Presence

Subscribing to a presence- channel sends channel_data containing a user_id and optional user_info. The subscriber receives the current membership:

{
  "event": "pusher_internal:subscription_succeeded",
  "channel": "presence-room-1",
  "data": "{\"presence\":{\"ids\":[\"42\"],\"hash\":{\"42\":{\"name\":\"Ada\"}},\"count\":1}}"
}

Everyone else in the channel gets pusher_internal:member_added, and pusher_internal:member_removed when someone leaves.

Members are users, not connections

One person with two tabs is one member. A second connection for a user id that is already present produces no member_added, and member_removed fires only when that user's last connection goes away. Getting this wrong is how member lists drift, so Arc counts references per user id and the webhooks follow the same rule.

Across nodes

Membership is replicated between nodes as a CRDT, so it converges after a network partition without any node coordinating the others. A node that dies has its members removed from everyone else's view within about five seconds.

Limits

An app caps presence channels at 100 members by default. Joining a full channel returns a subscription error with status 403 rather than a truncated member list — the client learns it is not in the room instead of believing it is. You can raise the ceiling or switch it off per app in the dashboard.

channel_data over 10 KB is rejected.

Reading membership from your backend

GET /apps/:app_id/channels/presence-room-1/users
GET /apps/:app_id/channels/presence-room-1?info=user_count

Both are part of the HTTP API.