What is the benefits of using spawn and channels to receive streams?

I am learning async and streams in Rust. I see some patterns like using spawn and channel to receive streaming response, e.g.

What are the benefits of this? Why spawn is used? Why not just use stream combinators to map the stream and return the original stream? Is this a common code pattern in (async) Rust or just because the author wants to wrap the original streams (into TextStream in this case)?

Thanks.

At a guess from what I see;
Receiving from the stream will likely pause each time when stream.next() is not waited upon.
Using spawn takes away having a choice of causing a wait. (Can't say if good or bad.)

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Yes. Streams are β€œlazy” β€” they perform computation when you ask them to. By spawning a task dedicated to polling the stream, it will be processed as fast as possible. I don't know whether in this case that's necessary, or whether the author is simply using the channel as a means to hide the type of the stream.

Note however that this transformation removes backpressure β€” the channel is unbounded, so if there are many responses quickly, memory will be allocated for all of them, to hold them until they are consumed from the receiving side of the channel. This can be bad because bugs or DoS attacks can cause arbitrary amounts of allocation. I don't know enough about this particular application to say whether it's a risk here.

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Does this mean the actual data transmission from server to client will only happen if the stream is polled? If the stream is drop before polling starts, no data is transmission is ever happen?

That depends entirely on what self.client.completions().create_stream(request) does. Probably, some amount of data will be received and buffered, but that could be anything from "1 byte" to "the entire response". You will have to consult create_stream()'s documentation or code to find out.

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