Support Lambda response streaming

[jabrks]

It would be great if LLRT supported Lambda response streaming as per https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html#runtimes-custom-response-streaming.

[jabrks]

I had a go at implementing this myself (my Rust isn't quite up to scratch but the code that interacts with the Lambda runtime API is written in TypeScript for the moment) but I don't think we have a way to stream a request body in LLRT currently. I would've ordinarily used the http API in Node.js but that isn't implemented here with a recommendation to use fetch instead, which only seems to allow a string, Array, ArrayBuffer or Uint8Array.

I'd be happy to take another look if there's a preferred way forward here?

[richarddavison]

Hi @jabrks. We want to support response streams but first we have to implement native streaming for performance reasons https://github.com/awslabs/llrt/issues/178

[gc-victor]

Hey @richarddavison, have you considered web-streams-polyfill? It's used in Lagon.

References:

• https://github.com/MattiasBuelens/web-streams-polyfill
• https://github.com/lagonapp/lagon/blob/aee172d548fb70d47083165f7b80867574c284e1/packages/js-runtime/src/runtime/streams.ts#L9

[richarddavison]

Thanks for the suggestion but we already have a streams polyfill but that won't perform as well as we desire due to lack of a JIT. Basically every byte that flows through a stream implemented will cause multiple function calls, branches, allocations etc.

[jabrks]

Thanks @richarddavison, I'll sit tight for now in that case!