Channels

The Channels engine carries messages to whoever listens at that moment: a new reply to every open tab, a "typing" indicator, a signal to reload. Messages live in memory and are delivered at most once. A channel has branches, such as one per chat, and a message published inside a transaction is delivered after its commit.

const events = store.channel<ChatEvent>('chat-events')

const sub = await events.under(chatId).subscribe(m => send(m.value), {
	onMissed: () => send({ kind: 'reload' }),
})
await events.under(chatId).publish({ kind: 'message', id: 41 })
await sub.stop()

events = store.channel("chat-events", ChatEvent)

sub = await events.under(chat_id).subscribe(lambda m: send(m.value), on_missed=lambda missed: send({"kind": "reload"}))
await events.under(chat_id).publish(ChatEvent(kind="message", id=41))
await sub.stop()

events, err := channels.OpenChannel[ChatEvent](store, "chat-events")

sub, err := events.Under(chatID).Subscribe(ctx, func(ctx context.Context, m channels.Message[ChatEvent]) error {
	send(m.Value)
	return nil
}, channels.OnMissed(func(channels.Missed) { send(reload) }))
err = events.Under(chatID).Publish(ctx, ChatEvent{Kind: "message", ID: 41})
err = sub.Stop(ctx)

let events = store.channel::<ChatEvent>("chat-events").open()?;

let sub = events.under(chat_id).on_missed(|_| reload()).subscribe(|m| send(m.value))?;
events.under(chat_id).publish(&ChatEvent { kind: "message".into(), id: 41 })?;
sub.stop();

#What it guarantees

• A subscriber receives what is published after it subscribed, in order, and nothing after it stops (contract CHAN-01).
• A publisher never waits for a subscriber; one that falls behind is told exactly how many messages it missed (contract CHAN-02).
• A message published in a transaction is delivered after its commit, and never after a rollback (contract CHAN-03).
• A handler that fails is reported, and the next message follows (contract CHAN-04).
• Not promised: a message nobody listens to, or one before a restart, is gone (NOT-02). What must not be lost belongs in a queue.

#In these guides

New to Channels? Live updates in the Handbook tells every open tab about a new reply.

#Reference

Bun and Node · Python · Go · Rust