Message

A sealed class for all the messages defined in the protocol.

Basics

The protocol is separated in two roles : a sender of Incoming messages, and a receiver of Outgoing messages. At a high-level, each role does the following :

• The receiver originates the sync, and tells which events it wants to receive.

• The sender answers the requests with the right events, and advertises when new events are available that the receiver might not know about.

In a real-world Exchange, both sites play both roles simultaneously. Because messages and roles are clearly separated between the server and the receiver, it's possible for sites to perform as a sender and a receiver simultaneously with a single communication channel.

Backpressure and termination

This iteration of the protocol introduces a notion of backpressure by the receiver. Instead of receiving all the events of the sender as they are produced, it becomes the responsibility of the receiver to clearly indicate how many events they want delivered from the sender.

This has a few benefits :

• The receiver may not request any new events, and it's therefore possible to have a "sender-only" implementation with this flexible behavior.

• Sites that need to process messages before requesting additional events can actually do so.

Additionally, the sender and the receiver protocol messages offer some Done messages. This allows the following :

• The sender may tell that it will not generate any additional Incoming.Advertisement and Incoming.Event.

• the receiver may tell that it not generate any additional Outgoing.Request.

Termination is cooperative :

• If you're the sender and you receive a Outgoing.Done event, you should not send any additional advertisement, nor should you send any additional event (event if they were requested). In-flight messages will still be processed by the receiver but they might be ignored. You should send a Done message right away, before emptying the in-flight queue.

• If you're the receiver and you receive an Incoming.Done event, you should not send any additional request. You are still allowed to process in-flight messages for advertisements and events, but may not issue any additional request. You should send a Done message right away, before emptying the in-flight queue.