# Realtime Channels — Supabase

Source: https://www.skillbyai.com/en/supabase/s-realtime

> Postgres changes, broadcast and presence.

## Three realtime features

Clients join a **channel** and listen for three kinds of events. **Postgres Changes** streams inserts, updates and deletes from tables added to the `supabase_realtime` publication, and respects RLS for what each user receives. **Broadcast** sends low-latency messages between clients (cursors, typing indicators) without touching tables. **Presence** keeps a shared state of who is connected. Call `subscribe()` to start and `removeChannel()` to clean up.

## Listening and broadcasting

supabase-js v2 channels.

```typescript
const channel = supabase
  .channel('project-1')
  .on(
    'postgres_changes',
    { event: 'INSERT', schema: 'public', table: 'tasks', filter: 'project_id=eq.1' },
    (payload) => console.log('new task', payload.new)
  )
  .on('broadcast', { event: 'cursor' }, ({ payload }) => console.log(payload))
  .on('presence', { event: 'sync' }, () => console.log(channel.presenceState()))
  .subscribe(async (status) => {
    if (status === 'SUBSCRIBED') {
      await channel.track({ user: 'asha', online_at: new Date().toISOString() })
      await channel.send({ type: 'broadcast', event: 'cursor', payload: { x: 10, y: 20 } })
    }
  })

// when the component unmounts
// supabase.removeChannel(channel)
```

## A radio station with three programmes

Postgres Changes is the news bulletin about the database, Broadcast is listeners calling each other, and Presence is the attendance list of who is tuned in.

**Quiz:** Which feature shares cursor positions between clients without writing to a table?

- [ ] pg_cron
- [ ] Postgres Changes
- [ ] Storage
- [x] Broadcast

*Answer:* Broadcast. Broadcast is ephemeral messaging.