# Broadcast

Signed, public ephemeral messages.

## Send and receive


```ts
const room = db.channel("editor", { broadcast: { self: true } });

room
  .on("broadcast", { event: "cursor" }, ({ payload, pubkey, sessionId }) => {
    console.log(payload, pubkey, sessionId);
  })
  .subscribe();

const { data, error, meta } = await room.send({
  type: "broadcast",
  event: "cursor",
  payload: { x: 120, y: 80 },
});
```

- Use `{ event: "*" }` to receive all broadcast event names.
- Self delivery is off by default. Other sessions with the same signing key still receive messages.
- A channel can subscribe and send without receive handlers.
- A listener needs no signer. Sending requires an active signer.
- `data` contains a write receipt. `meta.relays` records relay acknowledgements. The client's `minWriteAcks` applies.
- Payloads must contain JSON values. Duplicate events are ignored during their lifetime.


## Delivery and privacy


- Broadcast payloads and Presence state are public. Signing proves the author; it does not limit access.
- Relays need not retain ephemeral events. The SDK does not put them in its event cache or offline write queue.
- Relay acknowledgement proves acceptance, not delivery to every subscriber.
- Presence is approximate. Disconnects, clock differences, dropped messages, or relay policies can delay joins and leaves.
- Relays must accept and deliver kind `20078` and the channel tags. This is a nostrbase wire format, not a general Presence standard.
- Negentropy recovers stored record events. It cannot recover ephemeral Broadcast or Presence history.