opencensus - explicit context management

Question

I am implementing an opencensus tracing in my (asynchronous) JVM app.

However I don't understand how is the context passed. Sometimes it seems to work fine, sometimes traces from different requests appear nested for no reason.

I also have this warning appearing in the logs along with a stacktrace: SEVERE: Context was not attached when detaching

How do I explicitly create a root span, and how can I explicitly pass a parent/context to the child spans?

It looks like it should be possible ( https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/Span.md#span-creation ) but I haven't been able to find an example that works for me — nha, Nov 20 '18 at 16:52

score 3 · Accepted Answer · answered Nov 20 '18 at 19:37

In OpenCensus we have a concept of context independent of the "Span" or "Tags". It represents a Map that is propagated with the request (it is implemented as a thread-local so in sync calls automatically gets propagated). For callbacks/async calls just for propagation (we are using io.grpc.Context as the implementation of the context) use the wrap functions defined here https://github.com/grpc/grpc-java/blob/master/context/src/main/java/io/grpc/Context.java#L589. This will ensure just the context propagation, so entries in the context map will be propagated between different threads.

If you want to start a Span in one thread and end it in a different thread, use the withSpan methods from the tracer https://www.javadoc.io/doc/io.opencensus/opencensus-api/0.17.0 :

class MyClass {
  private static Tracer tracer = Tracing.getTracer();
  void handleRequest(Executor executor) {
    Span span = tracer.spanBuilder("MyRunnableSpan").startSpan();
    // do some work before scheduling the async
    executor.execute(Context.wrap(tracer.withSpan(span, new Runnable() {
      @Override      
      public void run() {
        try {
          sendResult();
        } finally {
          span.end();
        }
      }
    })));
  }
}

A bit more information about this here https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/Span.md#span-creation

Thanks a lot for your reply! Will read it carefully and report :) — nha, Nov 20 '18 at 20:06
the first line``` Span span = tracer.spanBuilder("MyRunnableSpan").startSpan(); ``` Is it possible that "MyRunnableSpan" is created and attached to an existing Context? In my case, if it gets it from the thread it would be wrong since the thread could be the same as another request thread. — nha, Nov 21 '18 at 15:24
Is there another way? If I don't have access to the executor because it is part of a framework for instance? — nha, Dec 07 '18 at 14:01

opencensus - explicit context management

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