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Realtime

Supabase provides a Realtime API using Realtime.

You can use this to:

  • broadcast messages between users
  • manage shared state between users
  • listen to database changes over websockets

Realtime Channels

In Realtime, everything is a channel. Channels are like chatrooms where participants can join and leave—similar to Slack or Discord channels.

Broadcast

Broadcast follows the classic Publisher/Subscriber pattern. A client (publisher) “broadcasts” messages using a unique identifier. For example, a user could send a message to a #random channel.

Other clients (subscribers) can “listen” to these messages in real-time, using the unique identifier. If they are listening to the #random channel, then they will receive the message.

A common use-case for Broadcast is sharing a user's cursor position with other clients in an online game.

Presence

Presence synchronizes shared state between users. Presence is very similar to Broadcast, except that the state is “persisted” in the Channel so that new joiners immediately receive the state without waiting for another user to broadcast it. In our Slack example, when you turn on Slack you can see who's online without each individual user sending an “I'm online” message to the new-joiner. Presence utilizes Broadcast as the transport layer.

The neat thing about Presence is that if a user is suddenly disconnected (for example, they go offline), their state will be automatically removed from the shared state. If you've ever tried to build an “I'm online” feature which handles unexpected disconnects, you'll appreciate how useful this is.

Database changes

You can subscribe to database changes using Realtime. The Realtime API works through PostgreSQL's replication functionality. Postgres sends database changes to a publication called supabase_realtime, and by managing this publication you can control which data is broadcast.