[wildfly-dev] A look at Eclipse MicroProfile Healthcheck

[Jeff Mesnil]

Hi,

I had a look at the Eclipse MicroProfile Healthcheck spec[1] and wanted to share some thoughts and experiments about it, how it relates to WildFly and its use in containers (such as OpenShift).

# Eclipse MicroProfile Healthcheck

The Eclipse MicroProfile Healthcheck (MPHC for short) is a specification to determine the healthiness of an application.
It defines a Health Check Procedure (HCP for short) interface that can be implemented by application to determine its healthiness. It’s a single method that returns a Health Status: either UP or DOWN (+ some metadata).
Typically, an application would provide one or more HCP to check healthiness of its parts.
The overall healthiness of the application is determined by the aggregation of all the HCP provided by the application. If any HCP is DOWN, the overall outcome is DOWN. Else the application is considered as UP.

The MPHC spec has a companion document[2] that specifies an HTTP format to check the healthiness of an application.

Heiko is leading the spec and Swarm is the sample implementation for it (MicroProfile does not have the notion of reference implementation).
The spec is still in flux and we have a good opportunity to contribute to it to ensure that it meets our requirements and use cases.

# Use case

Using the HTTP endpoint, a container can ask an application whether it is healthy. If it is not healthy, the container could stop the application and respin a new instance.
For example, OpenShift/Kubernetes can configure liveness probes[3][4].

Supporting MPHC in WildFly would allow a better integration with containers and ensure that any unhealthy WildFly  process is restarted promptly.

# Prototype

I’ve written a prototype of a WildFly extension to support MPHC for applications deployed in WildFly *and* add health check procedures inside WildFly:

https://github.com/jmesnil/wildfly-microprofile-health

and it passes the MPHC tck :)

The microprofile-health subsystem supports an operation to check the health of the app server:

[standalone at localhost:9990 /] /subsystem=microprofile-health:check
{
    "outcome" => "success",
    "result" => {
        "checks" => [{
            "id" => "heap-memory",
            "result" => "UP",
            "data" => {
                "max" => "477626368",
                "used" => "156216336"
            }
        }],
        "outcome" => "UP"
    }
}

It also exposes an (unauthenticated) HTTP endpoint:

$ curl http://localhost:8080/health/:
{
   "checks":[
      {
         "id":"heap-memory",
         "result":"UP",
         "data":{
            "max":"477626368",
            "used":"160137128"
         }
      }
   ],
   "outcome":"UP"
}

This HTTP endpoint can be used by OpenShift for its liveness probe.

Any deployment that defines Health Check Procedures will have them registered to determine the overall healthiness of the process.

# WildFly health check procedures

The MPHC specification mainly targets user applications that can apply application logic to determine their healthiness.
However I wonder if we could reuse the concepts *inside* WildFly. There are things that we could check to determine if the App server runtime is healthy, e.g.:
* The amount of heap memory is close to the max
* some deployments have failed
* Excessive GC
* Running out of disk space

Subsystems inside WildFly could provide Health check procedures that would be queried to check the overall healthiness.
We could for example provide a health check that the used heap memory is less that 90% of the max:

        HealthCheck.install(context, "heap-memory", () -> {
            MemoryMXBean memoryBean = ManagementFactory.getMemoryMXBean();
            long memUsed = memoryBean.getHeapMemoryUsage().getUsed();
            long memMax = memoryBean.getHeapMemoryUsage().getMax();
            HealthResponse response = HealthResponse.named("heap-memory")
                    .withAttribute("used", memUsed)
                    .withAttribute("max", memMax);
            // status is is down is used memory is greater than 90% of max memory.
            HealthStatus status = (memUsed < memMax * 0.9) ? response.up() : response.down();
            return status;
        });

HealthCheck.install creates a MSC service and makes sure that is is registered by the health monitor that queries all the procedures.
A subsystem would just have to call HealthCheck.install/uninstall with a Health check procedures to help determine the healthiness of the app server.

What do you think about this use case?

I even wonder if this is something that should be instead provided by our core-management subsystem with a private API (1 interface and some data structures).
The microprofile-health extension would then map our private API to the MPHC spec and handled health check procedures coming from deployments.

# Summary

To better integrate WildFly with OpenShift, we should provide a way to let OpenShift checks the healthiness of WildFly. The MPHC spec is a good candidate to provide such feature.
It is worth exploring how we could leverage it for user deployments and also for WildFly internals (when that makes sense).
Swarm is providing an implementation of the MPHC, we also need to see how we can collaborate between WildFly and Swarm to avoid duplicating code and efforts from providing the same feature to our users.

jeff


[1] https://github.com/eclipse/microprofile-evolution-process/blob/master/proposals/0003-health-checks.md
[2] https://github.com/eclipse/microprofile-evolution-process/blob/master/proposals/0003-spec.md
[3] https://docs.openshift.com/enterprise/3.0/dev_guide/application_health.html
[4] https://kubernetes.io/v1.0/docs/user-guide/walkthrough/k8s201.html#health-checking
-- 
Jeff Mesnil
JBoss, a division of Red Hat
http://jmesnil.net/