Sync Server & Client UI States
Server to client communications contain the synchronization tokens — syncId (server-to-client) and clientId (client-to-server) — for communicating and UI state tracking. This tracking is meant for ordering messages. It’s also used to verify that both sides are in the same known state.
The syncId token marks the state of the server and is returned as received by the client. Whereas, the clientId token is incremented for each message sent by the client, with the server incrementing to match the next id in the response. Both syncId and clientId are integers.
syncId
The syncId holds the latest communication state identifier given by the server. It’s generated as a strictly increasing id for each response to every request from the client.
This identifier is replayed to the server on each request from the client. It helps the server to know the state of the client and to compare it against its own state.
The initial value when no response has been received from the server is UNDEFINED_SYNC_ID(-1). This is the value after the bootstrap HTML loading and before the first UI is rendered.
The syncId is always incremented by 1 after a new UIDL response is generated. The client then handles the received messages according to the syncId and the latest handled syncId, so that changes are handled in the correct order.
clientId
The clientId holds the latest communication state identifier given by the client. The client token is incremented on the client after sending the message. The server increments the value in the response to match the next expected clientId (i.e., client updated after message sent).
On the client, pending messages are removed from the queue as handled when clientId from server matches the next expected value. If the id is less than expected, the client waits since the server has not yet seen all messages. The response should arrive later. Otherwise, the client trusts the server and updates the id to the server’s expectations.
On the server, in cases where the id is the previous one with the same request payload, the server re-sends the latest response since the client probably didn’t receive the message. If the client-sent id doesn’t match the server-expected id, a re-synchronization is initiated.