Presence

Phoenix Presence is a feature which allows you to register process information on a topic and replicate it transparently across a cluster. It's a combination of both a server-side and client-side library which makes it simple to implement. A simple use-case would be showing which users are currently online in an application.

Phoenix Presence is special for a number of reasons. It has no single point of failure, no single source of truth, relies entirely on the standard library with no operational dependencies and self heals. This is all handled with a conflict-free replicated data type (CRDT) protocol.

To get started with Presence we'll first need to generate a presence module. We can do this with the mix phx.gen.presence task:

$ mix phx.gen.presence
* creating lib/hello_web/channels/presence.ex

Add your new module to your supervision tree,
in lib/hello/application.ex:

    children = [
      ...
      HelloWeb.Presence,
    ]

You're all set! See the Phoenix.Presence docs for more details:
http://hexdocs.pm/phoenix/Phoenix.Presence.html

If we open up the lib/hello_web/channels/presence.ex file, we will see the following line:

use Phoenix.Presence, otp_app: :hello,
                      pubsub_server: Hello.PubSub

This sets up the module for presence, defining the functions we require for tracking presences. As mentioned in the generator task, we should add this module to our supervision tree in application.ex:

children = [
  ...
  HelloWeb.Presence,
]

Next we will create a channel that Presence can communicate on. For this example we will create a RoomChannel (see the channels guide for more details on this):

$ mix phx.gen.channel Room
* creating lib/hello_web/channels/room_channel.ex
* creating test/hello_web/channels/room_channel_test.exs

Add the channel to your `lib/hello_web/channels/user_socket.ex` handler, for example:

    channel "room:lobby", HelloWeb.RoomChannel

and register it in lib/hello_web/channels/user_socket.ex:

defmodule HelloWeb.UserSocket do
  use Phoenix.Socket

  channel "room:lobby", HelloWeb.RoomChannel
end

We also need to change our connect function to take a user_id from the params and assign it on the socket. In production you may want to use Phoenix.Token if you have real users that are authenticated.

def connect(params, socket) do
  {:ok, assign(socket, :user_id, params["user_id"])}
end

Next, we will create the channel that we'll communicate presence over. After a user joins we can push the list of presences down the channel and then track the connection. We can also provide a map of additional information to track.

Note that we provide the user_id from the connection in order to uniquely identify the client. You can use whatever identifier you like, but you'll see how this is provided to the socket in the client-side example below.

To learn more about channels, read the channel documentation in the guide.

defmodule HelloWeb.RoomChannel do
  use Phoenix.Channel
  alias HelloWeb.Presence

  def join("room:lobby", _params, socket) do
    send(self(), :after_join)
    {:ok, socket}
  end

  def handle_info(:after_join, socket) do
    push(socket, "presence_state", Presence.list(socket))
    {:ok, _} = Presence.track(socket, socket.assigns.user_id, %{
      online_at: inspect(System.system_time(:second))
    })
    {:noreply, socket}
  end
end

Finally we can use the client-side Presence library included in phoenix.js to manage the state and presence diffs that come down the socket. It listens for the "presence_state" and "presence_diff" events and provides a simple callback for you to handle the events as they happen, with the onSync callback.

The onSync callback allows you to easily react to presence state changes, which most often results in re-rendering an updated list of active users. You can use the list method is to format and return each individual presence based on the needs of your application.

To iterate users, we use the presences.list() function which accepts a callback. The callback will be called for each presence item with 2 arguments, the presence id and a list of metas (one for each presence for that presence id). We use this to display the users and the number of devices they are online with.

We can see presence working by adding the following to assets/js/app.js:

import {Socket, Presence} from "phoenix"

let socket = new Socket("/socket", {
  params: {user_id: window.location.search.split("=")[1]}
})

let channel = socket.channel("room:lobby", {})
let presence = new Presence(channel)

function renderOnlineUsers(presence) {
  let response = ""

  presence.list((id, {metas: [first, ...rest]}) => {
    let count = rest.length + 1
    response += `<br>${id} (count: ${count})</br>`
  })

  document.querySelector("main[role=main]").innerHTML = response
}

socket.connect()

presence.onSync(() => renderOnlineUsers(presence))

channel.join()

We can ensure this is working by opening 3 browser tabs. If we navigate to http://localhost:4000/?name=Alice on two browser tabs and http://localhost:4000/?name=Bob then we should see:

Alice (count: 2)
Bob (count: 1)

If we close one of the Alice tabs, then the count should decrease to 1. If we close another tab, the user should disappear from the list entirely.