Adds support for "current span" formerly supported with thread binders

This implements a "current span" concept which represents the in-flight
operation. `Tracer.currentSpan()` can be used to add custom tags to a
span and `Tracer.nextSpan()` can be used to create a child of whatever
is in-flight.

The backend of this is `CurrentTraceContext` which can be extended to
support SLF4J or Finagle synchronization. It also exposes concurrent
wrappers for types like `ExecutorService`.

Particularly, this includes the ability to get the "current span" and assign is to a scope. In most cases the implementation will be backed by thread locals, whether that's directly (as brave's former thread state was) or indirectly via a GRPC or Finagle Context.

The (power) user apis are located in the Tracer api:
* `Span currentSpan` - retrieves any span in scope or null
* `Span nextSpan` - creates a new child or trace if there's no current span
* `SpanInScope withSpanInScope(span)` - makes a span current and available to calls within the scope

The SPI for this is called `CurrentTraceContext`, and it can be implemented to support MDC integration or alternate span storage
* `TraceContext get()` - returns the current trace context or null
*  `Scope newScope(context)` - creates a scope where calls to get will return the input

By default, `CurrentTraceContext` is implemented by a thread local, though there are tests to show it can be implemented by Finagle (or gRPC or otherwise).

brave-benchmarks/pom.xml

@@ -50,7 +50,7 @@
     <dependency>
       <groupId>com.twitter</groupId>
       <artifactId>finagle-core_2.12</artifactId>
-      <version>6.41.0</version>
+      <version>${finagle.version}</version>
     </dependency>
   </dependencies>
 

brave-jersey/src/test/java/com/github/kristofa/brave/jersey/ITJerseyClientTraceFilter.java

@@ -35,18 +35,11 @@ Client configureClient(JerseyClientTraceFilter filter) {
 
   @Override protected void get(Client client, String pathIncludingQuery)
       throws IOException {
-    client.resource(server.url("/foo").uri()).get(String.class);
+    client.resource(server.url(pathIncludingQuery).uri()).get(String.class);
   }
 
   @Override protected void getAsync(Client client, String pathIncludingQuery) throws Exception {
-    client.asyncResource(server.url("/foo").uri()).get(String.class);
-  }
-
-
-  @Override
-  @Test(expected = AssertionError.class) // query params are not logged in jersey
-  public void httpUrlTagIncludesQueryParams() throws Exception {
-    super.httpUrlTagIncludesQueryParams();
+    client.asyncResource(server.url(pathIncludingQuery).uri()).get(String.class);
   }
 
