[GitHub] [arrow] lidavidm commented on a change in pull request #11964: ARROW-15067: [C++] Add tracing spans to the scanner

[GitBox <gi...@apache.org>]

lidavidm commented on a change in pull request #11964:
URL: https://github.com/apache/arrow/pull/11964#discussion_r778517602



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File path: cpp/src/arrow/util/tracing_internal.h
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@@ -97,6 +98,58 @@ AsyncGenerator<T> WrapAsyncGenerator(AsyncGenerator<T> wrapped,
     return fut;
   };
 }
+
+/// \brief Start a new span for each invocation of a generator.
+///
+/// The parent span of the new span will be the currently active span
+/// (if any) as of when WrapAsyncGenerator was itself called.
+template <typename T>
+AsyncGenerator<T> WrapAsyncGenerator(AsyncGenerator<T> wrapped,
+                                     const std::string& span_name) {
+  opentelemetry::trace::StartSpanOptions options;
+  options.parent = GetTracer()->GetCurrentSpan()->GetContext();
+  return WrapAsyncGenerator(std::move(wrapped), std::move(options), span_name);
+}
+
+/// \brief End the given span when the given async generator ends.
+///
+/// The span will be made the active span each time the generator is called.
+template <typename T>
+AsyncGenerator<T> TieSpanToAsyncGenerator(
+    AsyncGenerator<T> wrapped,
+    opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span> span) {
+  return [=]() mutable -> Future<T> {
+    auto scope = GetTracer()->WithActiveSpan(span);
+    return wrapped().Then(
+        [span](const T& result) -> Result<T> {
+          span->SetStatus(opentelemetry::trace::StatusCode::kOk);
+          return result;
+        },
+        [span](const Status& status) -> Result<T> {
+          MarkSpan(status, span.get());
+          return status;
+        });
+  };
+}
+
+/// \brief Activate the given span on each invocation of an async generator.
+template <typename T>
+AsyncGenerator<T> PropagateSpanThroughAsyncGenerator(
+    AsyncGenerator<T> wrapped,
+    opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span> span) {
+  return [=]() mutable -> Future<T> {
+    auto scope = GetTracer()->WithActiveSpan(span);
+    return wrapped();
+  };
+}

Review comment:
       Yes, active span is thread local. `wrapped()` must manually propagate the active span (or manually pass the span through) if it itself spawns a thread. That is a disadvantage and it does make it hard to use OpenTelemetry while making it possible to completely remove it at compile time.
   
   One way to get around this would be to instrument the Executor and possibly Future classes themselves, but I worry this would have more overhead than is desirable. (Or maybe not. I haven't tried.)




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