# Vertx OpenTracing

Vert.x integrates with OpenTracing thanks to the Jaeger client.

You can configure Vert.x to use the Jaeger client configured via Environment

    Vertx vertx = Vertx.vertx(new VertxOptions()
      .setTracingOptions(
        new OpenTracingOptions()
      )
    );

You can also pass a custom `Tracer` allowing for greater control over the configuration.

    Vertx vertx = Vertx.builder()
      .withTracer(new OpenTracingTracerFactory(tracer))
      .build();

## Tracing policy

The tracing policy defines the behavior of a component when tracing is enabled:

- `PROPAGATE`: the component reports a span in the active trace

- `ALWAYS`: the component reports a span in the active trace or creates a new active trace

- `IGNORE`: the component will not be involved in any trace.

The tracing policy is usually configured in the component options.

## HTTP tracing

The Vert.x HTTP server and client reports span around HTTP requests:

- `operationName`: the HTTP method

- tags

- `http.method`: the HTTP method

- `http.url`: the request URL

- `http.status_code`: the HTTP status code

The default HTTP server tracing policy is `ALWAYS`, you can configure the policy with `setTracingPolicy`

    HttpServer server = vertx.createHttpServer(new HttpServerOptions()
      .setTracingPolicy(TracingPolicy.IGNORE)
    );

The default HTTP client tracing policy is `PROPAGATE`, you can configure the policy with `setTracingPolicy`

    HttpClient client = vertx.createHttpClient(new HttpClientOptions()
      .setTracingPolicy(TracingPolicy.IGNORE)
    );

To initiate a trace for a client call, you need to create it first and make Vert.x aware of it by using `OpenTracingUtil.setSpan`:

    Span span = tracer.buildSpan("my-operation")
      .withTag("some-key", "some-value")
      .start();
    OpenTracingUtil.setSpan(span);
    // Do something, e.g. client request
    span.finish();

In an HTTP scenario between two Vert.x services, a span will be created client-side, then the trace context will be propagated server-side and another span will be added to the trace.

## EventBus tracing

The Vert.x EventBus reports spans around message exchanges.

The default sending policy is `PROPAGATE`, you can configure the policy with `setTracingPolicy`.

    DeliveryOptions options = new DeliveryOptions().setTracingPolicy(TracingPolicy.ALWAYS);
    vertx.eventBus().send("the-address", "foo", options);

## Obtain current Span

Vert.x stores current `Span` object in local context. To obtain it, use method `OpenTracingUtil.getSpan()`.

This method will work only on Vert.x threads (instances of `VertxThread`). Obtaining from non-Vert.x thread doesn’t work by design, method will return null.

## Coroutines support

There is no direct support for coroutines, but it can be achieved with minimal instrumentation.

There are several steps to achieve this.

1. Use `CoroutineVerticle`.

2. Convert every route handler you have to a coroutine.

3. Use CoroutineContext to store `Tracer` and current `Span` object

Example code:

    class TracedVerticle(private val tracer: Tracer): CoroutineVerticle() {
       override suspend fun start() {
           val router = Router.router(vertx)

           router.route("/hello1")
               .method(HttpMethod.GET)
               .coroutineHandler { ctx ->                          // (1)
                   launch { println("Hello to Console") }
                   ctx.end("Hello from coroutine handler")
               }

           router.route("/hello2")
               .method(HttpMethod.GET)
               .coroutineHandler(::nonSuspendHandler)              // (2)

           vertx.createHttpServer()
               .requestHandler(router)
               .listen(8080)
               .await()
       }

       private fun nonSuspendHandler(ctx: RoutingContext) {
           ctx.end("Hello from usual handler")
       }

       private fun Route.coroutineHandler(handler: Handler<RoutingContext>): Route = // (3)
           this.coroutineHandler(handler::handle)

       private fun Route.coroutineHandler(                                           // (4)
           handler: suspend (RoutingContext) -> (Unit)
       ): Route = handler { ctx ->
           val span: Span = OpenTracingUtil.getSpan()                                // (5)
           launch(ctx.vertx().dispatcher() + SpanElement(tracer, span)) {            // (6)
               val spanElem = coroutineContext[SpanElement]                          // (7)
               if (spanElem == null) {
                   handler(ctx)
               } else {
                   val span = spanElem.span
                   val tracer = spanElem.tracer
                   val childSpan = span                                                // (8)
                   try {
                       withContext(SpanElement(tracer, childSpan)) { handler(ctx) }    // (9)
                   } finally {
                       // childSpan.finish()                                           // (10)
                   }
               }
               // OR create a helper method for further reuse
               withContextTraced(coroutineContext) {
                   try {
                       handler(ctx)
                   } catch (t: Throwable) {
                       ctx.fail(t)
                   }
               }
           }
       }
    }

1. Creates a coroutine handler with `coroutineHandler` extension method.

2. Creates usual async handler, which is then wrapped to a coroutine.

3. Extension method to convert `Handler<RoutingContext>` to suspendable function.

4. Extension method which creates and launches a coroutine on Vert.x EventLoop.

5. Get current `Span` from Vert.x local context (populated automatically).

6. Create a wrapper coroutine, add current Span to `CoroutineContext`.

7. Retrieve a `Span` from coroutine context.

8. Either reuse `span` or create a new span with `tracer.buildSpan("").asChildOf(span).start()`.

9. Put a new `Span` to a context.

10. Finish `childSpan`, if you created a new one.

Helper code, your implementation may vary:

    /**
    * Keeps references to a tracer and current Span inside CoroutineContext
    */
    class SpanElement(val tracer: Tracer, val span: Span) :
       ThreadContextElement<Scope>,
       AbstractCoroutineContextElement(SpanElement) {

       companion object Key : CoroutineContext.Key<SpanElement>

       /**
       *  Will close current [Scope] after continuation's pause.
       */
       override fun restoreThreadContext(context: CoroutineContext, oldState: Scope) {
           oldState.close()
       }

       /**
       * Will create a new [Scope] after each continuation's resume, scope is activated with provided [span] instance.
       */
       override fun updateThreadContext(context: CoroutineContext): Scope {
           return tracer.activateSpan(span)
       }
    }

    /**
    * Advanced helper method with a few options, also shows how to use MDCContext to pass a Span to a logger.
    */
    suspend fun <T> withContextTraced(
       context: CoroutineContext,
       reuseParentSpan: Boolean = true,
       block: suspend CoroutineScope.() -> T
    ): T {
       return coroutineScope {
           val spanElem = this.coroutineContext[SpanElement]

           if (spanElem == null) {
               logger.warn { "Calling 'withTracer', but no span found in context" }
               withContext(context, block)
           } else {
               val childSpan = if (reuseParentSpan) spanElem.span
               else spanElem.tracer.buildSpan("").asChildOf(spanElem.span).start()

               try {
                   val mdcSpan = mapOf(MDC_SPAN_KEY to childSpan.toString())
                   withContext(context + SpanElement(spanElem.tracer, childSpan) + MDCContext(mdcSpan), block)
               } finally {
                   if (!reuseParentSpan) childSpan.finish()
               }
           }
       }
    }
    private const val MDC_SPAN_KEY = "request.span.id"