Consider adding an abstract HealthCheck base class that provides caching

[sleberknight]

CachingHealthCheck (introduced in version 3.7.0, see #492 ) decorates a HealthCheck to provide caching. In many cases this works great, for example health checks that are defined directly in a service. But if a health check is provided by a third-party library which provides its own initialization code, including health check registration, you may not have control over the health check in order to apply CachingHealthCheck. In such situations, it would be nice if the library could (easily) support caching in its health check(s) and expose those as configuration options to users.

Concretely, I have recently found several cases where I wanted to provide a health check (e.g., in a separate library) that can support caching as an optional feature. You can't use CachingHealthCheck in this manner.

One way to support this functionality would be to create a base class that provides caching functionality, but allows the user to control whether it caches or not. For example, we could have a AbstractCachingHealthCheck that custom health check classes extend. Here is some pseudo-code:

// in dropwizard-service-utilities

package org.kiwiproject.dropwizard.util.health;

public abstract class AbstractCachingHealthCheck extends HealthCheck {

   // there is probably a constructor that accepts caching-related arguments

    private final AtomicReference<Result> cachedResult;

    protected Result check() {
        // this provides the caching functionality, and delegates to doCheck when necessary
        if (upToDate()) {
            return cachedResult.get();
        }

        // the real implementation should catch exceptions and create an unhealthy Result
        var newResult = doCheck();
        cachedResult.set(newResult);
        return newResult;
    }

    // sub-classes implement the actual health check logic here
    protected abstract Result doCheck() throws Exception;

    private boolean upToDate() {
        // logic to determine whether the cached value needs to be refreshed
    }
}

// in some custom library

package com.acme.dynamic.health;

public class DynamicConfigurationHealthCheck extends AbstractCachingHealthCheck {

    // constructor that may (or may not) accept caching arguments

    // override the abstract method and provide the custom health check logic
    protected Result doCheck() {
        // custom, possibly expensive-to-compute, logic
        var result = ...;

        return result;  // this will be cached for a certain amount of time
    }
}