How Do I Sync Chat State After a User Was Offline?
Blog post from Stream
Chat application synchronization involves managing different types of state—durable content, mutable derived state, and ephemeral state—each requiring distinct synchronization strategies. Durable content such as messages and edits must be reliably replayed and survive app reinstallation, while mutable derived states like read receipts necessitate order-sensitive merging to ensure accuracy. Ephemeral states, including presence and typing indicators, are relevant only in real-time and should expire rather than be replayed. Proper synchronization involves first pulling updates from the server to avoid conflicts, followed by replaying queued local writes with idempotent transaction IDs to ensure seamless reconnection. Push notifications should not be used as a transport mechanism for data synchronization but merely as a wake-up signal to initiate a delta sync. The architecture for chat applications should rely on a server-authoritative log for durable content, ensuring that clients can reconcile changes accurately upon reconnection, with mechanisms in place to handle offline storage and ensure retries are safe, particularly in unreliable network conditions.
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