   @Override

brave/README.md

@@ -230,6 +230,121 @@ span = contextOrFlags.context() != null
     : tracer.newTrace(contextOrFlags.samplingFlags());
 ```
 
+## Current Span
+
+Brave supports a "current span" concept which represents the in-flight
+operation. `Tracer.currentSpan()` can be used to add custom tags to a
+span and `Tracer.nextSpan()` can be used to create a child of whatever
+is in-flight.
+
+### Setting a span in scope via custom executors
+
+Many frameworks allow you to specify an executor which is used for user
+callbacks. The type `CurrentTraceContext` implements all functionality
+needed to support the current span. It also exposes utilities which you
+can use to decorate executors.
+
+```java
+CurrentTraceContext currentTraceContext = new CurrentTraceContext.Default();
+tracer = Tracer.newBuilder()
+               .currentTraceContext(currentTraceContext)
+               ...
+               .build();
+
+Client c = Client.create();
+c.setExecutorService(currentTraceContext.executorService(realExecutorService));
+```
+
+### Setting a span in scope manually
+
+When writing new instrumentation, it is important to place a span you
+created in scope as the current span. Not only does this allow users to
+access it with `Tracer.currentSpan()`, but it also allows customizations
+like SLF4J MDC to see the current trace IDs.
+
+`Tracer.withSpanInScope(Span)` facilitates this and is most conveniently
+employed via the try-with-resources idiom. Whenever external code might
+be invoked (such as proceeding an interceptor or otherwise), place the
+span in scope like this.
+
+```java
+try (SpanInScope ws = tracer.withSpanInScope(span)) {
+  return inboundRequest.invoke();
+} finally { // note the scope is independent of the span
+  span.finish();
+}
+```
+
+### Working with callbacks
+
+Many libraries expose a callback model as opposed to an interceptor one.
+When creating new instrumentation, you may find places where you need to
+place a span in scope in one callback (like `onStart()`) and end the scope
+in another callback (like `onFinish()`).
+
+Provided the library guarantees these run on the same thread, you can
+simply propagate the result of `Tracer.withSpanInScope(Span)` from the
+starting callback to the closing one. This is typically done with a
+request-scoped attribute.
+
+Here's an example:
+```java
+class MyFilter extends Filter {
+  public void onStart(Request request, Attributes attributes) {
+    // Assume you have code to start the span and add relevant tags...
+
+    // We now set the span in scope so that any code between here and
+    // the end of the request can see it with Tracer.currentSpan()
+    SpanInScope spanInScope = tracer.withSpanInScope(span);
+
+    // We don't want to leak the scope, so we place it somewhere we can
+    // lookup later
+    attributes.put(SpanInScope.class, spanInScope);
+  }
+
+  public void onFinish(Response response, Attributes attributes) {
+    // as long as we are on the same thread, we can read the span started above
+    Span span = tracer.currentSpan();
+
+    // Assume you have code to complete the span
+
+    // We now remove the scope (which implicitly detaches it from the span)
+    attributes.get(SpanInScope.class).close();
+  }
+}
+```
+
+### Working with callbacks that occur on different threads
+
+The examples above work because the callbacks happen on the same thread.
+You should not set a span in scope if you cannot close that scope on the
+same thread. This may be the case in some asynchronous libraries. Often,
+you will need to propagate the span directly in a custom attribute. This
+will allow you to trace the RPC, even if this approach doesn't facilitate
+use of `Tracer.currentSpan()` from external code.
+
+Here's an example of explicit propagation:
+```java
+class MyFilter extends Filter {
+  public void onStart(Request request, Attributes attributes) {
+    // Assume you have code to start the span and add relevant tags...
+
+    // We can't open a scope as onFinish happens on another thread.
+    // Instead, we propagate the span manually so at least basic tracing
+    // will work.
+    attributes.put(Span.class, span);
+  }
+
+  public void onFinish(Response response, Attributes attributes) {
+    // We can't rely on Tracer.currentSpan(), but we can rely on explicit
+    // propagation
+    Span span = attributes.get(Span.class);
+
+    // Assume you have code to complete the span
+  }
+}
+```
+
 ## Performance
 Brave has been built with performance in mind. Using the core Span api,
 you can record spans in sub-microseconds. When a span is sampled, there's
@@ -360,6 +475,20 @@ direct integration with carrier types (such as http request) vs routing
 through an intermediate (such as a map). Brave also considers propagation
 a separate api from the tracer.
 
+
+### CurrentTraceContext Api
