chore: added ssr telemetry

lib/ssr.ex

@@ -11,6 +11,22 @@ defmodule LiveSvelte.SSR do
   To define a custom renderer, change the application config in `config.exs`:
 
       config :live_svelte, ssr_module: MyCustomSSRModule
+
+  ## Telemetry
+
+  Exposes a telemetry span for each render under the key `[:live_svelte, :ssr]`.
+
+  The following events are emitted:
+
+    * `[:live_svelte, :ssr, :start]` — fired when a render begins.
+      Metadata: `%{component: name, props: props, slots: slots}`.
+
+    * `[:live_svelte, :ssr, :stop]` — fired when a render completes successfully.
+      Metadata: same. Measurements include `%{duration: duration}` in native time units
+      (convert with `:erlang.convert_time_unit(duration, :native, :millisecond)`).
+
+    * `[:live_svelte, :ssr, :exception]` — fired when the renderer raises.
+      The exception is re-raised after the event is emitted.
   """
 
   @type component_name :: String.t()
@@ -41,8 +57,11 @@ defmodule LiveSvelte.SSR do
   @spec render(component_name, props, slots) :: render_response | no_return
   def render(name, props, slots) do
     mod = Application.get_env(:live_svelte, :ssr_module, LiveSvelte.SSR.NodeJS)
+    meta = %{component: name, props: props, slots: slots}
 
-    mod.render(name, props, slots)
+    :telemetry.span([:live_svelte, :ssr], meta, fn ->
+      {mod.render(name, props, slots), meta}
+    end)
   end
 
   @deprecated "Use LiveSvelte.SSR.NodeJS.server_path/0 instead."

mix.exs

@@ -67,6 +67,7 @@ defmodule LiveSvelte.MixProject do
       {:jason, "~> 1.2", optional: true},
       {:lazy_html, ">= 0.1.0", only: :test},
       {:nodejs, "~> 3.1"},
+      {:telemetry, "~> 0.4 or ~> 1.0"},
       {:phoenix, ">= 1.7.0"},
       {:phoenix_html, ">= 3.3.1"},
       {:phoenix_live_view, ">= 0.18.0"}

test/ssr_telemetry_test.exs

@@ -0,0 +1,108 @@
+defmodule LiveSvelte.SSRTelemetryTest do
+  # must be synchronous — modifies global config and attaches telemetry handlers
+  use ExUnit.Case, async: false
+
+  alias LiveSvelte.SSR
+
+  # Named handler to avoid telemetry "local function" performance penalty log warning.
+  # test_pid is threaded through the telemetry config argument.
+  def handle_telemetry(event, measurements, metadata, test_pid) do
+    send(test_pid, {:telemetry_event, event, measurements, metadata})
+  end
+
+  defmodule MockSSRRenderer do
+    @moduledoc false
+    @behaviour SSR
+
+    @impl true
+    def render("Success", _props, _slots) do
+      %{"head" => "", "html" => "<div>Hello</div>"}
+    end
+
+    def render("Failing", _props, _slots) do
+      raise RuntimeError, "SSR render failed"
+    end
+
+    # Catch-all: prevents unhelpful FunctionClauseError on typos in test component names
+    def render(name, _props, _slots) do
+      raise "MockSSRRenderer: no handler for #{inspect(name)}"
+    end
+  end
+
+  setup do
+    original_module = Application.get_env(:live_svelte, :ssr_module)
+    Application.put_env(:live_svelte, :ssr_module, MockSSRRenderer)
+
+    test_pid = self()
+    handler_id = {__MODULE__, :telemetry_test, make_ref()}
+
+    :telemetry.attach_many(
+      handler_id,
+      [
+        [:live_svelte, :ssr, :start],
+        [:live_svelte, :ssr, :stop],
+        [:live_svelte, :ssr, :exception]
+      ],
+      &__MODULE__.handle_telemetry/4,
+      test_pid
+    )
+
+    on_exit(fn ->
+      :telemetry.detach(handler_id)
+
+      if original_module do
+        Application.put_env(:live_svelte, :ssr_module, original_module)
+      else
+        Application.delete_env(:live_svelte, :ssr_module)
+      end
+    end)
+
+    :ok
+  end
+
+  describe "SSR telemetry" do
+    test "emits :start event with component, props, and slots metadata" do
+      props = %{"count" => 42}
+      slots = %{"default" => "<span>slot content</span>"}
+
+      SSR.render("Success", props, slots)
+
+      assert_receive {:telemetry_event, [:live_svelte, :ssr, :start], _measurements, metadata}
+      assert metadata.component == "Success"
+      assert metadata.props == %{"count" => 42}
+      assert metadata.slots == %{"default" => "<span>slot content</span>"}
+    end
+
+    test "emits :stop event with duration measurement and passes through render result" do
+      result = SSR.render("Success", %{}, %{})
+
+      assert result == %{"head" => "", "html" => "<div>Hello</div>"}
+      assert_receive {:telemetry_event, [:live_svelte, :ssr, :stop], measurements, metadata}
+      assert metadata.component == "Success"
+      assert is_integer(measurements.duration)
+      assert measurements.duration >= 0
+    end
+
+    test "emits :exception event with metadata and duration when renderer raises" do
+      assert_raise RuntimeError, "SSR render failed", fn ->
+        SSR.render("Failing", %{}, %{})
+      end
+
+      assert_receive {:telemetry_event, [:live_svelte, :ssr, :exception], measurements, metadata}
+      assert metadata.component == "Failing"
+      assert is_integer(measurements.duration)
+      assert measurements.duration >= 0
+    end
+
+    test "emits independent telemetry events for each render call" do
+      SSR.render("Success", %{"call" => 1}, %{})
+      SSR.render("Success", %{"call" => 2}, %{})
+
+      assert_receive {:telemetry_event, [:live_svelte, :ssr, :start], _,
+                      %{component: "Success", props: %{"call" => 1}}}
+
+      assert_receive {:telemetry_event, [:live_svelte, :ssr, :start], _,
+                      %{component: "Success", props: %{"call" => 2}}}
+    end
+  end
+end