View Source Phoenix.Tracker behaviour (Phoenix.PubSub v2.1.1)

Provides distributed presence tracking to processes.

Tracker shards use a heartbeat protocol and CRDT to replicate presence information across a cluster in an eventually consistent, conflict-free manner. Under this design, there is no single source of truth or global process. Each node runs a pool of trackers and node-local changes are replicated across the cluster and handled locally as a diff of changes.

implementing-a-tracker
Implementing a Tracker

To start a tracker, first add the tracker to your supervision tree:

children = [
  # ...
  {MyTracker, [name: MyTracker, pubsub_server: MyApp.PubSub]}
]

Next, implement MyTracker with support for the Phoenix.Tracker behaviour callbacks. An example of a minimal tracker could include:

defmodule MyTracker do
  use Phoenix.Tracker

  def start_link(opts) do
    opts = Keyword.merge([name: __MODULE__], opts)
    Phoenix.Tracker.start_link(__MODULE__, opts, opts)
  end

  def init(opts) do
    server = Keyword.fetch!(opts, :pubsub_server)
    {:ok, %{pubsub_server: server, node_name: Phoenix.PubSub.node_name(server)}}
  end

  def handle_diff(diff, state) do
    for {topic, {joins, leaves}} <- diff do
      for {key, meta} <- joins do
        IO.puts "presence join: key \"#{key}\" with meta #{inspect meta}"
        msg = {:join, key, meta}
        Phoenix.PubSub.direct_broadcast!(state.node_name, state.pubsub_server, topic, msg)
      end
      for {key, meta} <- leaves do
        IO.puts "presence leave: key \"#{key}\" with meta #{inspect meta}"
        msg = {:leave, key, meta}
        Phoenix.PubSub.direct_broadcast!(state.node_name, state.pubsub_server, topic, msg)
      end
    end
    {:ok, state}
  end
end

Trackers must implement start_link/1, init/1, and handle_diff/2. The init/1 callback allows the tracker to manage its own state when running within the Phoenix.Tracker server. The handle_diff callback is invoked with a diff of presence join and leave events, grouped by topic. As replicas heartbeat and replicate data, the local tracker state is merged with the remote data, and the diff is sent to the callback. The handler can use this information to notify subscribers of events, as done above.

An optional handle_info/2 callback may also be invoked to handle application specific messages within your tracker.

special-considerations
Special Considerations

Operations within handle_diff/2 happen in the tracker server's context. Therefore, blocking operations should be avoided when possible, and offloaded to a supervised task when required. Also, a crash in the handle_diff/2 will crash the tracker server, so operations that may crash the server should be offloaded with a Task.Supervisor spawned process.

Link to this section Summary

Types

presence()

topic()

Callbacks

handle_diff(map, state)

handle_info(message, state)

init(t)

Functions

child_spec(init_arg)

Returns a specification to start this module under a supervisor.

get_by_key(tracker_name, topic, key)

Gets presences tracked under a given topic and key pair.

graceful_permdown(tracker_name)

Gracefully shuts down by broadcasting permdown to all replicas.

list(tracker_name, topic)

Lists all presences tracked under a given topic.

start_link(tracker, tracker_arg, pool_opts)

Starts a tracker pool.

track(tracker_name, pid, topic, key, meta)

Tracks a presence.

untrack(tracker_name, pid)

untrack(tracker_name, pid, topic, key)

Untracks a presence.

update(tracker_name, pid, topic, key, meta)

Updates a presence's metadata.

Link to this section Types

presence()

@type presence() :: {key :: String.t(), meta :: map()}

topic()

@type topic() :: String.t()

Link to this section Callbacks

handle_diff(map, state)

@callback handle_diff(
  %{required(topic()) => {joins :: [presence()], leaves :: [presence()]}},
  state :: term()
) :: {:ok, state :: term()}

handle_info(message, state)

(optional)

@callback handle_info(message :: term(), state :: term()) :: {:noreply, state :: term()}

init(t)

@callback init(Keyword.t()) :: {:ok, state :: term()} | {:error, reason :: term()}

Link to this section Functions

child_spec(init_arg)

Returns a specification to start this module under a supervisor.

See Supervisor.

get_by_key(tracker_name, topic, key)

@spec get_by_key(atom(), topic(), term()) :: [presence()]

Gets presences tracked under a given topic and key pair.

server_name - The registered name of the tracker server
topic - The Phoenix.PubSub topic
key - The key of the presence

Returns a lists of presence metadata.

examples
Examples

iex> Phoenix.Tracker.get_by_key(MyTracker, "lobby", "user1")
[{#PID<0.88.0>, %{name: "User 1"}, {#PID<0.89.0>, %{name: "User 1"}]

graceful_permdown(tracker_name)

@spec graceful_permdown(atom()) :: :ok

Gracefully shuts down by broadcasting permdown to all replicas.

examples
Examples

iex> Phoenix.Tracker.graceful_permdown(MyTracker)
:ok

list(tracker_name, topic)

@spec list(atom(), topic()) :: [presence()]

Lists all presences tracked under a given topic.

server_name - The registered name of the tracker server
topic - The Phoenix.PubSub topic

Returns a lists of presences in key/metadata tuple pairs.

