- At t = T6 , Alice re-sends M1
What Bob sees is consistent with what the server sees at T6 but there’s a divergence (inconsistency) between Alice’s chat history and Bob’s chat history. This is because when Alice comes back online at T3 , Alice’s client does not download a fresh copy of the chat history from the server.
We avoid the need to solve the conflict resolution problem by keeping the client version after the network connection is established again and not forcing it to be consistent with the server version. As there’s no polling, the only server-driven update to the client replica is from WebSocket events.
The OkCupid chat app lets you go offline for an arbitrary amount of time and continue sending new messages. However, when you are online again, it doesn’t automatically download all the messages sent to you when you were offline and re-apply your offline edits on top of the latest state.
Choosing an appropriate final state when concurrent updates have occurred is called reconciliation and can be quite tricky to implement.
For instance, there’s a downside to simply syncing the replicas with the server state when the system reaches steady-state: It can violate the invariant for our collection wherein messages are always ordered by the time they were created. This has some usability implications as it can create a jarring user experience to see the messages in the chat history suddenly change order.
optimistic replication allows replicas to diverge. Replicas will reach eventual consistency the next time Alice and Bob sync their replicas with the server state, which only happens when they refresh their chat apps (reload the page).
This seems like kind of a cheat but convergence upon system quiescence is a common strategy to achieve eventual consistency. This relieves us from having to implement an explicit reconciliation policy for the replicas which could be unnecessarily complex for our problem space. Continue reading „It only syncs the messages sent after a new WebSocket connection is established“