2023-02-24 | Custom Cowboy Stream Handler for a Phoenix app

(Turn your phone sideways if you are on mobile)

Here is an example of how to write a custom cowboy stream handler. This could be useful to debug/add additional loggin when your application does not behave the way you'd want. Sometimes, things just get nuked and this is a way to get some type of warning/control when that happens.

Custom Cowboy Stream Handler

diff --git a/lib/your_app_web/endpoint.ex b/lib/your_app_web/endpoint.ex
index 199ceaf..c7bd7b8 100644
--- a/lib/your_app_web/endpoint.ex
+++ b/lib/your_app_web/endpoint.ex
@@ -59,7 +59,22 @@ defmodule YourAppWeb.Endpoint do
        config
        |> Keyword.put(:secret_key_base, secret_key_base)
        |> Keyword.merge(
-        http: [port: port, protocol_options: [idle_timeout: 180_000]]
+        http: [
+          port: port,
+          protocol_options: [idle_timeout: 180_000],
+          # Here is more info on `stream_handlers`. We are using the
+          # `cowboy_telemetry_h`
+          # (https://github.com/beam-telemetry/cowboy_telemetry#usage) the
+          # default `cowboy_stream_h`
+          # (https://ninenines.eu/docs/en/cowboy/2.5/manual/cowboy_stream/) and
+          # our own for logging purposes (which delegates to `:cowboy_stream`
+          # on all calls)
+          stream_handlers: [
+            :cowboy_telemetry_h,
+            :cowboy_stream_h,
+            YourApp.StreamHandlers.YourAppStreamHandler
+          ]
+        ]
        )
        |> Keyword.merge(
          url: [
diff --git a/lib/your_app_web/stream_handlers/your_app_stream_handler.ex b/lib/your_app_web/stream_handlers/your_app_stream_handler.ex
new file mode 100644
index 0000000..3de7e62
--- /dev/null
+++ b/lib/your_app_web/stream_handlers/your_app_stream_handler.ex
@@ -0,0 +1,37 @@
+defmodule YourApp.StreamHandlers.YourAppStreamHandler do
+  @moduledoc """
+  Delegates to :cowboy_stream.
+
+  At the moment, used for adding logs and introspection to the app.
+  """
+
+  @behaviour :cowboy_stream
+
+  require Logger
+
+  @impl :cowboy_stream
+  def init(stream_id, req, opts) do
+    :cowboy_stream.init(stream_id, req, opts)
+  end
+
+  @impl :cowboy_stream
+  def data(stream_id, is_fin, data, state) do
+    :cowboy_stream.data(stream_id, is_fin, data, state)
+  end
+
+  @impl :cowboy_stream
+  def info(stream_id, info, state) do
+    :cowboy_stream.info(stream_id, info, state)
+  end
+
+  @impl :cowboy_stream
+  def terminate(stream_id, reason, state) do
+    Logger.info("#{__MODULE__} terminating connection, reason: #{inspect(reason)}")
+    :cowboy_stream.terminate(stream_id, reason, state)
+  end
+
+  @impl :cowboy_stream
+  def early_error(stream_id, reason, partial_req, resp, opts) do
+    :cowboy_stream.early_error(stream_id, reason, partial_req, resp, opts)
+  end
+end

Thanks for reading, PDG