Ably’s presence feature allows clients or devices to announce their presence on a channel. Other devices or services may then subscribe to these presence events (such as entering, updating their state, or leaving the channel) in real time using our realtime SDKs, or via the Reactor service. You can also request a list of clients or devices that are online/offline on a channel at a particular point in time via the REST API. Presence representation

Furthermore, if persistence is enabled on the presence channel, you can also retrieve presence history for the channel, i.e, static data about historical presence states of your clients/devices. This operation also can be done using both Ably’s Realtime and REST libraries.
API reference

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