Lambda support for Python servers

[unexge]

This PR exposes app.run_lambda() function to allow Python servers to run on Lambda. Closes #1750.

Why we use custom Rust runtime

We could also use builtin Python runtime, I decided to use custom Rust runtime because there was some performance overheads.

If you use Python runtime:

• Receive requests with Python (Python)
• Pass requests to Rust (Rust)
• You basically receive a JSON object in API Gateway schema and you need to convert it to Hyper Request, this is handled by lambda_http crate but it is not designed to be reusable and requires some modification/copying (Rust)
• Call Python handler to get result (Python)
• Convert that result to Http response (Rust)
• Pass that result to Python (Python)
• Finally return response to Lambda runtime to complete (Python)

If you use Rust runtime:

• Receive Hyper Request with Rust (Conversion from JSON object to Hyper Request is already handled by lambda_http crate) (Rust)
• Call Python handler to get result (Python)
• Convert that result to Http response (Rust)
• Finally return response to Lambda runtime to complete (Rust)

Testing

It can be tested with a Dockerfile similar to this:

FROM public.ecr.aws/lambda/python:3.8-x86_64

COPY app.py ${LAMBDA_TASK_ROOT}
# Shared library should be on the same folder with application code, so it can be imported
COPY lib_pokemon_service_server_sdk.so ${LAMBDA_TASK_ROOT}

RUN ln -s ${LAMBDA_TASK_ROOT}/app.py /app.py

# override `ENTRYPOINT` to not call Python runtime, so we can use Rust runtime
ENTRYPOINT [ "/var/lang/bin/python3.8" ] 
CMD [ "/app.py" ]

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[crisidev]

What do you think about renaming the entrypoint of the standard server from run() to run_server() ?

[unexge]

I think run() is clearer. I don't know if it is idiomatic but, if we decide to unify run_lambda and any possible run_* versions into a big run function with keyword arguments like run(lambda=True, foo="bar", ...) that would allow us to do without renaming it again.

[crisidev]

Ok, makes sense!

[crisidev]

Not a blocker for this PR, but I would probably create an issue to feature-gate lambda in the future so that we do not impose this dependency on all customers. I am not even sure if its worth it thou.. Thoughts?

[unexge]

Yes good point. But I'm not sure if we currently support to generate SDKs with specific features on dependencies?

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[crisidev]

This looks good and I am very happy to see we can use this from the pure python lambda context! Are you planning to add an example of the Python code here?

[unexge]

This looks good and I am very happy to see we can use this from the pure python lambda context! Are you planning to add an example of the Python code here?

By example code you mean like having Lambda example of Pokemon service? If so, I think there is no need for that because only code change is changing app.run() to app.run_lambda(). What do you think?

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