Tutorial - Enabling Tracing for a Java Application and Datadog Agent in Containers

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Overview

This tutorial walks you through the steps for enabling tracing on a sample Java application installed in a container. In this scenario, the Datadog Agent is also installed in a container.

For other scenarios, including the application and Agent on a host, the application in a container and Agent on a host, the application and Agent on cloud infrastructure, and on applications written in other languages, see the other Enabling Tracing tutorials.

See Tracing Java Applications for general comprehensive tracing setup documentation for Java.

Prerequisites

A Datadog account and organization API key
Git
Docker
Curl

Install the sample Dockerized Java application

The code sample for this tutorial is on GitHub, at github.com/DataDog/apm-tutorial-java-host. To get started, clone the repository:

git clone https://github.com/DataDog/apm-tutorial-java-host.git

The repository contains a multi-service Java application pre-configured to be run within Docker containers. The sample app is a basic notes app with a REST API to add and change data. The docker-compose YAML files are located in the docker directory.

This tutorial uses the all-docker-compose.yaml file, which builds containers for both the application and the Datadog Agent.

In each of the notes and calendar directories, there are two sets of Dockerfiles for building the applications, either with Maven or with Gradle. This tutorial uses the Maven build, but if you are more familiar with Gradle, you can use it instead with the corresponding changes to build commands.

Starting and exercising the sample application

Build the application’s container by running the following from inside the /docker directory:
```
docker-compose -f all-docker-compose.yaml build notes
```
If the build gets stuck, exit with Ctrl+C and re-run the command.
Start the container:
```
docker-compose -f all-docker-compose.yaml up notes
```
You can verify that it’s running by viewing the running containers with the docker ps command.
Open up another terminal and send API requests to exercise the app. The notes application is a REST API that stores data in an in-memory H2 database running on the same container. Send it a few commands:

curl localhost:8080/notes: []
curl -X POST 'localhost:8080/notes?desc=hello': {"id":1,"description":"hello"}
curl localhost:8080/notes/1: {"id":1,"description":"hello"}
curl localhost:8080/notes: [{"id":1,"description":"hello"}]

Stop the application

After you’ve seen the application running, stop it so that you can enable tracing on it.

Stop the containers:

docker-compose -f all-docker-compose.yaml down

Remove the containers:

docker-compose -f all-docker-compose.yaml rm

Enable tracing

Now that you have a working Java application, configure it to enable tracing.

Add the Java tracing package to your project. Because the Agent runs in a container, ensure that the Dockerfiles are configured properly, and there is no need to install anything. Open the notes/dockerfile.notes.maven file and uncomment the line that downloads dd-java-agent:
```
RUN curl -Lo dd-java-agent.jar 'https://dtdg.co/latest-java-tracer'
```
Within the same notes/dockerfile.notes.maven file, comment out the ENTRYPOINT line for running without tracing. Then uncomment the ENTRYPOINT line, which runs the application with tracing enabled:
```
ENTRYPOINT ["java" , "-javaagent:../dd-java-agent.jar", "-Ddd.trace.sample.rate=1", "-jar" , "target/notes-0.0.1-SNAPSHOT.jar"]
```
This automatically instruments the application with Datadog services.
Note: The flags on these sample commands, particularly the sample rate, are not necessarily appropriate for environments outside this tutorial. For information about what to use in your real environment, read Tracing configuration.
Universal Service Tags identify traced services across different versions and deployment environments so that they can be correlated within Datadog, and so you can use them to search and filter. The three environment variables used for Unified Service Tagging are DD_SERVICE, DD_ENV, and DD_VERSION. For applications deployed with Docker, these environment variables can be added within the Dockerfile or the docker-compose file. For this tutorial, the all-docker-compose.yaml file already has these environment variables defined:
```
  environment:
    - DD_SERVICE=notes
    - DD_ENV=dev
    - DD_VERSION=0.0.1
```
You can also see that Docker labels for the same Universal Service Tags service, env, and version values are set in the Dockerfile. This allows you also to get Docker metrics once your application is running.
```
  labels:
    - com.datadoghq.tags.service="notes"
    - com.datadoghq.tags.env="dev"
    - com.datadoghq.tags.version="0.0.1"
```

Add the Agent container

Add the Datadog Agent in the services section of your all-docker-compose.yaml file to add the Agent to your build:

Uncomment the Agent configuration, and specify your own Datadog API key and site:

  datadog-agent:
    container_name: datadog-agent
    image: "gcr.io/datadoghq/agent:latest"
    pid: host
    environment:
      - DD_API_KEY=<DD_API_KEY_HERE>
      - DD_SITE=datadoghq.com  # Default. Change to eu.datadoghq.com, us3.datadoghq.com, us5.datadoghq.com as appropriate for your org
      - DD_APM_ENABLED=true
      - DD_APM_NON_LOCAL_TRAFFIC=true
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /proc/:/host/proc/:ro
      - /sys/fs/cgroup:/host/sys/fs/cgroup:ro

Uncomment the depends_on fields for datadog-agent in the notes container.

