Health

Overview

Microservices expose their health status primarily so external tools (for example, an orchestrator such as Kubernetes) can monitor each service and take action, such as restarting a service instance if it has failed or temporarily shunting traffic away from the instance if the service is unable to process incoming requests normally.

Maven Coordinates

To enable MicroProfile Health add the helidon-microprofile bundle dependency to your project’s pom.xml (see Managing Dependencies).

<dependency>
  <groupId>io.helidon.microprofile.bundles</groupId>
  <artifactId>helidon-microprofile</artifactId>
</dependency>

MicroProfile Health is already included in the bundle.

If full control over the dependencies is required, and you want to minimize the quantity of the dependencies - Helidon MicroProfile Core bundle should be used. In this case the following dependencies should be included in your project’s pom.xml:

<dependency>
  <groupId>io.helidon.microprofile.bundles</groupId>
  <artifactId>helidon-microprofile-core</artifactId>
</dependency>

<dependency>
  <groupId>io.helidon.microprofile.health</groupId>
  <artifactId>helidon-microprofile-health</artifactId>
</dependency>

To enable built-in health checks add the following dependency (or use the helidon-microprofile bundle )

<dependency>
  <groupId>io.helidon.health</groupId>
  <artifactId>helidon-health-checks</artifactId>
</dependency>

Usage

Helidon implements MicroProfile Health Specification. The spec prescribes how external tools probe a service’s health checks and how you implement health checks as part of your microservice that are specific to your service’s needs.

Concepts - Liveness, Readiness, and Startup Checks

MicroProfile Health supports three types of health checks:

• Liveness checks report whether the runtime environment in which the service is running is sufficient to support the work the service performs. The environment is beyond the control of the service itself and typically cannot improve without outside intervention. If a microservice instance reports a DOWN liveness check, it should never report UP later. It will need to be stopped and a replacement instance created.
• Readiness checks report whether the service is currently capable of performing its work. A service that reports DOWN for its readiness cannot at the moment do its job, but at some future point it might become able to do so without requiring a restart.
• Startup checks indicate whether the service has started to the point where liveness and readiness checks even make sense. A service reporting DOWN for a startup check is still initializing itself and normally will report UP soon, assuming it is able to start successfully.

REST Endpoints

A MicroProfile-compliant service reports its health via known REST endpoints. Helidon MP provides these endpoints automatically as part of every MP microservice that includes health support..

External management tools (or curl or browsers) retrieve health checks using the REST endpoints in the table below which summarizes the types of health checks in MicroProfile Health. Responses from the health endpoints report 200 (OK), 204 (no content), or 503 (service unavailable) depending on the outcome of running the health checks. HTTP GET responses include JSON content showing the detailed results of all the health checks which the server executed after receiving the request. HTTP HEAD requests return only the status with no payload.

Types of Health Checks

Configuration

Health checks may be configured using the following properties.

The class responsible for configuration is:

Configuration options

Properties may be set in application.yaml or in microprofile-config.properties, in both cases using the health prefix.

For example, you can specify a custom port and root context for the root health endpoint path. However, you cannot use different ports, such as http://localhost:8080/myhealth and http://localhost:8081/myhealth/live. Likewise, you cannot use different paths, such as http://localhost:8080/health and http://localhost:8080/probe/live. The example below will change the root path.

Create a file named microprofile-config.properties in the resources/META-INF directory with the following contents:

health.endpoint=/myhealth1

Built-In Health Checks

You can use Helidon-provided health checks to report various common health check statuses:

Simply adding the built-in health check dependency is sufficient to register all the built-in health checks automatically. If you want to use only some of the built-in checks in your application, you can disable automatic discovery of the built-in health checks and register only the ones you want.

By setting the config properties listed in the table you can influence the behavior of the health checks.

Further, you can suppress one or more health checks by setting the configuration item server.features.observe.observers.health.exclude to a comma-separated list of the health check names you want to exclude. The table above lists the names for the built-in health checks.

Examples

Generate Helidon MP Quickstart project following these instructions.

Using the Built-In Health Checks

Helidon has a set of built-in health checks that can report various conditions:

• deadlock detection
• available disk space
• available heap memory

The following example will demonstrate how to use the built-in health checks. These examples are all executed from the root directory of your project (helidon-quickstart-mp).

Include the built-in health checks dependency in your pom.xml:

<dependency>
  <groupId>io.helidon.health</groupId>
  <artifactId>helidon-health-checks</artifactId>
</dependency>

Build the application, then run it:

mvn package
java -jar target/helidon-quickstart-mp.jar

Verify the health endpoint in a new terminal window:

curl http://localhost:8080/health

{
  "status": "UP",
  "checks": [
    {
      "name": "deadlock",
      "status": "UP"
    },
    {
      "name": "diskSpace",
      "status": "UP",
      "data": {
        "free": "325.54 GB",
        "freeBytes": 349543358464,
        "percentFree": "69.91%",
        "total": "465.63 GB",
        "totalBytes": 499963174912
      }
    },
    {
      "name": "heapMemory",
      "status": "UP",
      "data": {
        "free": "230.87 MB",
        "freeBytes": 242085696,
        "max": "3.56 GB",
        "maxBytes": 3817865216,
        "percentFree": "98.90%",
        "total": "271.00 MB",
        "totalBytes": 284164096
      }
    }
  ]
}

Custom Liveness Health Checks

You can create application-specific custom health checks and integrate them with Helidon using CDI. The following example shows how to add a custom liveness health check.

