<commit_before>:github-tag: master
:github-repo: spring-cloud/spring-cloud-sleuth
:github-raw: http://raw.github.com/{github-repo}/{github-tag}
:github-code: http://github.com/{github-repo}/tree/{github-tag}
:toc: left
:nofooter:

Spring Cloud Sleuth
====================
<NAME>, <NAME>, <NAME>, <NAME>

*{spring-cloud-version}*

:doctype: book

include::intro.adoc[]

include::features.adoc[]

== Sampling

In distributed tracing the data volumes can be very high so sampling
can be important (you usually don't need to export all spans to get a
good picture of what is happening). Spring Cloud Sleuth has a
`Sampler` strategy that you can implement to take control of the
sampling algorithm. Samplers do not stop span (correlation) ids from
being generated, but they do prevent the tags and events being
attached and exported. By default you get a strategy that continues to
trace if a span is already active, but new ones are always marked as
non-exportable. If all your apps run with this sampler you will see
traces in logs, but not in any remote store. For testing the default
is often enough, and it probably is all you need if you are only using
the logs (e.g. with an ELK aggregator). If you are exporting span data
to Zipkin or Spring Cloud Stream, there is also an `AlwaysSampler`
that exports everything and a `PercentageBasedSampler` that samples a
fixed fraction of spans.

NOTE: the `PercentageBasedSampler` is the default if you are using
`spring-cloud-sleuth-zipkin` or `spring-cloud-sleuth-stream`. You can
configure the exports using `spring.sleuth.sampler.percentage`.

A sampler can be installed just by creating a bean definition, e.g:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=always_sampler,indent=0]
----

== Instrumentation

Spring Cloud Sleuth instruments all your Spring application
automatically, so you shouldn't have to do anything to activate
it. The instrumentation is added using a variety of technologies
according to the stack that is available, e.g. for a servlet web
application we use a `Filter`, and for Spring Integration we use
`ChannelInterceptors`.

You can customize the keys used in span tags. To limit the volume of
span data, by default an HTTP request will be tagged only with a
handful of metadata like the status code, host and URL. You can add
request headers by configuring `spring.sleuth.keys.http.headers` (a
list of header names).

NOTE: Remember that tags are only collected and exported if there is a
`Sampler` that allows it (by default there is not, so there is no
danger of accidentally collecting too much data without configuring
something).

NOTE: Currently the instrumentation in Spring Cloud Sleuth is eager - it means that
we're actively trying to pass the tracing context between threads. Also timing events
are captured even when sleuth isn't exporting data to a tracing system.
This approach may change in the future towards being lazy on this matter.

== Span lifecycle

You can do the following operations on the Span by means of *Tracer* interface:

- <<creating-and-closing-spans, start>> - when you start a span its name is assigned and start timestamp is recorded.
- <<creating-and-closing-spans, close>> - the span gets finished (the end time of the span is recorded) and if
the span is *exportable* then it will be eligible for collection to Zipkin.
The span is also removed from the current thread.
- <<continuing-spans, continue>> - a new instance of span will be created whereas it will be a copy of the
one that it continues.
- <<continuing-spans, detach>> - the span doesn't get stopped or closed. It only gets removed from the current thread.
- <<creating-spans-with-explicit-parent, create with explicit parent>> - you can create a new span and set an explicit parent to it

=== Creating and closing spans [[creating-and-closing-spans]]

You can manually create spans by using the *Tracer* interface.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_creation,indent=0]
----

In this example we could see how to create a new instance of span. Assuming that there already
was a span present in this thread then it would become the parent of that span.

IMPORTANT: Always clean after you create a span! Don't forget to close a span if you want to send it to Zipkin.

=== Continuing spans [[continuing-spans]]

Sometimes you don't want to create a new span but you want to continue one. Example of such a
situation might be (of course it all depends on the use-case):

    - *AOP* - If there was already a span created before an aspect was reached then you might not want to create a new span.
    - *Hystrix* - executing a Hystrix command is most likely a logical part of the current processing. It's in fact
    only a technical implementation detail that you wouldn't necessarily want to reflect in tracing as a separate being.

The continued instance of span is equal to the one that it continues:
[source,java]
----
Span continuedSpan = this.tracer.continueSpan(spanToContinue);
assertThat(continuedSpan).isEqualTo(spanToContinue);
----

To continue a span you can use the *Tracer* interface.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_continuation,indent=0]
----

IMPORTANT: Always clean after you create a span! Don't forget to detach a span if some work was done started in one
 thread (e.g. thread X) and it's waiting for other threads (e.g. Y, Z) to finish.
 Then the spans in the threads Y, Z should be detached at the end of their work. When the results are collected
 the span in thread X should be closed.

=== Creating spans with an explicit parent [[creating-spans-with-explicit-parent]]

There is a possibility that you want to start a new span and provide an explicit parent of that span.
Let's assume that the parent of a span is in one thread and you want to start a new span in another thread. The
`startSpan` method of the `Tracer` interface is the method you are looking for.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_joining,indent=0]
----

IMPORTANT: After having created such a span remember to close it. Otherwise you will see a lot of warnings in your logs
 related to the fact that you have a span present in the current thread other than the one you're trying to close.
 What's worse your spans won't get closed properly thus will not get collected to Zipkin.