Observe that in the notes service section, the DD_AGENT_HOST environment variable is set to the hostname of the Agent container. Your notes container section looks like this:

notes:
  container_name: notes
  restart: always
  build:
    context: ../
    dockerfile: notes/dockerfile.notes.maven
  ports:
    - 8080:8080
  labels:
    - com.datadoghq.tags.service="notes"
    - com.datadoghq.tags.env="dev"
    - com.datadoghq.tags.version="0.0.1"
  environment:
    - DD_SERVICE=notes
    - DD_ENV=dev
    - DD_VERSION=0.0.1
    - DD_AGENT_HOST=datadog-agent
  # - CALENDAR_HOST=calendar
  depends_on:
  # - calendar
    - datadog-agent

You’ll configure the calendar sections and variables later in this tutorial.

Launch the containers to see automatic tracing

Now that the Tracing Library is installed, restart your application and start receiving traces. Run the following commands:

docker-compose -f all-docker-compose.yaml build notes
docker-compose -f all-docker-compose.yaml up notes

You can tell the Agent is working by observing continuous output in the terminal, or by opening the Events Explorer in Datadog and seeing the start event for the Agent:

Agent start event shown in Events Explorer

With the application running, send some curl requests to it:

curl localhost:8080/notes: []
curl -X POST 'localhost:8080/notes?desc=hello': {"id":1,"description":"hello"}
curl localhost:8080/notes/1: {"id":1,"description":"hello"}
curl localhost:8080/notes: [{"id":1,"description":"hello"}]

Wait a few moments, and go to APM > Traces in Datadog, where you can see a list of traces corresponding to your API calls:

Traces from the sample app in APM Trace Explorer

The h2 is the embedded in-memory database for this tutorial, and notes is the Spring Boot application. The traces list shows all the spans, when they started, what resource was tracked with the span, and how long it took.

If you don’t see traces after several minutes, clear any filter in the Traces Search field (sometimes it filters on an environment variable such as ENV that you aren’t using).

Examine a trace

On the Traces page, click on a POST /notes trace to see a flame graph that shows how long each span took and what other spans occurred before a span completed. The bar at the top of the graph is the span you selected on the previous screen (in this case, the initial entry point into the notes application).

The width of a bar indicates how long it took to complete. A bar at a lower depth represents a span that completes during the lifetime of a bar at a higher depth.

The flame graph for a POST trace looks something like this:

A GET /notes trace looks something like this:

Tracing configuration

The Java tracing library uses Java’s built-in agent and monitoring support. The flag -javaagent:../dd-java-agent.jar in the Dockerfile tells the JVM where to find the Java tracing library so it can run as a Java Agent. Learn more about Java Agents at https://www.baeldung.com/java-instrumentation.

The dd.trace.sample.rate flag sets the sample rate for this application. The ENTRYPOINT command in the Dockerfile sets its value to 1, which means that 100% of all requests to the notes service are sent to the Datadog backend for analysis and display. For a low-volume test application, this is fine. Do not do this in production or in any high-volume environment, because this results in a very large volume of data. Instead, sample some of your requests. Pick a value between 0 and 1. For example, -Ddd.trace.sample.rate=0.1 sends traces for 10% of your requests to Datadog. Read more about tracing configuration settings and sampling mechanisms.

Notice that the sampling rate flag in the command appears before the -jar flag. That’s because this is a parameter for the Java Virtual Machine, not your application. Make sure that when you add the Java Agent to your application, you specify the flag in the right location.

Add manual instrumentation to the Java application

Automatic instrumentation is convenient, but sometimes you want more fine-grained spans. Datadog’s Java DD Trace API allows you to specify spans within your code using annotations or code.

The following steps walk you through adding annotations to the code to trace some sample methods.

Open /notes/src/main/java/com/datadog/example/notes/NotesHelper.java. This example already contains commented-out code that demonstrates the different ways to set up custom tracing on the code.

Uncomment the lines that import libraries to support manual tracing:

import datadog.trace.api.Trace;
import datadog.trace.api.DDTags;
import io.opentracing.Scope;
import io.opentracing.Span;
import io.opentracing.Tracer;
import io.opentracing.tag.Tags;
import io.opentracing.util.GlobalTracer;
import java.io.PrintWriter;
import java.io.StringWriter

Uncomment the lines that manually trace the two public processes. These demonstrate the use of @Trace annotations to specify aspects such as operationName and resourceName in a trace:

@Trace(operationName = "traceMethod1", resourceName = "NotesHelper.doLongRunningProcess")
// ...
@Trace(operationName = "traceMethod2", resourceName = "NotesHelper.anotherProcess")

You can also create a separate span for a specific code block in the application. Within the span, add service and resource name tags and error handling tags. These tags result in a flame graph showing the span and metrics in Datadog visualizations. Uncomment the lines that manually trace the private method:

