public Optional<ApplicationInstance> getApplicationInstance(ApplicationInstanceReference reference) {
        return Optional.ofNullable(applicationsByReference.get(reference));
    }
The service model is the union of the duper model and the service monitor, and presented
as classes from the {@code application-model} module.

<p>The duper model contains the latest {@link ApplicationInfo} of both tenant and infrastructure
applications. The service monitor provides Slobrok or {@code /state/v1/health} information
on (most) services. The application model presents an application as a set of clusters, and
each cluster a set of services, and each service is associated with a particular host
and has a health status.</p>

@author hakonhall
service-monitor/src/main/java/com/yahoo/vespa/service/monitor/ServiceModel.java