spring-boot-with-metrics

This demo app shows how a Spring Boot application can expose a Prometheus metrics endpoint for scraping.

• For Java 11+

🚼 The app was initially created with Spring Initializr and then by following the RESTful service tutorial on spring.io.

To run

To run the app:

mvn clean spring-boot:run

This will expose Prometheus metrics at /actuator/prometheus. This is a simple key value listing of metrics. You can check them out using curl, for example:

$ curl http://localhost:8080/actuator/prometheus
# HELP process_uptime_seconds The uptime of the Java virtual machine
# TYPE process_uptime_seconds gauge
process_uptime_seconds 10.284
# HELP jvm_threads_states_threads The current number of threads having NEW state
# TYPE jvm_threads_states_threads gauge
jvm_threads_states_threads{state="runnable",} 9.0
jvm_threads_states_threads{state="blocked",} 0.0
jvm_threads_states_threads{state="waiting",} 11.0
...
# TYPE tomcat_sessions_alive_max_seconds gauge
tomcat_sessions_alive_max_seconds 0.0
...    

Next, try accessing the API in the app at http://localhost:8080/greeting either in a web browser or using curl. Check the /actuator/prometheus endpoint again. You should see the http_server_requests_seconds_sum metric increase. You will also see a new metric greeting_time_seconds which is a custom metric added to the app:

$ curl http://localhost:8080/actuator/prometheus | grep greeting_time_seconds
# HELP greeting_time_seconds Time taken to return greeting
# TYPE greeting_time_seconds summary
greeting_time_seconds{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="none",method="greeting",quantile="0.5",} 0.0
greeting_time_seconds{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="none",method="greeting",quantile="0.9",} 0.0
...

Timing a custom method

The method in the application which responds to the REST request has been annotated with @Timed, so Micrometer will capture the execution time of this method:

@GetMapping("/greeting")
@Timed(value = "greeting.time", description = "Time taken to return greeting",
        percentiles = {0.5, 0.90})
public Greeting greeting(@RequestParam(value = "name", defaultValue = "World") String name) {
    return new Greeting(counter.incrementAndGet(), String.format(template, name));
}

Once you make a request to the service at http://localhost:8080/greeting, you will also see a new metric exposed, greeting_time_seconds exposed, which shows the execution time of the greeting method:

# HELP greeting_time_seconds Time taken to return greeting
# TYPE greeting_time_seconds summary
greeting_time_seconds{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="none",met
hod="greeting",quantile="0.9",} 0.02097152
greeting_time_seconds_count{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="non
e",method="greeting",} 1.0
greeting_time_seconds_sum{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="none"
,method="greeting",} 0.021689345
# HELP greeting_time_seconds_max Time taken to return greeting
# TYPE greeting_time_seconds_max gauge
greeting_time_seconds_max{class="com.tutorialworks.demos.springbootwithmetrics.GreetingController",exception="none",method="greeting",} 0.021689345

From the Micrometer docs:

All implementations of Timer report at least the total time and count of events as separate time series.

Getting metrics into Prometheus

Now we need to get these metrics into Prometheus.

In another terminal, use Podman to start an ephemeral Prometheus in a container, and use the host networking option, which will allow the container to access the Spring Boot app on localhost:

podman run --net=host \
    -v ./prometheus.yml:/etc/prometheus/prometheus.yml:Z \
    prom/prometheus

Check the Prometheus console at http://localhost:9090.

• Go to Targets, you should see the Spring Boot app being scraped successfully.

• On the Graph page, you should be able to type in a metric from the application (e.g. tomcat_sessions_active_current_sessions, or greeting_time_seconds) and see the raw data, or plot a graph

Troubleshooting

Inside the Prometheus container, you can check that you can access the Spring Boot metrics:

$ podman exec -it <container-name> sh
$$ wget -S -O - http://localhost:8080/actuator/prometheus