        Tracer tracer = GlobalTracer.get();
        // Tags can be set when creating the span
        Span span = tracer.buildSpan("manualSpan1")
            .withTag(DDTags.SERVICE_NAME, "NotesHelper")
            .withTag(DDTags.RESOURCE_NAME, "privateMethod1")
            .start();
        try (Scope scope = tracer.activateSpan(span)) {
            // Tags can also be set after creation
            span.setTag("postCreationTag", 1);
            Thread.sleep(30);
            Log.info("Hello from the custom privateMethod1");

And also the lines that set tags on errors:

     } catch (Exception e) {
         // Set error on span
         span.setTag(Tags.ERROR, true);
         span.setTag(DDTags.ERROR_MSG, e.getMessage());
         span.setTag(DDTags.ERROR_TYPE, e.getClass().getName());

         final StringWriter errorString = new StringWriter();
         e.printStackTrace(new PrintWriter(errorString));
         span.setTag(DDTags.ERROR_STACK, errorString.toString());
         Log.info(errorString.toString());
     } finally {
         span.finish();
     }

Update your Maven build by opening notes/pom.xml and uncommenting the lines configuring dependencies for manual tracing. The dd-trace-api library is used for the @Trace annotations, and opentracing-util and opentracing-api are used for manual span creation.

Rebuild the containers:

docker-compose -f all-docker-compose.yaml build notes
docker-compose -f all-docker-compose.yaml up notes

Resend some HTTP requests, specifically some GET requests.
On the Trace Explorer, click on one of the new GET requests, and see a flame graph like this:
Note the higher level of detail in the stack trace now that the getAll function has custom tracing.
The privateMethod around which you created a manual span now shows up as a separate block from the other calls and is highlighted by a different color. The other methods where you used the @Trace annotation show under the same service and color as the GET request, which is the notes application. Custom instrumentation is valuable when there are key parts of the code that need to be highlighted and monitored.

For more information, read Custom Instrumentation.

Add a second application to see distributed traces

Tracing a single application is a great start, but the real value in tracing is seeing how requests flow through your services. This is called distributed tracing.

The sample project includes a second application called calendar that returns a random date whenever it is invoked. The POST endpoint in the Notes application has a second query parameter named add_date. When it is set to y, Notes calls the calendar application to get a date to add to the note.

Configure the calendar app for tracing by adding dd-java-agent to the startup command in the Dockerfile, like you previously did for the notes app. Open calendar/Dockerfile.calendar.maven and see that it is already downloading dd-java-agent:
```
RUN curl -Lo dd-java-agent.jar 'https://dtdg.co/latest-java-tracer'
```
Within the same calendar/dockerfile.calendar.maven file, comment out the ENTRYPOINT line for running without tracing. Then uncomment the ENTRYPOINT line, which runs the application with tracing enabled:
```
ENTRYPOINT ["java" , "-javaagent:../dd-java-agent.jar", "-Ddd.trace.sample.rate=1", "-jar" , "target/calendar-0.0.1-SNAPSHOT.jar"]
```
Note: Again, the flags, particularly the sample rate, are not necessarily appropriate for environments outside this tutorial. For information about what to use in your real environment, read Tracing configuration.

Open docker/all-docker-compose.yaml and uncomment the environment variables for the calendar service to set up the Agent host and Unified Service Tags for the app and for Docker:

  calendar:
    container_name: calendar
    restart: always
    build:
      context: ../
      dockerfile: calendar/dockerfile.calendar.maven
    labels:
      - com.datadoghq.tags.service="calendar"
      - com.datadoghq.tags.env="dev"
      - com.datadoghq.tags.version="0.0.1"
    environment:
      - DD_SERVICE=calendar
      - DD_ENV=dev
      - DD_VERSION=0.0.1
      - DD_AGENT_HOST=datadog-agent
   ports:
     - 9090:9090
   depends_on:
     - datadog-agent

In the notes service section, uncomment the CALENDAR_HOST environment variable and the calendar entry in depends_on to make the needed connections between the two apps:

  notes:
  ...
    environment:
      - DD_SERVICE=notes
      - DD_ENV=dev
      - DD_VERSION=0.0.1
      - DD_AGENT_HOST=datadog-agent
      - CALENDAR_HOST=calendar
    depends_on:
      - calendar
      - datadog-agent

Build the multi-service application by restarting the containers. First, stop all running containers:
```
docker-compose -f all-docker-compose.yaml down
```
Then run the following commands to start them:
```
docker-compose -f all-docker-compose.yaml build
docker-compose -f all-docker-compose.yaml up
```
After all the containers are up, send a POST request with the add_date parameter:

curl -X POST 'localhost:8080/notes?desc=hello_again&add_date=y': {"id":1,"description":"hello_again with date 2022-11-06"}

In the Trace Explorer, click this latest trace to see a distributed trace between the two services:

Note that you didn’t change anything in the notes application. Datadog automatically instruments both the okHttp library used to make the HTTP call from notes to calendar, and the Jetty library used to listen for HTTP requests in notes and calendar. This allows the trace information to be passed from one application to the other, capturing a distributed trace.