AWS Lambda calling API gateway getting 500 internal server error

Question

I'm developing several AWS serverless applications using Lambda and API gateway.

At one point, I tried to execute an API request on one application (using requests python lib) from code running inside a Lambda function in another application. I get 500 server error. From the logs it appears that the Lambda function behind the API gateway is not starting at all. I don't find any logs that can tell me what happened.

Additional details:

• The API gateway is protected by IAM auth.
• The calling lambda has permission to "execute_api"
• The request is signed according to Signature V4 - I followed the example here: https://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html#sig-v4-examples-post .

Answer 1

The issue was that the calling lambda needed permission to execute the second lambda behind the API gateway. I don't know why that is. I also still don't know how I should have debugged this - where are the logs that should tell me what was the issue. Eventually it was a lucky guess on my side.