@Override
    public void onApplicationActivate(ApplicationInstanceReference reference, Set<HostName> hostnames) {
        Map<HostName, HostInfo> currentHostInfos = hostInfos.computeIfAbsent(reference, (ignored) -> new HashMap<>());
        hostnames.forEach(currentHostInfos::remove);
    }
In-memory implementation of StatusService.

@author hakon
orchestrator/src/test/java/com/yahoo/vespa/orchestrator/status/InMemoryStatusService.java