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?

Answer 1

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