Add option for annotating pipeline plug function calls with OpenCensu…

…s spans Once a Plug pipeline gets long enough and/or executes slowly enough, a person begins to wonder, "What plug functions are taking so much time?" This patch helps answer that question by adding an option to add OpenCensus span annotations to all plug function executions in a pipeline. The new OpenCensus spans can tell you the names of each pipeline function executed and how much time elapsed in each span. Nested pipelines are fully supported.
blockfi · Dec 24, 2020 · 70962d3 · 70962d3
1 parent 3193ab3
commit 70962d3
Showing 1 changed file with 44 additions and 5 deletions.
diff --git a/lib/plug/builder.ex b/lib/plug/builder.ex
@@ -38,6 +38,9 @@ defmodule Plug.Builder do
     * `:log_on_halt` - accepts the level to log whenever the request is halted
     * `:init_mode` - the environment to initialize the plug's options, one of
       `:compile` or `:runtime`. Defaults `:compile`.
+    * `:pipeline_tracing` - if true, add OpenCensus span telemetry to all
+       pipeline function calls.  The names of the spans will be prefixed by
+       `"Pipline plug: ` and will include the plug's name or module name.
 
   ## Plug behaviour
 
@@ -338,7 +341,7 @@ defmodule Plug.Builder do
   # `acc` is a series of nested plug calls in the form of plug3(plug2(plug1(conn))).
   # `quote_plug` wraps a new plug around that series of calls.
   defp quote_plug(plug_type, plug, opts, guards, acc, env, builder_opts) do
-    call = quote_plug_call(plug_type, plug, opts)
+    call = quote_plug_call(plug_type, plug, opts, builder_opts)
 
     error_message =
       case plug_type do
@@ -369,12 +372,48 @@ defmodule Plug.Builder do
     {fun, meta, [arg, [do: clauses]]}
   end
 
-  defp quote_plug_call(:function, plug, opts) do
-    quote do: unquote(plug)(conn, unquote(opts))
+  defp quote_plug_call(:function, plug, opts, builder_opts) do
+    if builder_opts[:pipeline_tracing] do
+      quote do
+        ctx =
+          :oc_trace.start_span(
+            "Pipeline plug: #{inspect(unquote(plug))}",
+            :ocp.current_span_ctx()
+          )
+
+        try do
+          unquote(plug)(conn, unquote(opts))
+        after
+          :oc_trace.finish_span(ctx)
+        end
+      end
+    else
+      quote do
+        unquote(plug)(conn, unquote(opts))
+      end
+    end
   end
 
-  defp quote_plug_call(:module, plug, opts) do
-    quote do: unquote(plug).call(conn, unquote(opts))
+  defp quote_plug_call(:module, plug, opts, builder_opts) do
+    if builder_opts[:pipeline_tracing] do
+      quote do
+        ctx =
+          :oc_trace.start_span(
+            "Pipeline plug: #{inspect(unquote(plug))}.call/2",
+            :ocp.current_span_ctx()
+          )
+
+        try do
+          unquote(plug).call(conn, unquote(opts))
+        after
+          :oc_trace.finish_span(ctx)
+        end
+      end
+    else
+      quote do
+        unquote(plug).call(conn, unquote(opts))
+      end
+    end
   end
 
   defp compile_guards(call, true) do