Add rate-limiter mail delivery strategy

config/prod.exs

@@ -15,4 +15,8 @@ config :prison_rideshare, PrisonRideshare.Repo,
   pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
   ssl: true
 
+config :prison_rideshare, PrisonRideshare.Mailer,
+  deliver_later_strategy: PrisonRideshare.MailerRateLimiter,
+  rate_limit_ms: String.to_integer(System.get_env("MAILER_RATE_LIMIT_MS") || "30000")
+
 config :prison_rideshare, gas_price_endpoint: System.get_env("GAS_PRICE_ENDPOINT")

lib/prison_rideshare/application.ex

@@ -10,6 +10,7 @@ defmodule PrisonRideshare.Application do
       {Phoenix.PubSub, name: PrisonRideshare.PubSub},
       # Start the Ecto repository
       PrisonRideshare.Repo,
+      PrisonRideshare.MailerRateLimiter,
       # Start the endpoint when the application starts
       PrisonRideshareWeb.Endpoint,
       # Start your own worker by calling: PrisonRideshare.Worker.start_link(arg1, arg2, arg3)

lib/prison_rideshare/mailer_rate_limiter.ex

@@ -0,0 +1,82 @@
+defmodule PrisonRideshare.MailerRateLimiter do
+  @behaviour Bamboo.DeliverLaterStrategy
+  use GenServer
+
+  require Logger
+
+  @default_interval_ms 30000
+
+  def start_link(opts \\ []) do
+    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  @impl Bamboo.DeliverLaterStrategy
+  def deliver_later(adapter, email, config) do
+    GenServer.cast(__MODULE__, {:enqueue, adapter, email, config})
+  end
+
+  @impl GenServer
+  def init(_opts) do
+    {:ok,
+     %{
+       queue: :queue.new(),
+       interval_ms: delivery_interval_ms(),
+       in_flight: false
+     }}
+  end
+
+  @impl GenServer
+  def handle_cast({:enqueue, adapter, email, config}, state) do
+    state = %{state | queue: :queue.in({adapter, email, config}, state.queue)}
+    {:noreply, maybe_schedule(state)}
+  end
+
+  @impl GenServer
+  def handle_info(:deliver_next, state) do
+    case :queue.out(state.queue) do
+      {{:value, {adapter, email, config}}, queue} ->
+        deliver(adapter, email, config)
+        state = %{state | queue: queue}
+
+        if :queue.is_empty(queue) do
+          {:noreply, %{state | in_flight: false}}
+        else
+          Process.send_after(self(), :deliver_next, state.interval_ms)
+          {:noreply, state}
+        end
+
+      {:empty, _queue} ->
+        {:noreply, %{state | in_flight: false}}
+    end
+  end
+
+  defp maybe_schedule(%{in_flight: true} = state), do: state
+
+  defp maybe_schedule(state) do
+    send(self(), :deliver_next)
+    %{state | in_flight: true}
+  end
+
+  defp deliver(adapter, email, config) do
+    try do
+      case adapter.deliver(email, config) do
+        {:error, error} ->
+          Logger.error("Email delivery failed: #{inspect(error)}")
+
+        _ ->
+          :ok
+      end
+    rescue
+      exception ->
+        Logger.error(Exception.format(:error, exception, __STACKTRACE__))
+    catch
+      kind, reason ->
+        Logger.error(Exception.format(kind, reason, __STACKTRACE__))
+    end
+  end
+
+  defp delivery_interval_ms do
+    config = Application.get_env(:prison_rideshare, PrisonRideshare.Mailer, [])
+    config[:rate_limit_ms] || @default_interval_ms
+  end
+end