examples
Examples

iex> Phoenix.Tracker.list(MyTracker, "lobby")
[{123, %{name: "user 123"}}, {456, %{name: "user 456"}}]

start_link(tracker, tracker_arg, pool_opts)

Starts a tracker pool.

tracker - The tracker module implementing the Phoenix.Tracker behaviour
tracker_arg - The argument to pass to the tracker handler init/1
pool_opts - The list of options used to construct the shard pool

required-pool_opts
Required `pool_opts`:

:name - The name of the server, such as: MyApp.Tracker This will also form the common prefix for all shard names
:pubsub_server - The name of the PubSub server, such as: MyApp.PubSub

optional-pool_opts
Optional `pool_opts`:

:broadcast_period - The interval in milliseconds to send delta broadcasts across the cluster. Default 1500
:max_silent_periods - The max integer of broadcast periods for which no delta broadcasts have been sent. Default 10 (15s heartbeat)
:down_period - The interval in milliseconds to flag a replica as temporarily down. Default broadcast_period * max_silent_periods * 2 (30s down detection). Note: This must be at least 2x the broadcast_period.
:permdown_period - The interval in milliseconds to flag a replica as permanently down, and discard its state. Note: This must be at least greater than the down_period. Default 1_200_000 (20 minutes)
:clock_sample_periods - The numbers of heartbeat windows to sample remote clocks before collapsing and requesting transfer. Default 2
:max_delta_sizes - The list of delta generation sizes to keep before falling back to sending entire state. Defaults [100, 1000, 10_000].
:log_level - The log level to log events, defaults :debug and can be disabled with false
:pool_size - The number of tracker shards to launch. Default 1

track(tracker_name, pid, topic, key, meta)

@spec track(atom(), pid(), topic(), term(), map()) ::
  {:ok, ref :: binary()} | {:error, reason :: term()}

Tracks a presence.

server_name - The registered name of the tracker server
pid - The Pid to track
topic - The Phoenix.PubSub topic for this presence
key - The key identifying this presence
meta - The map of metadata to attach to this presence

A process may be tracked multiple times, provided the topic and key pair are unique for any prior calls for the given process.

examples
Examples

iex> Phoenix.Tracker.track(MyTracker, self(), "lobby", u.id, %{stat: "away"})
{:ok, "1WpAofWYIAA="}

iex> Phoenix.Tracker.track(MyTracker, self(), "lobby", u.id, %{stat: "away"})
{:error, {:already_tracked, #PID<0.56.0>, "lobby", "123"}}

untrack(tracker_name, pid)

untrack(tracker_name, pid, topic, key)

@spec untrack(atom(), pid(), topic(), term()) :: :ok

Untracks a presence.

server_name - The registered name of the tracker server
pid - The Pid to untrack
topic - The Phoenix.PubSub topic to untrack for this presence
key - The key identifying this presence

All presences for a given Pid can be untracked by calling the Phoenix.Tracker.untrack/2 signature of this function.

examples
Examples

iex> Phoenix.Tracker.untrack(MyTracker, self(), "lobby", u.id)
:ok
iex> Phoenix.Tracker.untrack(MyTracker, self())
:ok

update(tracker_name, pid, topic, key, meta)

@spec update(atom(), pid(), topic(), term(), map() | (map() -> map())) ::
  {:ok, ref :: binary()} | {:error, reason :: term()}

Updates a presence's metadata.

server_name - The registered name of the tracker server
pid - The Pid being tracked
topic - The Phoenix.PubSub topic to update for this presence
key - The key identifying this presence
meta - Either a new map of metadata to attach to this presence, or a function. The function will receive the current metadata as input and the return value will be used as the new metadata

examples
Examples

iex> Phoenix.Tracker.update(MyTracker, self(), "lobby", u.id, %{stat: "zzz"})
{:ok, "1WpAofWYIAA="}

iex> Phoenix.Tracker.update(MyTracker, self(), "lobby", u.id, fn meta -> Map.put(meta, :away, true) end)
{:ok, "1WpAofWYIAA="}

Settings View Source Phoenix.Tracker behaviour (Phoenix.PubSub v2.1.1)

implementing-a-tracker Implementing a Tracker

special-considerations Special Considerations

Link to this section Summary

Types

Callbacks

Functions

Link to this section Types

presence()

topic()

Link to this section Callbacks

handle_diff(map, state)

handle_info(message, state)

init(t)

Link to this section Functions

child_spec(init_arg)

get_by_key(tracker_name, topic, key)

examples Examples

graceful_permdown(tracker_name)

examples Examples

list(tracker_name, topic)

examples Examples

start_link(tracker, tracker_arg, pool_opts)

required-pool_opts Required pool_opts:

optional-pool_opts Optional pool_opts:

track(tracker_name, pid, topic, key, meta)

examples Examples

untrack(tracker_name, pid)

untrack(tracker_name, pid, topic, key)

examples Examples

update(tracker_name, pid, topic, key, meta)

examples Examples

View Source Phoenix.Tracker behaviour (Phoenix.PubSub v2.1.1)

implementing-a-tracker
Implementing a Tracker

special-considerations
Special Considerations

examples
Examples

examples
Examples

examples
Examples

required-pool_opts
Required `pool_opts`:

optional-pool_opts
Optional `pool_opts`:

examples
Examples

examples
Examples

examples
Examples