== Naming spans

Picking a span name is not a trivial task. Span name should depict an operation name. The name should
be low cardinality (e.g. not include identifiers).

Since there is a lot of instrumentation going on some of the span names will be
artificial like:

- `controller-method-name` when received by a Controller with a method name `conrollerMethodName`
- `async` for asynchronous operations done via wrapped `Callable` and `Runnable`.
- `@Scheduled` annotated methods will return the simple name of the class.

Fortunately, for the asynchronous processing you can provide explicit naming.

=== @SpanName annotation

You can do name the span explicitly via the `@SpanName` annotation.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_annotation,indent=0]
----

In this case, when processed in the following manner:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_annotated_runnable_execution,indent=0]
----

The span will be named `calculateTax`.

=== toString() method

It's pretty rare to create separate classes for `Runnable` or `Callable`. Typically one creates an anonymous
instance of those classes. You can't annotate such classes thus to override that, if there is no `@SpanName` annotation present,
we're checking if the class has a custom implementation of the `toString()` method.

So executing such code:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_to_string_runnable_execution,indent=0]
----

will lead in creating a span named `calculateTax`.

== Customizations

Thanks to the `SpanInjector` and `SpanExtractor` you can customize the way spans
are created and propagated.

There are currently two built-in ways to pass tracing information between processes:

 * via Spring Integration
 * via HTTP

Span ids are extracted from Zipkin-compatible (B3) headers (either `Message`
or HTTP headers), to start or join an existing trace. Trace information is
injected into any outbound requests so the next hop can extract them.

=== Spring Integration

For Spring Integration these are the beans responsible for creation of a Span from a `Message`
 and filling in the `MessageBuilder` with tracing information.

[source,java]
----
@Bean
public SpanExtractor<Message> messagingSpanExtractor() {
    ...
}

@Bean
public SpanInjector<MessageBuilder> messagingSpanInjector() {
    ...
}
----

You can override them by providing your own implementation and by adding a `@Primary` annotation
to your bean definition.

=== HTTP

For HTTP these are the beans responsible for creation of a Span from a `HttpServletRequest`
  and filling in the `HttpServletResponse` with tracing information.

[source,java]
----
@Bean
public SpanExtractor<HttpServletRequest> httpServletRequestSpanExtractor() {
    ...
}

@Bean
public SpanInjector<HttpServletResponse> httpServletResponseSpanInjector() {
    ...
}
----

You can override them by providing your own implementation and by adding a `@Primary` annotation
to your bean definition.

=== Example

Let's assume that instead of the standard Zipkin compatible tracing HTTP header names
you have

* for trace id - `correlationId`
* for span id - `mySpanId`

This is a an example of a `SpanExtractor`

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=extractor,indent=0]
----

The following `SpanInjector` could be created

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=injector,indent=0]
----

And you could register them like this:

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=configuration,indent=0]
----

=== Custom SA tag in Zipkin

Sometimes you want to create a manual Span that will wrap a call to an external service which is not instrumented.
What you can do is to create a span with the `peer.service` tag that will contain a value of the service that you want to call.
Below you can see an example of a call to Redis that is wrapped in such a span.

[source,java]
----
include::../../../..//spring-cloud-sleuth-zipkin/src/test/java/org/springframework/cloud/sleuth/zipkin/HttpZipkinSpanReporterTest.java[tags=service_name,indent=0]
----

IMPORTANT: Remember not to add both `peer.service` tag and the `SA` tag! You have to add only `peer.service`.

== Span Data as Messages

You can accumulate and send span data over
http://cloud.spring.io/spring-cloud-stream[Spring Cloud Stream] by
including the `spring-cloud-sleuth-stream` jar as a dependency, and
adding a Channel Binder implementation
(e.g. `spring-cloud-starter-stream-rabbit` for RabbitMQ or
`spring-cloud-starter-stream-kafka` for Kafka). This will
automatically turn your app into a producer of messages with payload
type `Spans`.

=== Zipkin Consumer

There is a special convenience annotation for setting up a message consumer
for the Span data and pushing it into a Zipkin `SpanStore`. This application

[source,java]
----
include::../../../../spring-cloud-sleuth-zipkin-stream/src/test/java/org/springframework/cloud/sleuth/zipkin/stream/documentation/Consumer.java[tags=zipkin_consumer,indent=0]
----

will listen for the Span data on whatever transport you provide via a
Spring Cloud Stream `Binder` (e.g. include
`spring-cloud-starter-stream-rabbit` for RabbitMQ, and similar
starters exist for Redis and Kafka). If you add the following UI dependency

[source,xml]
----
<groupId>io.zipkin.java</groupId>
<artifactId>zipkin-autoconfigure-ui</artifactId>
----

Then you'll have your app a
https://github.com/openzipkin/zipkin[Zipkin server], which hosts
the UI and api on port 9411.


