Proxy Streams: individual GRPC streams to the backend for connection or subscription #680
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Replaced by #722 |
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Proposed changes
Proposing an experimental feature called Proxy Streams.
Proxy streams allow pushing data towards client channel subscription (and optionally connection itself) directly and individually from your application backend over the unidirectional or bidirectional GRPC stream.
The stream is established between Centrifugo and your application backend as soon as user subscribes to a channel (or connects to Centrifugo). The scheme may be useful if you want to generate individual streams and these streams should only work for a time while client is connected or subscribed to a channel.
In this case Centrifugo plays a role of WebSocket-to-GRPC streaming proxy – keeping numerous real-time connections from application's clients and establishing GRPC streams to the backend, multiplexing them using a pool of HTTP/2 (transport used by GRPC) connections.
This may be configured similar to our connect or subscribe proxies. For connection level stream proxy we open unidirectional or bidirectional GRPC stream for the time of connection liveness (backend can close stream at any point). For subscription level stream proxy we open unidirectional or bidirectional GRPC stream for the time of subscription liveness (backend can close stream at any point).
The downside of the idea is the number of additional goroutines we need for each such stream. Every stream will cost 3 extra goroutines. So for a WS connection with 5 streams the number of goroutines is 2 + 5*3 = 17. This will result into more CPU and more memory. This fact prevents me from proceeding right away. On the other hand the approach should scale horizontally (scaling GRPC on the backend is not a straightforward but definitely possible). Not every use case involves millions concurrent GRPC streams.