This section explains a few concepts that you need to understand before you get started with tracing.

• In the context of this document, a service is synonymous with an application.

• A span is the basic unit of work done within a single service, on a single host. Every span has a name, starting timestamp, and duration. For example, the work done by a REST endpoint is a span. A span is associated to a single service, but its descendants can belong to different services and hosts.

• A trace contains a collection of spans from one or more services, running on one or more hosts. For example, if you trace a service endpoint that calls another service, then the trace would contain spans from both services. Within a trace, spans are organized as a directed acyclic graph (DAG) and can belong to multiple services, running on multiple hosts. The OpenTracing Data Model describes the details at The OpenTracing Semantic Specification. Spans are automatically created by Helidon as needed during execution of the REST request.

Use the Helidon MP Maven archetype to create a simple project that can be used for the examples in this guide.

mvn -U archetype:generate -DinteractiveMode=false \
    -DarchetypeGroupId=io.helidon.archetypes \
    -DarchetypeArtifactId=helidon-quickstart-mp \
    -DarchetypeVersion=4.4.1 \
    -DgroupId=io.helidon.examples \
    -DartifactId=helidon-quickstart-mp \
    -Dpackage=io.helidon.examples.quickstart.mp

cd helidon-quickstart-mp

First, you need to run the Jaeger tracer. Helidon will communicate with this tracer at runtime.

docker run -d --name jaeger \                  
  -e COLLECTOR_OTLP_ENABLED=true \
  -p 6831:6831/udp \
  -p 6832:6832/udp \
  -p 5778:5778 \
  -p 16686:16686 \
  -p 4317:4317 \
  -p 4318:4318 \
  -p 14250:14250 \
  -p 14268:14268 \
  -p 14269:14269 \
  -p 9411:9411 \
  jaegertracing/all-in-one:1.50

• Run the Jaeger docker image.

http://localhost:16686/search

Update the pom.xml file and add the following Jaeger dependency to the <dependencies> section (not <dependencyManagement>). This will enable Helidon to use Jaeger at the default host and port, localhost:14250.

<dependencies>
    <dependency>
        <groupId>io.helidon.microprofile.tracing</groupId>
        <artifactId>helidon-microprofile-tracing</artifactId>
    </dependency>
    <dependency>
        <groupId>io.helidon.tracing.providers</groupId>
        <artifactId>helidon-tracing-providers-jaeger</artifactId>
    </dependency>
</dependencies>

All spans sent by Helidon to Jaeger need to be associated with a service. Specify the service name below.

tracing.service=helidon-mp-1

mvn package -DskipTests=true
java -jar target/helidon-quickstart-mp.jar

curl http://localhost:8080/greet

{
  "message": "Hello World!"
}

The tracing output data is verbose and can be difficult to interpret using the REST API, especially since it represents a structure of spans. Jaeger provides a web-based UI at http://localhost:16686/search, where you can see a visual representation of the same data and the relationship between spans within a trace. If you see a Lens UI button at the top center then click on it, and it will take you to the specific UI used by this guide.

Click on the UI Find traces button (the search icon) as shown in the image below.

Jaeger UI

The image below shows the trace summary, including start time and duration of each trace. There are several traces, each one generated in response to a curl http://localhost:8080/greet invocation. The oldest trace will have a much longer duration since there is one-time initialization that occurs.

Tracing list view

Click on a trace, and you will see the trace detail page where the spans are listed. You can clearly see the root span and the relationship among all the spans in the trace, along with timing information.

Trace detail page

You can examine span details by clicking on the span row. Refer to the image below, which shows the security span details, including timing information. You can see times for each space relative to the root span. These rows are annotated with Server Start and Server Finish, as shown in the third column.

So far in this tutorial, you have used tracing with JAX-RS without needing to annotate. You can enable tracing on other CDI beans, either at the class level or at the method level, as shown by the following examples.

Tracing at the Method Level

class GreetingProvider {
    @Traced 
    String getMessage() {
        return message.get();
    }
}

curl http://localhost:8080/greet

Tracing at the Class Level

@Traced 
@ApplicationScoped
public class GreetingProvider {

    String getMessage() { 
        return message.get();
    }
}

curl http://localhost:8080/greet

@ApplicationScoped
public class GreetingProvider {
    private final AtomicReference<String> message = new AtomicReference<>();

    @Inject
    public GreetingProvider(@ConfigProperty(name = "app.greeting") String message) {
        this.message.set(message);
    }

    @Traced 
    String getMessage() {
        return getMessage2();
    }

    @Traced 
    String getMessage2() {
        return message.get();
    }

    void setMessage(String message) {
        this.message.set(message);
    }
}

curl http://localhost:8080/greet

Helidon automatically traces across services as long as the services use the same tracer, for example, the same instance of Jaeger. This means a single trace can include spans from multiple services and hosts. OpenTracing uses a SpanContext to propagate tracing information across process boundaries. When you make client API calls, Helidon will internally call OpenTracing APIs to propagate the SpanContext. There is nothing you need to do in your application to make this work.

