Request response message ordering

[tqwewe]

Hi, does request response have guaranteed message ordering when sending two requests from the same peer?

Ideally, if I run this code:

swarm.behaviour_mut().req_resp.send_request(peer_id, Req::One);
swarm.behaviour_mut().req_resp.send_request(peer_id, Req::Two);

The peer receiving the messages would always handle them in the same order, where One is received first, and then Two is received next.

[elenaf9]

Generally, the request-response behavior handles requests in FIFO order. But the actual order in which the requests are send and received also depends on the underlying connection, mainly:

1. Number of connections: If there are multiple connections to the same peer, the requests are distributed among all connections. If one is faster than the other than that will impact the order in which requests are received.
2. The used transport and its multiplexing: Request are sent in separate streams on the transport. With transports like QUIC that don't have Head-of-Line blocking, two requests on the same transport can arrive if a different order than they were sent if e.g. some packets on the first stream are dropped and have to be retransmitted.

Does that help?

[tqwewe]

Okay that helps. So essentially if I want guaranteed ordering, I'd need to implement it myself I guess.

In my particular use case, I'll have three kind of requests:

• Req::AppendEvents { request_id: Uuid, ... }
• Req::ConfirmEventsAppended { request_id: Uuid }
• Req::ErrorAppendingEvents { request_id: Uuid, err: Error }

Its quite important in my case that AppendEvents is received before either of the other two, so I guess I'll need to implement some kind of special handling when either of the other requests are received before AppendEvents is ever received.

[elenaf9]

Another, quite simple solution would be to just wait on the sender side after sending AppendEvents until you received the response, before sending the other requests. That guarantees that the remote has received and processed the event.

[tqwewe]

Thank you! I'll go with your latest suggestion :)