Remove opentracing-api.md
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Laplie Anderson
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# OpenTracing API
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The OpenTracing group offers an API to instrument your code.
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This document is a kind of a "quick start" for the official specification: https://github.com/opentracing/specification
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There are several concepts exposed by the OpenTracing API:
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* The core API used for instrumenting the code
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* The tracer implementations are in charge of capturing and reporting the traces. For instance `dd-trace` generates and reports traces to the Datadog trace agent.
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* In-process trace propagation for trace consistency in a concurrent/asynchronous request context.
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* Distributed trace propagation for when receiving a request and making external calls.
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## OpenTracing Core API
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Official documentation link: [Opentracting Tracer specification](https://github.com/opentracing/specification/blob/master/specification.md)
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The core API exposes 3 main objects:
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* A [Tracer](https://github.com/opentracing/specification/blob/master/specification.md#tracer)
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* A [Span](https://github.com/opentracing/specification/blob/master/specification.md#span)
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* A collection of **Tags** associated with a Span
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### Tracers
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The tracer is in charge of instantiating new spans for a given context, and sending them to the appropriate sink when complete.
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The tracer instantiation depends of the implementation you chose. For instance, `dd-trace` allows you
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to send the traces to a logger or directly to a running Datadog agent.
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```java
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// Initialize the Datadog Java Tracer to write traces to the log:
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Tracer tracer = new DDTracer();
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```
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After a tracer is created, you will usually want to register it with the `GlobalTracer`
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to make it accessible all OpenTracing instrumentation in your JVM.
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```java
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io.opentracing.util.GlobalTracer.register(tracer);
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```
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### Spans
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Once a tracer is instantiated, you can use it to create and manage span. OpenTracing defines a SpanBuilder
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accessible through the method `buildSpan(String operationName)` to serve this purpose.
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```java
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// Create a new Span with the operation name "componentTracking"
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ActiveSpan current = tracer.buildSpan("componentTracking").startActive(true);
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```
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This example creates a simple span referenced "componentTracking". The `startActive()` method starts a new span and sets it
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as the active span. This means that any new span created going forward on the same thread will reference this as its' parent.
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If another span is already active, the new span will replace it.
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**A collection of related spans is called a trace.**
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Sometimes you need to create a span without promoting it as the active. If you want to do that, use the `startManual()`
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method instead.
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```java
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// Create a span, but do not promoting it as the active span
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Span anotherSpan = tracer.buildSpan("componentTracking").start();
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```
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Typically, span creations are made in the beginning of the methods you want to trace.
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And of course, you need to finish/close the span in order to get the operation duration.
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This is achieved using the `finish` method.
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```java
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// Finishing the tracing operation
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current.finish()
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```
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**Be careful!!** All children spans must be finished/closed before finishing the root span.
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If child spans are unfinished when the parent attempts to finish, the span will remain incomplete and risk being unreported.
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Now, you are able to create, start and stop very simple spans.
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You can manipulate them at any time and add extra contextual information using the tags.
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### Tags
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Tags are local to each span and no tags will be inherited from the parent. Information relevant to all spans in a trace
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should be stored as [baggage](#baggage).
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OpenTracing defines a [standard set of tags](https://github.com/opentracing/specification/blob/master/semantic_conventions.md#standard-span-tags-and-log-fields) and should be used appropriately. Custom tags can also be defined as needed.
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```java
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// Create a span and set it as the active span
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ActiveSpan valuableSpan = tracer.
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buildSpan("componentTracking")
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.withTag("custom-meta", "some-useful-value")
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.withTag(Tags.COMPONENT.getKey(), "my-component-mysql")
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.startActive(true);
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// Somewhere further in the code
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Tags.HTTP_URL.setTag(valuableSpan, "https://my-endpoint/resource/item");
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Tags.HTTP_STATUS.setTag(valuableSpan, 200);
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```
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### Baggage
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Information relevant for the entire trace is stored as baggage.
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Baggage is very similar to tags, but has important distinctions. Baggage is:
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* Associated with all spans for a trace.
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* Propagated outside the trace context via HTTP or Messaging protocols (depends of the tracer implementation).
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```java
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// Like tags, you can add baggage item to the span
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valuableSpan.setBaggageItem("username", "modernmajorgeneral");
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```
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### Errors
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Errors are manually captured in a span by setting the error flag and logging the error attributes.
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```java
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try {
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// your code
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} catch (Exception e) {
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// capture error
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ActiveSpan span = GlobalTracer.get().activeSpan();
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Tags.ERROR.set(span, Boolean.TRUE);
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span.log(Collections.singletonMap(io.opentracing.log.Fields.ERROR_OBJECT, e));
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// recovery code
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}
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```
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## OpenTracing Asynchronous API
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An `ActiveSpan` can generate a `Continuation` as a way of propagating traces from across a thread boundary.
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On the parent thread:
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```java
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Continuation spanContinuation = valuableSpan.capture();
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// pass the continuation onto a new thread
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```
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On a different thread:
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```java
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ActiveSpan valuableSpan = spanContinuation.activate();
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// span is now active on the new thread
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```
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## OpenTracing Cross Process Propagation
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Spans are associated across processes in a trace via the `Tracer.extract` and `Tracer.inject` methods.
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```java
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// On the start of a new trace in an application, associate incoming request with existing traces.
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SpanContext spanCtx = tracer.extract(Format.Builtin.HTTP_HEADERS, someTextMapInstance);
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ActiveSpan currentSpan = tracer.buildSpan("componentTracking").asChildOf(spanCtx).startActive(true);
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```
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```java
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// When making an external call propagate the trace by injecting it into the carrier...
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tracer.inject(currentSpan.context(), Format.Builtin.HTTP_HEADERS, someTextMapInstance);
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```
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