To demonstrate distributed tracing, you will need to create a second project, where the server listens on port 8081. Create a new root directory to hold this new project, then do the following steps, similar to what you did at the start of this guide:

Create a second service

mvn -U archetype:generate -DinteractiveMode=false \
    -DarchetypeGroupId=io.helidon.archetypes \
    -DarchetypeArtifactId=helidon-quickstart-mp \
    -DarchetypeVersion=4.4.1 \
    -DgroupId=io.helidon.examples \
    -DartifactId=helidon-quickstart-mp-2 \
    -Dpackage=io.helidon.examples.quickstart.mp

cd helidon-quickstart-mp-2

<dependency>
  <groupId>io.helidon.tracing.providers</groupId>
  <artifactId>helidon-tracing-providers-jaeger</artifactId>
</dependency>

app.greeting=Hello From MP-2
tracing.service=helidon-mp-2

# Microprofile server properties
server.port=8081
server.host=0.0.0.0

mvn package -DskipTests=true
java -jar target/helidon-quickstart-mp-2.jar

curl http://localhost:8081/greet

{
  "message": "Hello From MP-2 World!"
}

Modify the first service

Once you have validated that the second service is running correctly, you need to modify the original application to call it.

Replace the GreetResource class with the following code:

@Path("/greet")
@RequestScoped
public class GreetResource {

    @Uri("http://localhost:8081/greet")
    private WebTarget target; 

    private static final JsonBuilderFactory JSON = Json.createBuilderFactory(Map.of());
    private final GreetingProvider greetingProvider;

    @Inject
    public GreetResource(GreetingProvider greetingConfig) {
        this.greetingProvider = greetingConfig;
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public JsonObject getDefaultMessage() {
        return createResponse("World");
    }

    @GET
    @Path("/outbound") 
    public JsonObject outbound() {
        return target.request().accept(MediaType.APPLICATION_JSON_TYPE).get(JsonObject.class);
    }

    private JsonObject createResponse(String who) {
        String msg = String.format("%s %s!", greetingProvider.getMessage(), who);

        return JSON.createObjectBuilder().add("message", msg).build();
    }
}

content_copy Copied

• This is the WebTarget needed to send a request to the second service at port 8081.
• This is the new endpoint that will call the second service.

Build and run the application, then invoke the endpoint and check the response:

curl -i http://localhost:8080/greet/outbound 

content_copy Copied

• The request went to the service on 8080, which then invoked the service at 8081 to get the greeting.

{
  "message": "Hello From MP-2 World!" 
}

content_copy Copied

• Notice the greeting came from the second service.

Refresh the Jaeger UI trace listing page and notice that there is a trace across two services.

Tracing across multiple services detail view

In the image above, you can see that the trace includes spans from two services. You will notice there is a gap before the sixth span, which is a get operation. This is a one-time client initialization delay. Run the /outbound curl command again and look at the new trace to see that the delay no longer exists.

You can now stop your second service, it is no longer used in this guide.

apiVersion: v1
kind: Service
metadata:
  name: jaeger
spec:
  ports:
    - port: 16686
      protocol: TCP
  selector:
    app: jaeger
---
kind: Pod
apiVersion: v1
metadata:
  name: jaeger
  labels:
    app: jaeger
spec:
  containers:
    - name: jaeger
      image: jaegertracing/all-in-one
      imagePullPolicy: IfNotPresent
      ports:
        - containerPort: 16686

kubectl apply -f ./jaeger.yaml

kubectl expose pod jaeger --name=jaeger-external --port=16687 --target-port=16686 --type=LoadBalancer 

• Create a service so that you can access the Jaeger UI.

Navigate to http://localhost:16687/search to validate that you can access Jaeger running in Kubernetes. It may take a few seconds before it is ready.

kind: Service
apiVersion: v1
metadata:
  name: helidon-tracing 
  labels:
    app: helidon-tracing
spec:
  type: NodePort
  selector:
    app: helidon-tracing
  ports:
    - port: 8080
      targetPort: 8080
      name: http
---
kind: Deployment
apiVersion: apps/v1
metadata:
  name: helidon-tracing
spec:
  replicas: 1 
  selector:
    matchLabels:
      app: helidon-tracing
  template:
    metadata:
      labels:
        app: helidon-tracing
        version: v1
    spec:
      containers:
        - name: helidon-tracing
          image: helidon-tracing-mp
          imagePullPolicy: IfNotPresent
          ports:
            - containerPort: 8080

• A service of type NodePort that serves the default routes on port 8080.
• A deployment with one replica of a pod.

kubectl apply -f ./tracing.yaml

kubectl get service/helidon-tracing

NAME             TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)          AGE
helidon-tracing   NodePort   10.99.159.2   <none>        8080:31143/TCP   8s 

• A service of type NodePort that serves the default routes on port 31143.

curl http://localhost:31143/greet

{
  "message": "Hello World!"
}

Access the Jaeger UI at http://localhost:16687/search and click on the refresh icon to see the trace that was just created.

You can now delete the Kubernetes resources that were just created during this example.

kubectl delete -f ./jaeger.yaml
kubectl delete -f ./tracing.yaml
kubectl delete service jaeger-external
docker rm -f jaeger

A listener cannot affect the lifecycle of a span or scope it is notified about, but it can add tags and events and update the baggage associated with a span. Often a listener does additional work that does not change the span or scope such as logging a message.

When Helidon invokes the listener’s methods it passes proxies for the Span.Builder, Span, and Scope arguments. These proxies limit the access the listener has to the span builder, span, or scope, as summarized in the following table. If a listener method tries to invoke a forbidden operation, the proxy throws a SpanListener.ForbiddenOperationException and Helidon then logs a WARNING message describing the invalid operation invocation.

The following tables list specifically what operations the proxies permit.

† Helidon returns the unwrapped object, not a proxy for it.

† Helidon returns the unwrapped object, not a proxy to it.

Explicitly Registering a Listener on a Tracer

Automatically Registering a Listener on all Tracer Instances