The default `SpanStore` is in-memory (good for demos and getting
started quickly). For a more robust solution you can add MySQL and
`spring-boot-starter-jdbc` to your classpath and enable the JDBC
`SpanStore` via configuration, e.g.:

[source,yaml]
----
spring:
  rabbitmq:
    host: ${RABBIT_HOST:localhost}
  datasource:
    schema: classpath:/mysql.sql
    url: jdbc:mysql://${MYSQL_HOST:localhost}/test
    username: root
    password: <PASSWORD>
# Switch this on to create the schema on startup:
    initialize: true
    continueOnError: true
  sleuth:
    enabled: false
zipkin:
  storage:
    type: mysql
----

NOTE: The `@EnableZipkinStreamServer` is also annotated with
`@EnableZipkinServer` so the process will also expose the standard
Zipkin server endpoints for collecting spans over HTTP, and for
querying in the Zipkin Web UI.

=== Custom Consumer

A custom consumer can also easily be implemented using
`spring-cloud-sleuth-stream` and binding to the `SleuthSink`. Example:

[source,java]
----
@EnableBinding(SleuthSink.class)
@SpringBootApplication(exclude = SleuthStreamAutoConfiguration.class)
@MessageEndpoint
public class Consumer {

    @ServiceActivator(inputChannel = SleuthSink.INPUT)
    public void sink(Spans input) throws Exception {
        // ... process spans
    }
}
----

NOTE: the sample consumer application above explicitly excludes
`SleuthStreamAutoConfiguration` so it doesn't send messages to itself,
but this is optional (you might actually want to trace requests into
the consumer app).

== Metrics

Currently Spring Cloud Sleuth registers very simple metrics related to spans.
It's using the http://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-metrics.html#production-ready-recording-metrics[Spring Boot's metrics support]
to calculate the number of accepted and dropped spans. Each time a span gets
sent to Zipkin the number of accepted spans will increase. If there's an error then
the number of dropped spans will get increased.

== Integrations

=== Runnable and Callable

If you're wrapping your logic in `Runnable` or `Callable` it's enough to wrap those classes in their Sleuth representative.

Example for `Runnable`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=trace_runnable,indent=0]
----

Example for `Callable`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=trace_callable,indent=0]
----

That way you will ensure that a new Span is created and closed for each execution.

=== Hystrix

==== Custom Concurrency Strategy

We're registering a custom https://github.com/Netflix/Hystrix/wiki/Plugins#concurrencystrategy[`HystrixConcurrencyStrategy`]
that wraps all `Callable` instances into their Sleuth representative -
the `TraceCallable`. The strategy either starts or continues a span depending on the fact whether tracing was already going
on before the Hystrix command was called. To disable the custom Hystrix Concurrency Strategy set the `spring.sleuth.hystrix.strategy.enabled` to `false`.

==== Manual Command setting

Assuming that you have the following `HystrixCommand`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java[tags=hystrix_command,indent=0]
----

In order to pass the tracing information you have to wrap the same logic in the Sleuth version of the `HystrixCommand` which is the
`TraceCommand`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java[tags=trace_hystrix_command,indent=0]
----

=== RxJava

We're registering a custom https://github.com/ReactiveX/RxJava/wiki/Plugins#rxjavaschedulershook[`RxJavaSchedulersHook`]
that wraps all `Action0` instances into their Sleuth representative -
the `TraceAction`. The hook either starts or continues a span depending on the fact whether tracing was already going
on before the Action was scheduled. To disable the custom RxJavaSchedulersHook set the `spring.sleuth.rxjava.schedulers.hook.enabled` to `false`.

You can define a list of regular expressions for thread names, for which you don't want a Span to be created. Just provide a comma separated list
of regular expressions in the `spring.sleuth.rxjava.schedulers.ignoredthreads` property.

=== HTTP integration

Features from this section can be disabled by providing the `spring.sleuth.web.enabled` property with value equal to `false`.

==== HTTP Filter

Via the `TraceFilter` all sampled incoming requests result in creation of a Span. That Span's name is `http:` + the path to which
 the request was sent. E.g. if the request was sent to `/foo/bar` then the name will be `http:/foo/bar`. You can configure which URIs you would
 like to skip via the `spring.sleuth.web.skipPattern` property. If you have `ManagementServerProperties` on classpath then
 its value of `contextPath` gets appended to the provided skip pattern.

==== HandlerInterceptor

Since we want the span names to be precise we're using a `TraceHandlerInterceptor` that either wraps an
 existing `HandlerInterceptor` or is added directly to the list of existing `HandlerInterceptors`. The
 `TraceHandlerInterceptor` adds a special request attribute to the given `HttpServletRequest`. If the
 the `TraceFilter` doesn't see this attribute set it will create a "fallback" span which is an additional
 span created on the server side so that the trace is presented properly in the UI. Seeing that most likely
 signifies that there is a missing instrumentation. In that case please file an issue in Spring Cloud Sleuth.

==== Async Servlet support

If your controller returns a `Callable` or a `WebAsyncTask` Spring Cloud Sleuth will continue the existing span instead of creating a new one.

