Messages queued server-side during disconnect and delivered in order on reconnection

A user who loses connectivity for 30 seconds and reconnects finds the conversation with a gap — every message sent during their outage is gone with no indication it was missed. This is especially damaging in moderation workflows, incident channels, or any context where missing a message has operational consequences. ISO 25010 reliability specifically covers data persistence across failure events; dropping messages on disconnect is a reliability failure regardless of how brief the outage.

Persist messages for offline users and flush the queue in delivery order on reconnect. For production use, back the queue with Redis lists rather than in-process Maps.

// On connect, deliver queued messages in order
socket.on('connect_post_auth', async () => {
  const queued = await redis.lrange(`offline:${socket.userId}`, 0, -1);
  for (const raw of queued) {
    socket.emit('message', JSON.parse(raw));
  }
  await redis.del(`offline:${socket.userId}`);
});

// When a message is sent, queue for offline subscribers
const offlineUsers = await getOfflineChannelMembers(channel);
for (const uid of offlineUsers) {
  await redis.rpush(`offline:${uid}`, JSON.stringify(message));
  await redis.expire(`offline:${uid}`, 86400); // 24 h TTL
}

Set a TTL on the queue to prevent unbounded growth for users who never reconnect.