Delivery, Ordering, and Groups

Group fan-out strategies

• Small groups (< 256): push to every member inbox on send.
• Large groups: store single copy; members pull on chat open.
• Typing indicators: gossip or pub/sub channel, lossy is acceptable.

// Per-chat ordering with client ack
type ChatMessage = { id: string; chatId: string; seq: number; ciphertext: Buffer };

async function onClientAck(chatId: string, upToSeq: number) {
  await db.messages.deleteQueued({ chatId, seq: { lte: upToSeq } });
}
// Server assigns monotonic seq per chat; client dedupes by message id

Step-by-step walkthrough

Send path — persist message

• ① WebSocket SEND — Sender posts ciphertext over an open connection; payload stays small (no JSON bloat).
• ② Append seq=N — Chat server assigns a monotonic sequence number and appends to the replicated message store.
• ③ Update presence — Sender marked online in Redis with TTL; typing indicators use the same channel.

Delivery path — online or offline

• ④a Online: WebSocket PUSH — If receiver is connected, server pushes the message on their socket immediately.
• ④b Offline: notify — If receiver is disconnected, server triggers APNs/FCM instead of holding sockets open.
• ⑤ Push alert — Mobile OS shows notification; message ciphertext waits in the durable store.
• ⑥ ACK seq — Receiver acknowledges; server trims delivered messages from the hot queue.