Basic queue.

lib/pleroma/application.ex

@@ -18,7 +18,8 @@ def start(_type, _args) do
                          default_ttl: 25000,
                          ttl_interval: 1000,
                          limit: 2500
-                       ]])
+                       ]]),
+      worker(Pleroma.Web.Federator, [])
     ]
 
     # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html

lib/pleroma/web/federator/federator.ex

 defmodule Pleroma.Web.Federator do
+  use GenServer
   alias Pleroma.User
   alias Pleroma.Web.WebFinger
   require Logger
 
   @websub Application.get_env(:pleroma, :websub)
+  @max_jobs 10
+
+  def start_link do
+    GenServer.start_link(__MODULE__, {:sets.new(), :queue.new()}, name: __MODULE__)
+  end
 
   def handle(:publish, activity) do
     Logger.debug(fn -> "Running publish for #{activity.data["id"]}" end)
@@ -28,11 +34,38 @@ def handle(type, payload) do
   end
 
   def enqueue(type, payload) do
-    # for now, just run immediately in a new process.
     if Mix.env == :test do
       handle(type, payload)
     else
-      spawn(fn -> handle(type, payload) end)
+      GenServer.cast(__MODULE__, {:enqueue, type, payload})
+    end
+  end
+
+  def maybe_start_job(running_jobs, queue) do
+    if (:sets.size(running_jobs) < @max_jobs) && !:queue.is_empty(queue) do
+      {{:value, {type, payload}}, queue} = :queue.out(queue)
+      {:ok, pid} = Task.start(fn -> handle(type, payload) end)
+      mref = Process.monitor(pid)
+      {:sets.add_element(mref, running_jobs), queue}
+    else
+      {running_jobs, queue}
     end
   end
+
+  def handle_cast({:enqueue, type, payload}, {running_jobs, queue}) do
+    queue = :queue.in({type, payload}, queue)
+    {running_jobs, queue} = maybe_start_job(running_jobs, queue)
+    {:noreply, {running_jobs, queue}}
+  end
+
+  def handle_info({:DOWN, ref, :process, _pid, _reason}, {running_jobs, queue}) do
+    running_jobs = :sets.del_element(ref, running_jobs)
+    {running_jobs, queue} = maybe_start_job(running_jobs, queue)
+    {:noreply, {running_jobs, queue}}
+  end
+
+  def handle_cast(m, state) do
+    IO.inspect("Unknown: #{inspect(m)}, #{inspect(state)}")
+    {:noreply, state}
+  end
 end