Phoenix.PubSub

Front-end to Phoenix pubsub layer.

Used internally by Channels for pubsub broadcast but also provides an API for direct usage.

Adapters

Phoenix pubsub was designed to be flexible and support multiple backends. We currently ship with two backends:

• Phoenix.PubSub.PG2 - uses Distributed Elixir, directly exchanging notifications between servers

• Phoenix.PubSub.Redis - uses Redis to exchange data between servers

Pubsub adapters are often configured in your endpoint:

config :my_app, MyApp.Endpoint,
  pubsub: [adapter: Phoenix.PubSub.PG2]

The configuration above takes care of starting the pubsub backend and exposing its functions via the endpoint module.

Direct usage

It is also possible to use Phoenix.PubSub directly or even run your own pubsub backends outside of an Endpoint.

The first step is to start the adapter of choice in your supervision tree:

supervisor(Phoenix.PubSub.Redis, [:my_redis_pubsub, host: "192.168.100.1"])

The configuration above will start a Redis pubsub and register it with name :my_redis_pubsub.

You can know use the functions in this module to subscribe and broadcast messages:

iex> PubSub.subscribe MyApp.PubSub, self, "user:123"
:ok
iex> Process.info(self)[:messages]
[]
iex> PubSub.broadcast MyApp.PubSub, "user:123", {:user_update, %{id: 123, name: "Shane"}}
:ok
iex> Process.info(self)[:messages]
{:user_update, %{id: 123, name: "Shane"}}

Implementing your own adapter

PubSub adapters run isnide their own supervision tree. If you are interested in providing your own adapter, let’s call it Phoenix.PubSub.MyQueue, the first step is to provide a supervisor module that receives the server name and a bunch of options on start_link/2:

defmodule Phoenix.PubSub.MyQueue do
  def start_link(name, options) do
    Supervisor.start_link(__MODULE__, {name, options},
                          name: Module.concat(name, Supervisor))
  end

  def init({name, options}) do
    ...
  end
end

On init/1, you will define the supervision tree and use the given name to register the main pubsub process locally. This process must be able to handle the following GenServer calls:

• subscribe - subscribes the given pid to the given topic sends: {:subscribe, pid, topic, opts} respond with: :ok | {:error, reason} | {:perform, {m, f, a}}

• unsubscribe - unsubscribes the given pid from the given topic sends: {:unsubscribe, pid, topic} respond with: :ok | {:error, reason} | {:perform, {m, f, a}}

• broadcast - broadcasts a message on the given topic sends: {:broadcast, :none | pid, topic, message} respond with: :ok | {:error, reason} | {:perform, {m, f, a}}

Offloading work to clients via MFA response

The Phoenix.PubSub API allows any of its functions to handle a response from the adapter matching {:perform, {m, f, a}}. The PubSub client will recursively invoke all MFA responses until a result is returned. This is useful for offloading work to clients without blocking your PubSub adapter. See Phoenix.PubSub.PG2 implementation for examples.