Functional Web Development with Elixir, OTP, and Phoenix

Phoenix Presence

From the earliest days of Phoenix channels, developers have asked how to tell who is currently subscribed to a channel. Until recently, the answer was to create a custom solution that best fits an individual application’s circumstances. But now, we have Phoenix Presence to solve that problem in a general way for all of us.

Presence has one job to do: to keep track of the clients subscribed to a topic on a channel. For us, that means keeping track of the players in each game. Presence does this amazingly well. This might sound like a trivial task, but it’s deceptively difficult.

If you were to roll your own version of Presence, your first thought might be to maintain a list of the subscribers, adding clients to the list when ...