=== HTTP client integration

==== Synchronous Rest Template

We're injecting a `RestTemplate` interceptor that ensures that all the tracing information is passed to the requests. Each time a
call is made a new Span is created. It gets closed upon receiving the response. In order to block the synchronous `RestTemplate` features
just set `spring.sleuth.web.client.enabled` to `false`.

IMPORTANT: You have to register `RestTemplate` as a bean so that the interceptors will get injected.
If you create a `RestTemplate` instance with a `new` keyword then the instrumentation WILL NOT work.

==== Asynchronous Rest Template

Custom instrumentation is set to create and close Spans upon sending and receiving requests. You can customize the `ClientHttpRequestFactory`
and the `AsyncClientHttpRequestFactory` by registering your beans. Remember to use tracing compatible implementations (e.g. don't forget to
wrap `ThreadPoolTaskScheduler` in a `TraceAsyncListenableTaskExecutor`). Example of custom request factories:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/client/TraceWebAsyncClientAutoConfigurationTests.java[tags=async_template_factories,indent=0]
----

To block the `AsyncRestTemplate` features set `spring.sleuth.web.async.client.enabled` to `false`.
To disable creation of the default `TraceAsyncClientHttpRequestFactoryWrapper` set `spring.sleuth.web.async.client.factory.enabled`
to `false`. If you don't want to create `AsyncRestClient` at all set `spring.sleuth.web.async.client.template.enabled` to `false`.

=== Feign

By default Spring Cloud Sleuth provides integration with feign via the `TraceFeignClientAutoConfiguration`. You can disable it entirely
by setting `spring.sleuth.feign.enabled` to false. If you do so then no Feign related instrumentation will take place.

Part of Feign instrumentation is done via a `FeignBeanPostProcessor`. You can disable it by providing the `spring.sleuth.feign.processor.enabled` equal to `false`.
If you set it like this then Spring Cloud Sleuth will not instrument any of your custom Feign components. All the default instrumentation
however will be still there.

=== Asynchronous communication

==== @Async annotated methods

In Spring Cloud Sleuth we're instrumenting async related components so that the tracing information is passed between threads.
You can disable this behaviour by setting the value of `spring.sleuth.async.enabled` to `false`.

If you annotate your method with `@Async` then we'll automatically create a new Span with the following characteristics:

    - the Span name will be the annotated method name
    - the Span will be tagged with that method's class name and the method name too

==== @Scheduled annotated methods

In Spring Cloud Sleuth we're instrumenting scheduled method execution so that the tracing information is passed between threads. You can disable this behaviour
by setting the value of `spring.sleuth.scheduled.enabled` to `false`.

If you annotate your method with `@Scheduled` then we'll automatically create a new Span with the following characteristics:

    - the Span name will be the annotated method name
    - the Span will be tagged with that method's class name and the method name too

If you want to skip Span creation for some `@Scheduled` annotated classes you can set the
`spring.sleuth.scheduled.skipPattern` with a regular expression that will match the fully qualified name of the
`@Scheduled` annotated class.

==== Executor, ExecutorService and ScheduledExecutorService

We're providing `LazyTraceExecutor`, `TraceableExecutorService` and `TraceableScheduledExecutorService`. Those implementations
are creating Spans each time a new task is submitted, invoked or scheduled.

Here you can see an example of how to pass tracing information with `TraceableExecutorService` when working with `CompletableFuture`:

[source,java]
----

include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/async/TraceableExecutorServiceTests.java[tags=completablefuture,indent=0]
----

=== Messaging

Spring Cloud Sleuth integrates with http://projects.spring.io/spring-integration/[Spring Integration]. It creates spans for publish and
subscribe events. To disable Spring Integration instrumentation, set `spring.sleuth.integration.enabled` to false.

Spring Cloud Sleuth up till version 1.0.4 is sending invalid tracing headers when using messaging. Those headers are actually
the same as the ones sent in HTTP (they contain a `-`) in its name. For the sake of
backwards compatibility in 1.0.4 we've started sending both valid and invalid headers. Please upgrade to 1.0.4 because
in Spring Cloud Sleuth 1.1 we will remove the support for the deprecated headers.

Since 1.0.4 you can provide the `spring.sleuth.integration.patterns` pattern to explicitly
provide the names of channels that you want to include for tracing. By default all channels
are included.

=== Zuul

We're registering Zuul filters to propagate the tracing information (the request header is enriched with tracing data).
To disable Zuul support set the `spring.sleuth.zuul.enabled` property to `false`.

== Running examples

You can find the running examples deployed in the https://run.pivotal.io/[Pivotal Web Services]. Check them out in the following links:

- http://docssleuth-zipkin-server.cfapps.io/[Zipkin for apps presented in the samples to the top]
- http://docsbrewing-zipkin-web.cfapps.io/[Zipkin for Brewery on PWS], its https://github.com/spring-cloud-samples/brewery[Github Code]
<commit_msg>Added more explicit Tracer explanation<commit_after>:github-tag: master
:github-repo: spring-cloud/spring-cloud-sleuth
:github-raw: http://raw.github.com/{github-repo}/{github-tag}
:github-code: http://github.com/{github-repo}/tree/{github-tag}
:toc: left
:nofooter:

Spring Cloud Sleuth
====================
<NAME>, <NAME>, <NAME>, <NAME>

*{spring-cloud-version}*

:doctype: book

include::intro.adoc[]

include::features.adoc[]

== Sampling

In distributed tracing the data volumes can be very high so sampling
can be important (you usually don't need to export all spans to get a
good picture of what is happening). Spring Cloud Sleuth has a
`Sampler` strategy that you can implement to take control of the
sampling algorithm. Samplers do not stop span (correlation) ids from
being generated, but they do prevent the tags and events being
attached and exported. By default you get a strategy that continues to
trace if a span is already active, but new ones are always marked as
non-exportable. If all your apps run with this sampler you will see
traces in logs, but not in any remote store. For testing the default
is often enough, and it probably is all you need if you are only using
the logs (e.g. with an ELK aggregator). If you are exporting span data
to Zipkin or Spring Cloud Stream, there is also an `AlwaysSampler`
that exports everything and a `PercentageBasedSampler` that samples a
fixed fraction of spans.

NOTE: the `PercentageBasedSampler` is the default if you are using
`spring-cloud-sleuth-zipkin` or `spring-cloud-sleuth-stream`. You can
configure the exports using `spring.sleuth.sampler.percentage`.

A sampler can be installed just by creating a bean definition, e.g:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=always_sampler,indent=0]
----

== Instrumentation

Spring Cloud Sleuth instruments all your Spring application
automatically, so you shouldn't have to do anything to activate
it. The instrumentation is added using a variety of technologies
according to the stack that is available, e.g. for a servlet web
application we use a `Filter`, and for Spring Integration we use
`ChannelInterceptors`.

You can customize the keys used in span tags. To limit the volume of
span data, by default an HTTP request will be tagged only with a
handful of metadata like the status code, host and URL. You can add
request headers by configuring `spring.sleuth.keys.http.headers` (a
list of header names).

NOTE: Remember that tags are only collected and exported if there is a
`Sampler` that allows it (by default there is not, so there is no
danger of accidentally collecting too much data without configuring
something).

NOTE: Currently the instrumentation in Spring Cloud Sleuth is eager - it means that
we're actively trying to pass the tracing context between threads. Also timing events
are captured even when sleuth isn't exporting data to a tracing system.
This approach may change in the future towards being lazy on this matter.

== Span lifecycle

You can do the following operations on the Span by means of *org.springframework.cloud.sleuth.Tracer* interface:

- <<creating-and-closing-spans, start>> - when you start a span its name is assigned and start timestamp is recorded.
- <<creating-and-closing-spans, close>> - the span gets finished (the end time of the span is recorded) and if
the span is *exportable* then it will be eligible for collection to Zipkin.
The span is also removed from the current thread.
- <<continuing-spans, continue>> - a new instance of span will be created whereas it will be a copy of the
one that it continues.
- <<continuing-spans, detach>> - the span doesn't get stopped or closed. It only gets removed from the current thread.
- <<creating-spans-with-explicit-parent, create with explicit parent>> - you can create a new span and set an explicit parent to it

TIP: Spring creates the instance of `Tracer` for you. In order to use it all you need is to just autowire it.

=== Creating and closing spans [[creating-and-closing-spans]]

You can manually create spans by using the *Tracer* interface.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_creation,indent=0]
----

In this example we could see how to create a new instance of span. Assuming that there already
was a span present in this thread then it would become the parent of that span.

IMPORTANT: Always clean after you create a span! Don't forget to close a span if you want to send it to Zipkin.

=== Continuing spans [[continuing-spans]]

Sometimes you don't want to create a new span but you want to continue one. Example of such a
situation might be (of course it all depends on the use-case):

    - *AOP* - If there was already a span created before an aspect was reached then you might not want to create a new span.
    - *Hystrix* - executing a Hystrix command is most likely a logical part of the current processing. It's in fact
    only a technical implementation detail that you wouldn't necessarily want to reflect in tracing as a separate being.

The continued instance of span is equal to the one that it continues:
[source,java]
----
Span continuedSpan = this.tracer.continueSpan(spanToContinue);
assertThat(continuedSpan).isEqualTo(spanToContinue);
----

To continue a span you can use the *Tracer* interface.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_continuation,indent=0]
----

IMPORTANT: Always clean after you create a span! Don't forget to detach a span if some work was done started in one
 thread (e.g. thread X) and it's waiting for other threads (e.g. Y, Z) to finish.
 Then the spans in the threads Y, Z should be detached at the end of their work. When the results are collected
 the span in thread X should be closed.

=== Creating spans with an explicit parent [[creating-spans-with-explicit-parent]]

There is a possibility that you want to start a new span and provide an explicit parent of that span.
Let's assume that the parent of a span is in one thread and you want to start a new span in another thread. The
`startSpan` method of the `Tracer` interface is the method you are looking for.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=manual_span_joining,indent=0]
----

IMPORTANT: After having created such a span remember to close it. Otherwise you will see a lot of warnings in your logs
 related to the fact that you have a span present in the current thread other than the one you're trying to close.
 What's worse your spans won't get closed properly thus will not get collected to Zipkin.

== Naming spans

Picking a span name is not a trivial task. Span name should depict an operation name. The name should
be low cardinality (e.g. not include identifiers).

Since there is a lot of instrumentation going on some of the span names will be
artificial like:

- `controller-method-name` when received by a Controller with a method name `conrollerMethodName`
- `async` for asynchronous operations done via wrapped `Callable` and `Runnable`.
- `@Scheduled` annotated methods will return the simple name of the class.

Fortunately, for the asynchronous processing you can provide explicit naming.

=== @SpanName annotation

You can do name the span explicitly via the `@SpanName` annotation.

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_annotation,indent=0]
----

In this case, when processed in the following manner:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_annotated_runnable_execution,indent=0]
----

The span will be named `calculateTax`.

=== toString() method

It's pretty rare to create separate classes for `Runnable` or `Callable`. Typically one creates an anonymous
instance of those classes. You can't annotate such classes thus to override that, if there is no `@SpanName` annotation present,
we're checking if the class has a custom implementation of the `toString()` method.

So executing such code:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=span_name_to_string_runnable_execution,indent=0]
----

will lead in creating a span named `calculateTax`.

== Customizations

Thanks to the `SpanInjector` and `SpanExtractor` you can customize the way spans
are created and propagated.

There are currently two built-in ways to pass tracing information between processes:

 * via Spring Integration
 * via HTTP

Span ids are extracted from Zipkin-compatible (B3) headers (either `Message`
or HTTP headers), to start or join an existing trace. Trace information is
injected into any outbound requests so the next hop can extract them.

=== Spring Integration

For Spring Integration these are the beans responsible for creation of a Span from a `Message`
 and filling in the `MessageBuilder` with tracing information.

[source,java]
----
@Bean
public SpanExtractor<Message> messagingSpanExtractor() {
    ...
}

@Bean
public SpanInjector<MessageBuilder> messagingSpanInjector() {
    ...
}
----

You can override them by providing your own implementation and by adding a `@Primary` annotation
to your bean definition.

=== HTTP

For HTTP these are the beans responsible for creation of a Span from a `HttpServletRequest`
  and filling in the `HttpServletResponse` with tracing information.

[source,java]
----
@Bean
public SpanExtractor<HttpServletRequest> httpServletRequestSpanExtractor() {
    ...
}

@Bean
public SpanInjector<HttpServletResponse> httpServletResponseSpanInjector() {
    ...
}
----

You can override them by providing your own implementation and by adding a `@Primary` annotation
to your bean definition.

=== Example

Let's assume that instead of the standard Zipkin compatible tracing HTTP header names
you have

* for trace id - `correlationId`
* for span id - `mySpanId`

This is a an example of a `SpanExtractor`

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=extractor,indent=0]
----

The following `SpanInjector` could be created

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=injector,indent=0]
----

And you could register them like this:

[source,java]
----
include::../../../..//spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterCustomExtractorTests.java[tags=configuration,indent=0]
----

=== Custom SA tag in Zipkin

Sometimes you want to create a manual Span that will wrap a call to an external service which is not instrumented.
What you can do is to create a span with the `peer.service` tag that will contain a value of the service that you want to call.
Below you can see an example of a call to Redis that is wrapped in such a span.

[source,java]
----
include::../../../..//spring-cloud-sleuth-zipkin/src/test/java/org/springframework/cloud/sleuth/zipkin/HttpZipkinSpanReporterTest.java[tags=service_name,indent=0]
----

IMPORTANT: Remember not to add both `peer.service` tag and the `SA` tag! You have to add only `peer.service`.

== Span Data as Messages

You can accumulate and send span data over
http://cloud.spring.io/spring-cloud-stream[Spring Cloud Stream] by
including the `spring-cloud-sleuth-stream` jar as a dependency, and
adding a Channel Binder implementation
(e.g. `spring-cloud-starter-stream-rabbit` for RabbitMQ or
`spring-cloud-starter-stream-kafka` for Kafka). This will
automatically turn your app into a producer of messages with payload
type `Spans`.

=== Zipkin Consumer

There is a special convenience annotation for setting up a message consumer
for the Span data and pushing it into a Zipkin `SpanStore`. This application

[source,java]
----
include::../../../../spring-cloud-sleuth-zipkin-stream/src/test/java/org/springframework/cloud/sleuth/zipkin/stream/documentation/Consumer.java[tags=zipkin_consumer,indent=0]
----

will listen for the Span data on whatever transport you provide via a
Spring Cloud Stream `Binder` (e.g. include
`spring-cloud-starter-stream-rabbit` for RabbitMQ, and similar
starters exist for Redis and Kafka). If you add the following UI dependency

[source,xml]
----
<groupId>io.zipkin.java</groupId>
<artifactId>zipkin-autoconfigure-ui</artifactId>
----

Then you'll have your app a
https://github.com/openzipkin/zipkin[Zipkin server], which hosts
the UI and api on port 9411.


The default `SpanStore` is in-memory (good for demos and getting
started quickly). For a more robust solution you can add MySQL and
`spring-boot-starter-jdbc` to your classpath and enable the JDBC
`SpanStore` via configuration, e.g.:

[source,yaml]
----
spring:
  rabbitmq:
    host: ${RABBIT_HOST:localhost}
  datasource:
    schema: classpath:/mysql.sql
    url: jdbc:mysql://${MYSQL_HOST:localhost}/test
    username: root
    password: <PASSWORD>
# Switch this on to create the schema on startup:
    initialize: true
    continueOnError: true
  sleuth:
    enabled: false
zipkin:
  storage:
    type: mysql
----

NOTE: The `@EnableZipkinStreamServer` is also annotated with
`@EnableZipkinServer` so the process will also expose the standard
Zipkin server endpoints for collecting spans over HTTP, and for
querying in the Zipkin Web UI.

=== Custom Consumer

A custom consumer can also easily be implemented using
`spring-cloud-sleuth-stream` and binding to the `SleuthSink`. Example:

[source,java]
----
@EnableBinding(SleuthSink.class)
@SpringBootApplication(exclude = SleuthStreamAutoConfiguration.class)
@MessageEndpoint
public class Consumer {

    @ServiceActivator(inputChannel = SleuthSink.INPUT)
    public void sink(Spans input) throws Exception {
        // ... process spans
    }
}
----

NOTE: the sample consumer application above explicitly excludes
`SleuthStreamAutoConfiguration` so it doesn't send messages to itself,
but this is optional (you might actually want to trace requests into
the consumer app).

== Metrics

Currently Spring Cloud Sleuth registers very simple metrics related to spans.
It's using the http://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-metrics.html#production-ready-recording-metrics[Spring Boot's metrics support]
to calculate the number of accepted and dropped spans. Each time a span gets
sent to Zipkin the number of accepted spans will increase. If there's an error then
the number of dropped spans will get increased.

== Integrations

=== Runnable and Callable

If you're wrapping your logic in `Runnable` or `Callable` it's enough to wrap those classes in their Sleuth representative.

Example for `Runnable`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=trace_runnable,indent=0]
----

Example for `Callable`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/documentation/SpringCloudSleuthDocTests.java[tags=trace_callable,indent=0]
----

That way you will ensure that a new Span is created and closed for each execution.

=== Hystrix

==== Custom Concurrency Strategy

We're registering a custom https://github.com/Netflix/Hystrix/wiki/Plugins#concurrencystrategy[`HystrixConcurrencyStrategy`]
that wraps all `Callable` instances into their Sleuth representative -
the `TraceCallable`. The strategy either starts or continues a span depending on the fact whether tracing was already going
on before the Hystrix command was called. To disable the custom Hystrix Concurrency Strategy set the `spring.sleuth.hystrix.strategy.enabled` to `false`.

==== Manual Command setting

Assuming that you have the following `HystrixCommand`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java[tags=hystrix_command,indent=0]
----

In order to pass the tracing information you have to wrap the same logic in the Sleuth version of the `HystrixCommand` which is the
`TraceCommand`:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java[tags=trace_hystrix_command,indent=0]
----

=== RxJava

We're registering a custom https://github.com/ReactiveX/RxJava/wiki/Plugins#rxjavaschedulershook[`RxJavaSchedulersHook`]
that wraps all `Action0` instances into their Sleuth representative -
the `TraceAction`. The hook either starts or continues a span depending on the fact whether tracing was already going
on before the Action was scheduled. To disable the custom RxJavaSchedulersHook set the `spring.sleuth.rxjava.schedulers.hook.enabled` to `false`.

You can define a list of regular expressions for thread names, for which you don't want a Span to be created. Just provide a comma separated list
of regular expressions in the `spring.sleuth.rxjava.schedulers.ignoredthreads` property.

=== HTTP integration

Features from this section can be disabled by providing the `spring.sleuth.web.enabled` property with value equal to `false`.

==== HTTP Filter

Via the `TraceFilter` all sampled incoming requests result in creation of a Span. That Span's name is `http:` + the path to which
 the request was sent. E.g. if the request was sent to `/foo/bar` then the name will be `http:/foo/bar`. You can configure which URIs you would
 like to skip via the `spring.sleuth.web.skipPattern` property. If you have `ManagementServerProperties` on classpath then
 its value of `contextPath` gets appended to the provided skip pattern.

==== HandlerInterceptor

Since we want the span names to be precise we're using a `TraceHandlerInterceptor` that either wraps an
 existing `HandlerInterceptor` or is added directly to the list of existing `HandlerInterceptors`. The
 `TraceHandlerInterceptor` adds a special request attribute to the given `HttpServletRequest`. If the
 the `TraceFilter` doesn't see this attribute set it will create a "fallback" span which is an additional
 span created on the server side so that the trace is presented properly in the UI. Seeing that most likely
 signifies that there is a missing instrumentation. In that case please file an issue in Spring Cloud Sleuth.

==== Async Servlet support

If your controller returns a `Callable` or a `WebAsyncTask` Spring Cloud Sleuth will continue the existing span instead of creating a new one.

=== HTTP client integration

==== Synchronous Rest Template

We're injecting a `RestTemplate` interceptor that ensures that all the tracing information is passed to the requests. Each time a
call is made a new Span is created. It gets closed upon receiving the response. In order to block the synchronous `RestTemplate` features
just set `spring.sleuth.web.client.enabled` to `false`.

IMPORTANT: You have to register `RestTemplate` as a bean so that the interceptors will get injected.
If you create a `RestTemplate` instance with a `new` keyword then the instrumentation WILL NOT work.

==== Asynchronous Rest Template

Custom instrumentation is set to create and close Spans upon sending and receiving requests. You can customize the `ClientHttpRequestFactory`
and the `AsyncClientHttpRequestFactory` by registering your beans. Remember to use tracing compatible implementations (e.g. don't forget to
wrap `ThreadPoolTaskScheduler` in a `TraceAsyncListenableTaskExecutor`). Example of custom request factories:

[source,java]
----
include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/client/TraceWebAsyncClientAutoConfigurationTests.java[tags=async_template_factories,indent=0]
----

To block the `AsyncRestTemplate` features set `spring.sleuth.web.async.client.enabled` to `false`.
To disable creation of the default `TraceAsyncClientHttpRequestFactoryWrapper` set `spring.sleuth.web.async.client.factory.enabled`
to `false`. If you don't want to create `AsyncRestClient` at all set `spring.sleuth.web.async.client.template.enabled` to `false`.

=== Feign

By default Spring Cloud Sleuth provides integration with feign via the `TraceFeignClientAutoConfiguration`. You can disable it entirely
by setting `spring.sleuth.feign.enabled` to false. If you do so then no Feign related instrumentation will take place.

Part of Feign instrumentation is done via a `FeignBeanPostProcessor`. You can disable it by providing the `spring.sleuth.feign.processor.enabled` equal to `false`.
If you set it like this then Spring Cloud Sleuth will not instrument any of your custom Feign components. All the default instrumentation
however will be still there.

=== Asynchronous communication

==== @Async annotated methods

In Spring Cloud Sleuth we're instrumenting async related components so that the tracing information is passed between threads.
You can disable this behaviour by setting the value of `spring.sleuth.async.enabled` to `false`.

If you annotate your method with `@Async` then we'll automatically create a new Span with the following characteristics:

    - the Span name will be the annotated method name
    - the Span will be tagged with that method's class name and the method name too

==== @Scheduled annotated methods

In Spring Cloud Sleuth we're instrumenting scheduled method execution so that the tracing information is passed between threads. You can disable this behaviour
by setting the value of `spring.sleuth.scheduled.enabled` to `false`.

If you annotate your method with `@Scheduled` then we'll automatically create a new Span with the following characteristics:

    - the Span name will be the annotated method name
    - the Span will be tagged with that method's class name and the method name too

If you want to skip Span creation for some `@Scheduled` annotated classes you can set the
`spring.sleuth.scheduled.skipPattern` with a regular expression that will match the fully qualified name of the
`@Scheduled` annotated class.

==== Executor, ExecutorService and ScheduledExecutorService

We're providing `LazyTraceExecutor`, `TraceableExecutorService` and `TraceableScheduledExecutorService`. Those implementations
are creating Spans each time a new task is submitted, invoked or scheduled.

Here you can see an example of how to pass tracing information with `TraceableExecutorService` when working with `CompletableFuture`:

[source,java]
----

include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/async/TraceableExecutorServiceTests.java[tags=completablefuture,indent=0]
----

=== Messaging

Spring Cloud Sleuth integrates with http://projects.spring.io/spring-integration/[Spring Integration]. It creates spans for publish and
subscribe events. To disable Spring Integration instrumentation, set `spring.sleuth.integration.enabled` to false.

Spring Cloud Sleuth up till version 1.0.4 is sending invalid tracing headers when using messaging. Those headers are actually
the same as the ones sent in HTTP (they contain a `-`) in its name. For the sake of
backwards compatibility in 1.0.4 we've started sending both valid and invalid headers. Please upgrade to 1.0.4 because
in Spring Cloud Sleuth 1.1 we will remove the support for the deprecated headers.

Since 1.0.4 you can provide the `spring.sleuth.integration.patterns` pattern to explicitly
provide the names of channels that you want to include for tracing. By default all channels
are included.

=== Zuul

We're registering Zuul filters to propagate the tracing information (the request header is enriched with tracing data).
To disable Zuul support set the `spring.sleuth.zuul.enabled` property to `false`.

== Running examples

You can find the running examples deployed in the https://run.pivotal.io/[Pivotal Web Services]. Check them out in the following links:

- http://docssleuth-zipkin-server.cfapps.io/[Zipkin for apps presented in the samples to the top]
- http://docsbrewing-zipkin-web.cfapps.io/[Zipkin for Brewery on PWS], its https://github.com/spring-cloud-samples/brewery[Github Code]

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