Tracer is a library that manages custom trace identifiers and carries them through distributed systems. Traditionally traces are transported as custom HTTP headers, usually generated by clients on the very first request. Traces are added to any subsequent requests and responses, especially to transitive dependencies. Having a consistent trace across different services in a system allows to correlate requests and responses beyond the traditional single client-server communication.

This library historically originates from a closed-source implementation called Flow-ID. The goal was to create a clean open source version in which we could get rid of all the drawbacks of the old implementation, e.g. strong-coupling to internal libraries, single hard-coded header and limited testability.

• Status: Under development and used in production

• Tracing of HTTP requests and responses
• Customization by having a pluggable trace format and lifecycle listeners for easy integration
• Support for Servlet containers, Apache’s HTTP client, and (via its elegant API) other frameworks
• Convenient Spring Boot Auto Configuration
• Sensible defaults

• Java 8
• Any build tool using Maven Central, or direct download
• Servlet Container (optional)
• Apache HTTP Client (optional)
• Spring Boot (optional)

Selectively add the following dependencies to your project:

<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-core</artifactId>
    <version>${tracer.version}</version>
</dependency>
<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-servlet</artifactId>
    <version>${tracer.version}</version>
</dependency>
<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-httpclient</artifactId>
    <version>${tracer.version}</version>
</dependency>
<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-hystrix</artifactId>
    <version>${tracer.version}</version>
</dependency>
<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-aspectj</artifactId>
    <version>${tracer.version}</version>
</dependency>
<dependency>
    <groupId>org.zalando</groupId>
    <artifactId>tracer-spring-boot-starter</artifactId>
    <version>${tracer.version}</version>
</dependency>

After adding the dependency, create a Tracer and specify the name of the traces you want to manage:

Tracer tracer = Tracer.create("X-Trace-ID");

If you need access to the current trace's value, call getValue() on it:

Trace trace = tracer.get("X-Trace-ID"); // this is a live-view that can be a shared as a singleton
entity.setLastModifiedBy(trace.getValue());

By default there can only be one trace active at a time. Sometimes it can be useful to stack traces:

Tracer tracer = Tracer.builder()
    .stacked(true)
    .trace("X-Trace-ID")
    .build();

When starting a new trace, Tracer will create trace value by means of pre-configured generator. You can override this on a per-trace level by adding the following to your setup:

Tracer tracer = Tracer.builder()
        .trace("X-Trace-ID", new CustomGenerator())
        .build();

There are several generator implementations included.

This is the default generator implementation. It creates 36 characters long, random-based UUID string as a trace value.

This generator was created for historical reasons. It basically renders a UUID as a base64-encoded byte array, e.g. REcCvlqMSReeo7adheiYFA. The length of generated value is 22 characters.

Phrase generator provides over 10^9 different phrases like:

tender_goodall_likes_evil_panini
nostalgic_boyd_helps_agitated_noyce
pensive_allen_tells_fervent_einstein

The generated phrase is 22 to 61 character long.

For some use cases, e.g. integration with other frameworks and libraries, it might be useful to register a listener that gets notified every time a trace is either started or stopped.

Tracer tracer = Tracer.builder()
        .trace("X-Trace-ID")
        .listener(new CustomTraceListener())
        .build();

Tracer comes with a very useful listener by default, the MDCTraceListener:

Tracer tracer = Tracer.builder()
        .trace("X-Trace-ID")
        .listener(new MDCTraceListener())
        .build();

It allows you to add the trace id to every log line:

<PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} [%X{X-Trace-ID}] - %msg%n"/>

Another built-in listener is the LoggingTraceListener which logs the start and end of every trace.

On the server side is a single filter that you must be register in your filter chain. Make sure it runs very early — otherwise you might miss some crucial information when debugging.

You have to register the TracerFilter as a Filter in your filter chain:

context.addFilter("TracerFilter", new TracerFilter(tracer))
    .addMappingForUrlPatterns(EnumSet.of(REQUEST, ASYNC, ERROR), true, "/*");

Many client-side HTTP libraries on the JVM use the Apache HTTPClient, which is why Tracer comes with a request interceptor:

DefaultHttpClient client = new DefaultHttpClient();
client.addRequestInterceptor(new TracerHttpRequestInterceptor(tracer));

Tracer comes with built-in Hystrix support in form of a custom HystrixConcurrencyStrategy:

final HystrixPlugins plugins = HystrixPlugins.getInstance();
final HystrixConcurrencyStrategy delegate = HystrixConcurrencyStrategyDefault.getInstance(); // or another
plugins.registerConcurrencyStrategy(new TracerConcurrencyStrategy(tracer, delegate));

For background jobs and tests, you can use the built-in aspect:

@Traced
public void performBackgroundJob() {
    // do work
}

or you can manage the lifecycle yourself:

tracer.start();

try {
    // do work
} finally {
    tracer.stop();
}

Tracer comes with a convenient auto configuration for Spring Boot users that sets up aspect, servlet filter and MDC support automatically with sensible defaults:

tracer:
    aspect.enabled: true
    async.enabled: true
    filter.enabled: true
    logging:
        enabled: false
        category: org.zalando.tracer.Tracer
    mdc.enabled: true
    scheduling.enabled: true
    traces:
        X-Trace-ID: uuid
        X-Flow-ID: flow-id

The TracerAutoConfiguration will automatically pick up any TraceListener bound in the application context.

Be aware: The TracerAutoConfiguration will, as long as tracer.async.enabled is set to true, register a AsyncConfigurer and Spring only allows one per Application Context. If this turns out to be a problem please disable the async support and configure it manually. Let us know if you think this needs to be improved.

If you have questions, concerns, bug reports, etc., please file an issue in this repository's Issue Tracker.

To contribute, simply make a pull request and add a brief description (1-2 sentences) of your addition or change. For more details, check the contribution guidelines.

• Zipkin Support (debatable)