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Phoenix Pubsub Comms Across Nodes

Jun 4, 2021

Phoenix PubSub makes cross-nodes publish-subscribe extremely simple: setting up one for single node is the same as setting up multiple. This post covers the steps in doing so.

First, run mix new pub_sub --sup to generate a new project. Then add {:phoenix_pubsub, "~> 2.0"} as a dependency in mix.exs. Open lib/pub_sub/application.ex to add Phoenix PubSub to the supervision tree:

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defmodule PubSub.Application do
  @moduledoc false
  use Application

  @impl true
  def start(_type, _args) do
    children = [
      {Phoenix.PubSub, name: PS}      # add this line
    ]

    opts = [
      strategy: :one_for_one,
      name: PubSub.Supervisor
    ]

    Supervisor.start_link(children, opts)
  end
end

That’s all to it!

Start the application in one terminal, using node@localhost as the node’s short name and start interacting with it:

$ iex --sname node@localhost -S mix

iex(node@localhost)1> alias Phoenix.PubSub
iex(node@localhost)2> PubSub.subscribe(PS, "user:123")
iex(node@localhost)3> Process.info(self(), :messages)
{:messages, []}
iex(node@localhost)4> PubSub.broadcast(PS, "user:123", {node(), "hello"})
:ok
iex(node@localhost)5> Process.info(self(), :messages)
{:messages, [node@localhost: "hello"]}
iex(node@localhost)6> flush()
iex(node@localhost)7> Process.info(self(), :messages)
{:messages, []}

Before proceeding further, flush the process’ message box as shown above. Then in a separate terminal, start a node deno@localhost and connect it to the previous one:

$ iex --sname deno@localhost -S mix

iex(deno@localhost)1> Node.connect(:node@localhost)
iex(deno@localhost)2> alias Phoenix.PubSub
iex(deno@localhost)4> PubSub.broadcast(PS, "user:123", {node(), "goodbye"})
:ok
iex(deno@localhost)5> Process.info(self(), :messages)
{:messages, []}

In this second terminal, notice that its process’ message box is empty because this node doesn’t subscribe to the topic user:123. Switching back to the first terminal, query the process’ message box to confirm the receipt of the message broadcast from the second (deno) node:

iex(node@localhost)8> Process.info(self(), :messages)
{:messages, [deno@localhost: "goodbye"]}

Amazing, right?

A few things to note: