Pick the fix
Messages appear out of order when two people's phones have different clocks. Fix?
Who should decide the order?
Choose an answer, then open Show answer below to check it.
Answer: A Order by a server-assigned sequence number
Correct. One authority decides the order.
Keep a persistent connection, such as a WebSocket, while the app is in the foreground, and rely on FCM high-priority messages when it isn't. Store conversations and messages in Room and render from there. Send messages with a client ID and 'sending' state, retry through WorkManager, and mark them sent on server ack. Order by server sequence numbers, and sync missed messages on reconnect.
Design a chat client.
Say your answer out loud, then check which of these points you covered:
Don't keep a socket open in the background; Android will restrict it and it drains battery.
Pagination loads older messages from Room and the server.
Read receipts and typing indicators are lightweight events, not stored messages.
End-to-end encryption changes where search and backup can happen.
Split foreground (socket) from background (FCM), then the send pipeline and ordering.
Think of your answer first, then open the hint.
How do you order messages?
By server-assigned sequence numbers, not device time.
What happens when the app is killed?
FCM wakes it; it syncs from the last sequence number.
Compare on iOS: A chat timeline that survives retries The same design; background delivery uses APNs instead of FCM.
Suggest a fix if something here is wrong or out of date.