+The first design of `CurrentTraceContext` was borrowed from `ContextUtils`
+in Google's [instrumentation-java](https://github.com/google/instrumentation-java) project.
+This was possible because of high collaboration between the projects and
+an almost identical goal. Slight departures including naming (which prefers
+Guice's naming conventions), and that the object scoped is a TraceContext
+vs a Span.
+
+Propagating a trace context instead of a span is a right fit for several reasons:
+* `TraceContext` can be created without a reference to a tracer
+* Common tasks like making child spans and staining log context only need the context
+* Brave's recorder is keyed on context, so there's no feature loss in this choice
+
 ### Public namespace
 Brave 4's pubic namespace is more defensive that the past, using a package
 accessor design from [OkHttp](https://github.com/square/okhttp).

brave/pom.xml

@@ -78,5 +78,11 @@
       <version>${grpc.version}</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>com.twitter</groupId>
+      <artifactId>finagle-core_2.12</artifactId>
+      <version>${finagle.version}</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 </project>

brave/src/main/java/brave/NoopSpan.java

@@ -1,23 +1,20 @@
 package brave;
 
 import brave.propagation.TraceContext;
+import com.google.auto.value.AutoValue;
 import zipkin.Endpoint;
 
-final class NoopSpan extends Span {
-  final TraceContext context;
+@AutoValue
+abstract class NoopSpan extends Span {
 
-  NoopSpan(TraceContext context) {
-    this.context = context;
+  static NoopSpan create(TraceContext context) {
+    return new AutoValue_NoopSpan(context);
   }
 
   @Override public boolean isNoop() {
     return true;
   }
 
-  @Override public TraceContext context() {
-    return context;
-  }
-
   @Override public Span start() {
     return this;
   }
@@ -58,9 +55,4 @@ final class NoopSpan extends Span {
 
   @Override public void flush() {
   }
-
-  @Override
-  public String toString() {
-    return "NoopSpan(" + context + ")";
-  }
 }

brave/src/main/java/brave/RealSpan.java

@@ -2,81 +2,77 @@
 
 import brave.internal.recorder.Recorder;
 import brave.propagation.TraceContext;
+import com.google.auto.value.AutoValue;
 import zipkin.Endpoint;
 
 /** This wraps the public api and guards access to a mutable span. */
-final class RealSpan extends Span {
+@AutoValue
+abstract class RealSpan extends Span {
 
-  final TraceContext context;
-  final Clock clock;
-  final Recorder recorder;
+  abstract Clock clock();
 
-  RealSpan(TraceContext context, Clock clock, Recorder recorder) {
-    this.context = context;
-    this.clock = clock;
-    this.recorder = recorder;
+  abstract Recorder recorder();
+
+  static RealSpan create(TraceContext context, Clock clock, Recorder recorder) {
+    return new AutoValue_RealSpan(context, clock, recorder);
   }
 
   @Override public boolean isNoop() {
     return false;
   }
 
-  @Override public TraceContext context() {
-    return context;
-  }
-
   @Override public Span start() {
-    return start(clock.currentTimeMicroseconds());
+    return start(clock().currentTimeMicroseconds());
   }
 
   @Override public Span start(long timestamp) {
-    recorder.start(context, timestamp);
+    recorder().start(context(), timestamp);
     return this;
   }
 
   @Override public Span name(String name) {
-    recorder.name(context, name);
+    recorder().name(context(), name);
     return this;
   }
 
   @Override public Span kind(Kind kind) {
-    recorder.kind(context, kind);
+    recorder().kind(context(), kind);
     return this;
   }
 
   @Override public Span annotate(String value) {
-    return annotate(clock.currentTimeMicroseconds(), value);
+    return annotate(clock().currentTimeMicroseconds(), value);
   }
 
   @Override public Span annotate(long timestamp, String value) {
-    recorder.annotate(context, timestamp, value);
+    recorder().annotate(context(), timestamp, value);
     return this;
   }
 
   @Override public Span tag(String key, String value) {
-    recorder.tag(context, key, value);
+    recorder().tag(context(), key, value);
     return this;
   }
 
   @Override public Span remoteEndpoint(Endpoint remoteEndpoint) {
-    recorder.remoteEndpoint(context, remoteEndpoint);
+    recorder().remoteEndpoint(context(), remoteEndpoint);
     return this;
   }
 
   @Override public void finish() {
-    finish(clock.currentTimeMicroseconds());
+    finish(clock().currentTimeMicroseconds());
   }
 
   @Override public void finish(long timestamp) {
-    recorder.finish(context, timestamp);
+    recorder().finish(context(), timestamp);
   }
 
   @Override public void flush() {
-    recorder.flush(context);
+    recorder().flush(context());
   }
 
   @Override
   public String toString() {
-    return "RealSpan(" + context + ")";
+    return "RealSpan(" + context() + ")";
   }
 }