Contents

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 budnle 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.

Table 1. Types of Health Checks
Type Meaning REST endpoint Kubernetes response on failure

liveness

whether the runtime environment is suitable

/health/live

Restarts container.

readiness

whether the microservice is currently capable of doing its work

/health/ready

Diverts requests away from the instance; periodically rechecks readiness and resumes traffic once the microservice reports itself as ready.

startup

whether the microservice has initialized to the point where liveness and readiness checks might pass

/health/started

Treats the instance as still starting up; does not check liveness or readiness until the startup probe reports success or times out according to its configuration.

Configuration

Health checks may be configured using the following properties.

The class responsible for configuration is:

Type: io.helidon.webserver.observe.health.HealthObserver

This is a standalone configuration type, prefix from configuration root: health

This type provides the following service implementations:

  • io.helidon.webserver.observe.spi.ObserveProvider

Configuration options

Table 2. Optional configuration options
key type default value description

details

boolean

false

Whether details should be printed. By default, health only returns a io.helidon.http.Status.NO_CONTENT_204 for success, io.helidon.http.Status.SERVICE_UNAVAILABLE_503 for health down, and io.helidon.http.Status.INTERNAL_SERVER_ERROR_500 in case of error with no entity. When details are enabled, health returns io.helidon.http.Status.OK_200 for success, same codes otherwise and a JSON entity with detailed information about each health check executed.

endpoint

string

health

exclude

string[]

 

Health check names to exclude in computing the overall health of the server.

use-system-services

boolean

true

Whether to use services discovered by java.util.ServiceLoader. By default, all io.helidon.health.spi.HealthCheckProvider based health checks are added.

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=/myhealth  #(1)

  1. The endpoint setting specifies the root path for the health endpoint.

Built-In Health Checks

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

Built-in health check Health check name JavaDoc Config properties (within server.features.observe.observers.health) Default config value

deadlock detection †

deadlock

DeadlockHealthCheck

n/a

n/a

available disk space †

diskSpace

DiskSpaceHealthCheck

helidon.health.diskSpace.thresholdPercent

99.999

helidon.health.diskSpace.path

/

available heap memory

heapMemory

HeapMemoryHealthCheck

helidon.health.heapMemory.thresholdPercent

98

† Helidon cannot support the indicated health checks in the GraalVM native image environment, so with native image those health checks do not appear in the health output.

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
JSON response:
{
  "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:
@Liveness // (1)
@ApplicationScoped // (2)
public class GreetLivenessCheck implements HealthCheck {

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

  1. Annotation indicating this is a liveness health check.

  2. Annotation indicating this is a bean instantiated once per application (in Helidon this means just once per runtime).

  3. Build the HealthCheckResponse with status UP and the current time.

Build and run the application, then verify the custom liveness health endpoint:
curl http://localhost:8080/health/live
JSON response:
{
  "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:
@Readiness // (1)
@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;
    }
}

  1. Annotation indicating that this is a readiness health check.

  2. Build the HealthCheckResponse with status UP after five seconds, else DOWN.

  3. Record the time at startup.

  4. Become ready after 5 seconds.

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 response status
< HTTP/1.1 503 Service Unavailable // (1)

  1. The HTTP status is 503 since the application is not ready.

JSON response:
{
  "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 response status
< HTTP/1.1 200 OK // (1)

  1. The HTTP status is 200 indicating that the application is ready.

JSON response:
{
  "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:
@Startup // (1)
@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;
    }
}

  1. Annotation indicating that this is a startup health check.

  2. Build the HealthCheckResponse with status UP after eight seconds, else DOWN.

  3. Record the time at startup of Helidon; the application will declare itself as started eight seconds later.

  4. Become ready after 5 seconds.

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 response status:
< HTTP/1.1 503 Service Unavailable // (1)

  1. The HTTP status is 503 since the application has not started.

JSON response:
{
  "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 response status:
< HTTP/1.1 200 OK // (1)

  1. The HTTP status is 200 indicating that the application is started.

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

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

Get all the health check data, including custom data:
curl http://localhost:8080/health
JSON response:
{
  "status": "UP",
  "checks": [
    {
      "name": "LivenessCheck",
      "status": "UP",
      "data": {
        "time": 1566403431536
      }
    }
  ]
}

Full example code is available here.

Reference