Flow Control Implementation

[Linus Meierhoefer]

Hi,
I am trying to setup an rpc server in order to integrate it into my custom publisher-subscriber library. For this I need a flow control mechanism that doesn't depend on individual streams but on a global disk writing speed. That is: The flow limit should adapt to the in-flight-bytes of the disk IO. Moreover, if the flow limit is exceeded the rpc server should effectively stop reading packets from client sockets, pushing the backpressure logic to TCP. This should prevent memory overhead for the broker when a publisher tries to flood the server.
I saw that there is flow control logic implemented in capnp but wonder how it actually operates? The header-documentation of RPCSystem says that "the RpcSystem will not read further messages from the stream" but does this imply that these overflow messages will reside in the sockets buffer and don't get transferred to the receivers memory? And how is backpressure implemented then?
If the default flow control logic does not fit my use case, I guess I have to setup a custom event loop that stops polling from sockets fds when the disk flow limit is exceeded.
It would be really helpful if you could share some of your knowledge.
Thanks in advance

[Kenton Varda]

RpcSystem::setFlowLimit() should work for this. Yes, this means the RPC system will stop reading messages from the socket buffer if too many requests are in flight. That will naturally cause TCP backpressure to kick in. However, buffering still could happen on the client side if the client isn't careful to limit how much data is in flight.

Another, more advanced option is to declare your main payload methods as streaming with `-> stream`. This causes the RPC system *on the client side* to automatically keep track of how many calls are in flight and apply backpressure if it's too high. More info here: https://capnproto.org/news/2020-04-23-capnproto-0.8.html

You could also combine both approaches if you do not trust clients to apply flow control correctly. Note that `-> stream` flow control is only actually implemented in C++ so far, so clients in other languages might ignore it.

-Kenton

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