Create a new GreetLivenessCheck class with the following content:

@Liveness1
@ApplicationScoped2
public class GreetLivenessCheck implements HealthCheck {

    @Override
    public HealthCheckResponse call() {
        return HealthCheckResponse.named("LivenessCheck")3
                .up()
                .withData("time", System.currentTimeMillis())
                .build();
    }
}

Build and run the application, then verify the custom liveness health endpoint:

curl http://localhost:8080/health/live

{
  "status": "UP",
  "checks": [
    {
      "name": "LivenessCheck",
      "status": "UP",
      "data": {
        "time": 1566338255331
      }
    }
  ]
}

Custom Readiness Health Checks

You can add a readiness check to indicate that the application is ready to be used. In this example, the server will wait five seconds before it becomes ready.

Create a new GreetReadinessCheck class with the following content:

@Readiness1
@ApplicationScoped
public class GreetReadinessCheck implements HealthCheck {
    private final AtomicLong readyTime = new AtomicLong(0);

    @Override
    public HealthCheckResponse call() {
        return HealthCheckResponse.named("ReadinessCheck")2
                .status(isReady())
                .withData("time", readyTime.get())
                .build();
    }

    public void onStartUp(
            @Observes @Initialized(ApplicationScoped.class) Object init) {
        readyTime.set(System.currentTimeMillis());3
    }

    private boolean isReady() {4
        return Duration.ofMillis(System.currentTimeMillis() - readyTime.get()).getSeconds() >= 5;
    }
}

Build and run the application. Issue the curl command with -v within five seconds, and you will see that the application is not ready:

curl -v  http://localhost:8080/health/ready

< HTTP/1.1 503 Service Unavailable1

{
  "status": "DOWN",
  "checks": [
    {
      "name": "ReadinessCheck",
      "status": "DOWN",
      "data": {
        "time": 1566399775700
      }
    }
  ]
}

After five seconds you will see the application is ready:

curl -v http://localhost:8080/health/ready

< HTTP/1.1 200 OK1

{
  "status": "UP",
  "checks": [
    {
      "name": "ReadinessCheck",
      "status": "UP",
      "data": {
        "time": 1566399775700
      }
    }
  ]
}

Full example code is available here.

Custom Startup Health Checks

You can add a startup check to indicate whether or not the application has initialized to the point that the other health checks make sense. In this example, the server will wait eight seconds before it declares itself started.

Create a new GreetStartedCheck class with the following content:

@Startup1
@ApplicationScoped
public class GreetStartedCheck implements HealthCheck {
    private final AtomicLong readyTime = new AtomicLong(0);

    @Override
    public HealthCheckResponse call() {
        return HealthCheckResponse.named("StartedCheck")2
                .status(isStarted())
                .withData("time", readyTime.get())
                .build();
    }

    public void onStartUp(
            @Observes @Initialized(ApplicationScoped.class) Object init) {
        readyTime.set(System.currentTimeMillis());3
    }

    private boolean isStarted() {4
        return Duration.ofMillis(System.currentTimeMillis() - readyTime.get()).getSeconds() >= 8;
    }
}

Build and run the application. Issue the curl command with -v within five seconds, and you will see that the application has not yet started:

curl -v  http://localhost:8080/health/started

< HTTP/1.1 503 Service Unavailable1

{
  "status": "DOWN",
  "checks": [
    {
      "name": "StartedCheck",
      "status": "DOWN",
      "data": {
        "time": 1566399775700
      }
    }
  ]
}

After eight seconds you will see the application has started:

curl -v http://localhost:8080/health/started

< HTTP/1.1 200 OK1

{
  "status": "UP",
  "checks": [
    {
      "name": "StartedCheck",
      "status": "UP",
      "data": {
        "time": 1566399775700
      }
    }
  ]
}

When using the health check URLs, you can get the following health check data:

• liveness only - http://localhost:8080/health/live
• readiness only - http://localhost:8080/health/ready
• startup checks only - http://localhost:8080/health/started
• all health check data - http://localhost:8080/health

Get all the health check data, including custom data:

curl http://localhost:8080/health

{
  "status": "UP",
  "checks": [
    {
      "name": "LivenessCheck",
      "status": "UP",
      "data": {
        "time": 1566403431536
      }
    }
  ]
}

Full example code is available here.

Reference

• Helidon MicroProfile Health Javadoc
• Helidon Built-in Checks Javadoc
• MicroProfile Health Specification
• MicroProfile Health on GitHub