refactor: review remarks

lib/membership/settings_cache.ex

@@ -0,0 +1,85 @@
+defmodule Mv.Membership.SettingsCache do
+  @moduledoc """
+  Process-based cache for global settings to avoid repeated DB reads on hot paths
+  (e.g. RegistrationEnabled validation, Layouts.public_page, Plugs).
+
+  Uses a short TTL (default 60 seconds). Cache is invalidated on every settings
+  update so that changes take effect quickly. If no settings process exists
+  (e.g. in tests), get/1 falls back to direct read.
+  """
+  use GenServer
+
+  @default_ttl_seconds 60
+
+  def start_link(opts \\ []) do
+    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  @doc """
+  Returns cached settings or fetches and caches them. Uses TTL; invalidate on update.
+  """
+  def get do
+    case Process.whereis(__MODULE__) do
+      nil ->
+        # No cache process (e.g. test) – read directly
+        do_fetch()
+
+      _pid ->
+        GenServer.call(__MODULE__, :get, 10_000)
+    end
+  end
+
+  @doc """
+  Invalidates the cache so the next get/0 will refetch from the database.
+  Call after update_settings and any other path that mutates settings.
+  """
+  def invalidate do
+    case Process.whereis(__MODULE__) do
+      nil -> :ok
+      _pid -> GenServer.cast(__MODULE__, :invalidate)
+    end
+  end
+
+  @impl true
+  def init(opts) do
+    ttl = Keyword.get(opts, :ttl_seconds, @default_ttl_seconds)
+    state = %{ttl_seconds: ttl, cached: nil, expires_at: nil}
+    {:ok, state}
+  end
+
+  @impl true
+  def handle_call(:get, _from, state) do
+    now = System.monotonic_time(:second)
+    expired? = state.expires_at == nil or state.expires_at <= now
+
+    {result, new_state} =
+      if expired? do
+        fetch_and_cache(now, state)
+      else
+        {{:ok, state.cached}, state}
+      end
+
+    {:reply, result, new_state}
+  end
+
+  defp fetch_and_cache(now, state) do
+    case do_fetch() do
+      {:ok, settings} = ok ->
+        expires = now + state.ttl_seconds
+        {ok, %{state | cached: settings, expires_at: expires}}
+
+      err ->
+        result = if state.cached, do: {:ok, state.cached}, else: err
+        {result, state}
+    end
+  end
+
+  @impl true
+  def handle_cast(:invalidate, state) do
+    {:noreply, %{state | cached: nil, expires_at: nil}}
+  end
+
+  defp do_fetch do
+    Mv.Membership.get_settings_uncached()
+  end
+end