api_gateway_base_path works as expected for API Gateway events
• As far as I can tell, api_gateway_base_path does not work for ALB events.

I looked at https://mangum.io/http/ and the GitHub issues and couldn't find this mentioned anywhere.

My Setup

• Python 3.9
• mangum 0.12.4
• starlette 0.17.1

Simplified version of my lambda handler:

def public_foo_function:
    print("Public foo function")

def private_bar_function:
    print("Private bar function")

app = Starlette(
    routes=[
        Route(
            path="/public/foo",
            endpoint=public_foo_function,
        ),
        Route(
            path="/private/bar",
            endpoint=private_bar_function,
        ),
    ]
)

mangum = Mangum(
    app,
    api_gateway_base_path="/prefix",
)

def main(event, context):
    return mangum(event, context)

An external API Gateway v2 HTTP has a route /prefix/public/{proxy+} integrated with the lambda.
An internal ALB has a listener rule /prefix/private/* integrated with the lambda.

Requests to <api gateway url>/prefix/public/foo arrive in the lambda and public_foo_function is called, as expected.

From within my VPC, internal requests to <alb url>/prefix/private/bar arrive in the lambda and Starlette returns Not Found 404. This is unexpected.

As a workaround, if I change the private route in Starlette app from /private/bar to /prefix/private/bar, private_bar_function is called.

Considerations

api_gateway_base_path does have 'api gateway' in the name, and ALB is not an API Gateway. So there's an argument it could be intentional?

But, I expected mangum to behave consistently for all of its supported input events.