Add LocalTracer

[codefromthecrypt]

I've been playing around with what's needed for local tracing. Most notably, we need more precise timestamps than what System.currentTimeMillis gives. Here's an example

final class LocalTracer {
  private final ClientTracer clientTracer;
  private final ServerAndClientSpanState state;
  private final SpanCollector spanCollector;

  LocalTracer(ClientTracer clientTracer, ServerAndClientSpanState state, SpanCollector spanCollector) {
    this.clientTracer = clientTracer;
    this.state = state;
    this.spanCollector = spanCollector;
  }

  /**
   * Request a new local span, optionally providing the time it started.
   *
   * @param component component responsible for the operation
   * @param operation name of the operation that's begun
   * @param timestamp time the operation started, in epoch microseconds, or null if now.
   * @return metadata about the new span or null if one wasn't started due to sampling policy.
   * @see Constants#LOCAL_COMPONENT
   */
  SpanId startSpan(String component, String operation, @Nullable Long timestamp) {
    SpanId spanId = clientTracer.startNewSpan(operation);
    if (spanId == null) return null; // Don't calculate time if we aren't going to trace.

    com.twitter.zipkin.gen.BinaryAnnotation lc = new com.twitter.zipkin.gen.BinaryAnnotation();
    lc.setKey(Constants.LOCAL_COMPONENT);
    lc.setValue(component.getBytes(Util.UTF_8));
    lc.setAnnotation_type(AnnotationType.STRING);
    lc.setHost(state.getClientEndpoint());

    Span span = state.getCurrentClientSpan();
    synchronized (span) {
      span.setTimestamp(timestamp == null ? System.currentTimeMillis() * 1000 : timestamp);
      span.addToBinary_annotations(lc);
    }
    return spanId;
  }

  /** Log an annotation with a relative timestamp more precise than system time. */
  void submitAnnotation(String value, long timestamp) {
    Span span = state.getCurrentClientSpan();
    if (span == null) return;
    synchronized (span) {
      span.addToAnnotations(new Annotation(timestamp, value).setHost(state.getClientEndpoint()));
    }
  }

  /** Completes the span, which took {@code duration} microseconds. */
  public void finishSpan(long duration) {
    Span span = state.getCurrentClientSpan();
    if (span == null) return;

    synchronized (span) {
      span.setDuration(duration);
      spanCollector.collect(span);
    }

    state.setCurrentClientSpan(null);
    state.setCurrentClientServiceName(null);
  }
}

[kristofa]

👍