Streaming bevy/get and bevy/list with HTTP SSE

[LiamGallagher737]

Objective

Add a way to stream BRP requests when the data changes.

Solution

BRP Side (reusable for other transports)

Add a new "update checker" handler alongside a normal method handler that works a lot like a run condition. Streaming requests are stored in a resource, every request in it is checked every frame for updates and the main handler is run if the update condition returns true. The new response is sent over the same channel as normal. Transports decide how to decide if requests should be streamed or not as some like web sockets will always want to stream and others may want to be a hybrid (like I did with HTTP).

HTTP Side

If a request comes in with +stream in the method, the transport will invoke the method with streaming enabled. It will then reply with text/event-stream content-type (Server sent events) and keep the connection open for more responses.

Testing

I tested with the podman HTTP client. This client has good support for SSE's if you want to test it too.

Parts I want some opinions on

For selecting streaming in HTTP I chose to add a +stream suffix to the end kind of like content-type headers. A get would be bevy/get+stream. Anything after a '+' is then ignored when selecting a handler. I chose this as I don't think the functionality is different enough for a new method name but I also don't want to add to the body params.

Future work

• Currently the full response is resent, it would be nice to only send whats changed but for now I want to keep things as simple as I can keep them.
• The bevy/query method would also benefit from this but that condition will be quite complex so I will leave that to later.

[JMS55]

Really not sure how I feel about SSE vs a proper streaming protocol like QUIC or WebTransport...

[LiamGallagher737]

Really not sure how I feel about SSE vs a proper streaming protocol like QUIC or WebTransport...

I chose it just due to being easy to implement with the existing HTTP transport. I am happy to swap it for something else as it's the BRP part I care about, the SSE is just to have something to show it works.

[LiamGallagher737]

After a quick look at the wtransport crate, it is defiantly something I'd like to have a go at integrating. Until then I think the SSE serves as a good example of how to use the streaming.

[LiamGallagher737]

@notmd I'd also appreciate a review from you since you have been working on streaming stuff.