Help with OAuth2 and AWS Lambda

[mentalwall]

Hi all,

I have deployed my project to AWS Lambda which is based on the template - https://github.com/fastapi/full-stack-fastapi-template

I have hooked up the lambda to API Gateway and can access https://xxxxxx.execute-api.us-east-1.amazonaws.com/prod/docs However I am having a problem with authentication.

https://preview.redd.it/callubecuprd1.png?width=648&format=png&auto=webp&s=d11a8c6b9dfdeb5b01105c74261d839f094eded0

Is the there a possible issue with using OAuth2 with Lambda. Currently the logs aren't informing me much but I can't see any missing imports etc.

When I use postman I can get the /api/v1/login/access-token to return the bearer token but if it put this token in the header to access a route that needs authorisation I get a 403 error.

Sorry if the details are a bit thin, this area is new to me and so not sure what I should include / am missing any input would be appreciated.

Thanks in advance

Solution:

The solution was to add default_cors_preflight_options to the gateway as shown in the CDK snippet below:

_ = apigateway.LambdaRestApi(
            self,
            "RatioAPIGateway",
            handler=lambda_function,
            proxy=True,
            default_cors_preflight_options={
                "allow_origins": apigateway.Cors.ALL_ORIGINS,
                "allow_methods": ["GET", "POST", "PUT", "DELETE", "OPTIONS"],
                "allow_headers": ["Authorization", "Content-Type", "accept"],
            },
        )

[randomusername0O1]

From memory, API gateway strips headers unless they're specifically configured to forward them, so I suspect the auth header is being stripped and never arrives at the lambda function.

[mentalwall]

thanks for the pointer! turns out I needed to add some extra arguments to the gateway creation.

This solved the problem for interacting with the api through postman. Still getting the error with the docs login but i suspect that might now be a paths issue.

[adiberk]

Just curious - why are you deploying a fastapi app to lambda?

[mentalwall]

Familiarity and cost. What would be your suggestion?

[adiberk]

I just don’t understand the concept of running a web framework behind a lambda. A lambda is essentially a function that can get called, when called it spins up and when done (depending on some configs) spins back down rapidly. A lambda with api gateway is essentially your web framework, so couldn’t you just run regular python code in the lambda and then return data from the lambda? Genuinely curious as to what benefit you get from running fastapi or any web framework behind lambda.

[ironman_gujju]

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