Syncer

Syncer communication protocol enables communication between Syncer Server and Syncer Client, based on Chatter communication. It is aimed primarily for events synchronization between two, or more redundant Event Servers. Thus, each Event Server implements both server and client side of this communication. Generally, this communication is made not to be exclusively used within an Event Server: Syncer Client can be used to establish synchronization of any remote instance (e.g. an arbitrary database) with an Event Server.

Messages used in Syncer communication, defined in HatSyncer SBS module (see Chatter messages), are the following:

Message	Conversation			Direction
	First	Last	Token
MsgInitReq	T	F	T	c > s
MsgInitRes	F	T	T	s > c
MsgSynced	T	T	T	s > c
MsgEvents	T	T	T	s > c
MsgFlushReq	T	F	T	s > c
MsgFlushRes	F	T	T	c > s

Communication between Syncer Client and server starts with initialization request initiated by the client with MsgInitReq message. When Syncer Server receives MsgInitReq, it must immediately respond with MsgInitRes message. The connection between server and client is considered established when client receives MsgInitRes success message. If MsgInitRes error is received, chatter connection should be closed. With MsgInitReq message, client requests for server events in order to get “synchronized”. More specifically, by MsgInitReq client represents itself by clientName and requests all the events that have EventId such that session is same or greater and instance is greater (ie. newer) than lastEventId and server corresponds to Syncer Server’s Event Server.

Upon receiving MsgInitReq and responding with MsgInitRes, server communicates back the requested events by MsgEvents message. Events are grouped in MsgEvents such that all events in the message are from the same session (session of EventId). Once all of these events are sent, server sends MsgSynced message in order to signalize the client that synchronization phase ended.

All events created (by the Event Server with running Syncer Server) after the MsgInitReq (and before MsgSynced) are not included in the synchronization phase. They are buffered on the server side and sent to client only after the MsgSynced message, using the same MsgEvents message. In the remaining life of the connection between Syncer Server and client, only MsgEvents messages are sent from server to a client in order to notify client with the new events. All events in one MsgEvents message always belong to the same session.

In addition to lastEventId, MsgInitReq contains subscriptions and optional clientToken. subscription is used as additional filter for reporting only events that satisfy specified event types. clientToken, if configured by server, is used for identifying clients which are allowed to communicate to server. This parameter is usually used for checking configuration synchronization between client and server.

Implementation

Documentation is available as part of generated API reference:

• Python hat.event.syncer module

Chatter messages

module HatSyncer

MsgInitReq = Record {
    lastEventId:    HatEventer.EventId
    clientName:     String
    clientToken:    Optional(String)
    subscriptions:  Array(HatEventer.EventType)
}

MsgInitRes = Choice {
    success:  None
    error:    String
}

MsgSynced = None

MsgEvents = Array(HatEventer.Event)

MsgFlushReq = None

MsgFlushRes = None