Phoenix v1.2.0-rc.1 Phoenix.Presence behaviour

Provides Presence tracking to processes and channels.

This behaviour provides presence features such as fetching presences for a given topic, as well as handling diffs of join and leave events as they occur in real-time. Using this module defines a supervisor and allows the calling module to implement the Phoenix.Tracker behaviour which starts a tracker process to handle presence information.

Example Usage

Start by defining a presence module within your application which uses Phoenix.Presence and provide the :otp_app which holds your configuration, as well as the :pubsub_server.

defmodule MyApp.Presence do
  use Phoenix.Presence, otp_app: :my_app,
                        pubsub_server: MyApp.PubSub

The :pubsub_server must point to an existing pubsub server running in your application, which is included by default as MyApp.PubSub for new applications.

Next, add the new supervisor to your supervision tree in lib/my_app.ex:

children = [
  supervisor(MyApp.Presence, []),

Once added, presences can be tracked in your channel after joining:

defmodule MyApp.MyChannel do
  use MyApp.Web, :channel
  alias MyApp.Presence

  def join("some:topic", _params, socket) do
    send(self, :after_join)
    {:ok, assign(socket, :user_id, ...)}

  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(:seconds))
    {:noreply, socket}

In the example above, Presence.track is used to register this channel’s process as a presence for the socket’s user ID, with a map of metadata. Next, the current presence information for the socket’s topic is pushed to the client as a "presence_state" event.

Finally, a diff of presence join and leave events will be sent to the client as they happen in real-time with the “presence_diff” event. The diff structure will be a map of :joins and :leaves of the form:

%{joins: %{"123" => %{metas: [%{status: "away", phx_ref: ...}]},
  leaves: %{"456" => %{metas: [%{status: "online", phx_ref: ...}]},

See Phoenix.Presence.list/2 for more information on the presence datastructure.

Fetching Presence Information

Presence metadata should be minimized and used to store small, ephemeral state, such as a user’s “online” or “away” status. More detailed information, such as user details that need to be fetched from the database can be achieved overriding the fetch/2 function. The fetch/2 callback is triggered when using list/1 and serves as a mechanism to fetch presence information a single time, before broadcasting the information to all channel subscribers. This prevents N query problems and gives you a single place to group isolated data fetching to extend presence metadata. The function must return a map of data matching the outlined Presence datastructure, including the :metas key, but can extend the map of information to include any additional information. For example:

def fetch(_topic, entries) do
  query =
    from u in User,
      where: in ^Map.keys(entries),
      select: {, u}

  users = query |> Repo.all |> Enum.into(%{})

  for {key, %{metas: metas}} <- entries, into: %{} do
    {key, %{metas: metas, user: users[key]}}

The function above fetches all users from the database who have registered presences for the given topic. The fetched information is then extended with a :user key of the user’s information, while maintaining the required :metas field from the original presence data.



Returns presences for a topic


presence :: %{key: String.t, meta: map}
presences :: %{String.t => %{metas: [map]}}


list(module, topic)

Returns presences for a topic.

Presence datastructure

The presence information is returned as map with presences grouped by key and accumulated metadata, with the following form:

%{key => %{metas: [%{phx_ref: ..., ...}, ...]}}

For example, imagine a user with id 123 online from two different devices, as well as a user with id 456 online from just one device. The following presence information might be returned:

%{"123" => %{metas: [%{status: "away", phx_ref: ...},
                     %{status: "online", phx_ref: ...}]},
  "456" => %{metas: [%{status: "online", phx_ref: ...}]}}

The keys of the map will usually point to a resource ID. The value will contain a map with a :metas key containing a list of metadata for each resource. Additionally, every metadata entry will contain a :phx_ref key which can be used to uniquely identify metadata for a given key. In the event that the metadata was previously updated, a :phx_ref_prev key will be present containing the previous :phx_ref value.


fetch(topic, presences)


handle_diff(%{}, state)


handle_diff(%{topic => {joins :: presences, leaves :: presences}}, state :: term) :: {:ok, state :: term}


init(Keyword.t) ::
  {:ok, pid} |
  {:error, reason :: term}


list(topic) :: presences


start_link(Keyword.t) :: {:ok, pid} | {:error, reason :: term} :: :ignore
track(arg0, key, meta)


track(Phoenix.Socket.t, key :: String.t, meta :: map) :: :ok
track(pid, topic, key, meta)


track(pid, topic, key :: String.t, meta :: map) :: :ok
untrack(arg0, key)


untrack(Phoenix.Socket.t, key :: String.t) :: :ok
untrack(pid, topic, key)


untrack(pid, topic, key :: String.t) :: :ok
update(arg0, key, meta)


update(Phoenix.Socket.t, key :: String.t, meta :: map) :: :ok
update(pid, topic, key, meta)


update(pid, topic, key :: String.t, meta :: map) :: :ok