follows_from_scenario.java

// FOLLOWS_FROM use scenario.

public class Promise<T> {
  public Promise() {
    // 1. Capture the active Span.
    this.parentSpan = tracer.activeSpan();
  }
  
  // 2. Add callacks that will be invoked asynchronously
  //    upon success.
  public void onSuccess(Callback<T> callback) {
    successCallbacks.add(callback);
  }
  
  public void success(final T result) {
    // 3. Upon succcess, invoke the callbacks asynchronously
    //    through a service executor. Each one will do an independent,
    //    post-processing task that is *not* core part of the parent
    //    Span/task.
    for (Callback<T> callback: successCallbacks) {
      executorService.submit(new Runnable {
        @Override
        public void run() {
          Span span = tracer.buildSpan("postProcessingTask")
            .addReference(References.FOLLOWS_FROM, parentSpan.context())
            .start();
          try (Scope scope = tracer.activateSpan(span)) {
            // 4. Do a post-processing task and create a child Span.
          } finally {
            span.finish();
          }
        }
      });
    }
  }
}