message_middleware: make message passing from Replication.Server -> R…

…eplication.Publisher configurable

the aim of this is to allow the end user to implement a backpresure system if need be
for instance using broadway / dets / etc.

lib/walex/config/config.ex

@@ -4,9 +4,10 @@ defmodule WalEx.Config do
   """
   use Agent
 
+  alias WalEx.Replication.Publisher
   alias WalEx.Config.Registry, as: WalExRegistry
 
-  @allowed_config_value ~w(database hostname name password port publication username webhook_signing_secret slot_name durable_slot)a
+  @allowed_config_value ~w(database hostname name password port publication username webhook_signing_secret slot_name durable_slot message_middleware)a
   @allowed_config_values ~w(destinations event_relay modules subscriptions)a
 
   def start_link(opts) do
@@ -127,7 +128,8 @@ defmodule WalEx.Config do
       webhook_signing_secret: Keyword.get(configs, :webhook_signing_secret),
       event_relay: Keyword.get(configs, :event_relay),
       slot_name: Keyword.get(configs, :slot_name) |> parse_slot_name(name),
-      durable_slot: Keyword.get(configs, :durable_slot, false) == true
+      durable_slot: Keyword.get(configs, :durable_slot, false) == true,
+      message_middleware: Keyword.get(configs, :message_middleware) |> parse_message_middleware()
     ]
   end
 
@@ -200,6 +202,11 @@ defmodule WalEx.Config do
   defp parse_slot_name(nil, app_name), do: to_string(app_name) <> "_walex"
   defp parse_slot_name(slot_name, _), do: slot_name
 
+  defp parse_message_middleware(message_middleware) when is_function(message_middleware, 2),
+    do: message_middleware
+
+  defp parse_message_middleware(nil), do: &Publisher.process_message_async/2
+
   defp set_url_opts(username, password, database, info) do
     url_opts = [
       username: username,

lib/walex/replication/publisher.ex

@@ -27,12 +27,16 @@ defmodule WalEx.Replication.Publisher do
     GenServer.start_link(__MODULE__, opts, name: name)
   end
 
-  def process_message(message, app_name) do
+  def process_message_async(message, app_name) do
     name = registry_name(app_name)
-
     GenServer.cast(name, %{message: message, app_name: app_name})
   end
 
+  def process_message_sync(message, app_name) do
+    name = registry_name(app_name)
+    GenServer.call(name, %{message: message, app_name: app_name}, :infinity)
+  end
+
   defp registry_name(app_name) do
     Config.Registry.set_name(:set_gen_server, __MODULE__, app_name)
   end

lib/walex/replication/server.ex

@@ -9,7 +9,6 @@ defmodule WalEx.Replication.Server do
 
   alias WalEx.Config.Registry, as: WalExRegistry
   alias WalEx.Decoder
-  alias WalEx.Replication.Publisher
   alias WalEx.Replication.QueryBuilder
 
   def start_link(opts) do
@@ -48,20 +47,23 @@ defmodule WalEx.Replication.Server do
     [
       slot_name: slot_name,
       publication: publication,
-      durable_slot: durable_slot
+      durable_slot: durable_slot,
+      message_middleware: message_middleware
     ] =
       WalEx.Config.get_configs(app_name, [
         :slot_name,
         :publication,
-        :durable_slot
+        :durable_slot,
+        :message_middleware
       ])
 
     state = %{
       step: :disconnected,
       app_name: app_name,
       slot_name: slot_name,
       publication: publication,
-      durable_slot: durable_slot
+      durable_slot: durable_slot,
+      message_middleware: message_middleware
     }
 
     {:ok, state}
@@ -138,7 +140,7 @@ defmodule WalEx.Replication.Server do
   def handle_data(<<?w, _wal_start::64, _wal_end::64, _clock::64, rest::binary>>, state) do
     rest
     |> Decoder.decode_message()
-    |> Publisher.process_message(state.app_name)
+    |> state.message_middleware.(state.app_name)
 
     {:noreply, state}
   end