How to handle missed messages during WebSocket reconnection
message sequence numbers, last-seen sequence ID, catch-up on reconnect, server-side message buffer, cursor-based resume, event sourcing for WebSocket, offline queue
Connections Drop — Messages Get Lost
When a WebSocket reconnects, the client does not automatically receive messages sent during the disconnection. Fix this with sequence numbers:
// Server: attach a sequence number to every message let seq = 0; const buffer = []; // ring buffer, keep last N messages function broadcast(event, data) { const msg = { seq: ++seq, event, data }; buffer.push(msg); if (buffer.length > 500) buffer.shift(); // keep last 500 io.emit('msg', msg); } // Client sends its last known seq on reconnect socket.on('reconnect', () => { socket.emit('catch:up', { lastSeq: lastSeenSeq }); }); // Server replays missing messages socket.on('catch:up', ({ lastSeq }) => { const missed = buffer.filter(m => m.seq > lastSeq); missed.forEach(m => socket.emit('msg', m)); });
This works for transient disconnections under a few minutes. For longer outages, redirect to a REST endpoint to load missed state from the database rather than relying on an in-memory buffer. Always handle the case where lastSeq is behind the oldest buffered message — in that case, send a full state refresh instead of a partial replay.