public synchronized Collection<String> getHosts(ApplicationId key) {
        return host2ApplicationId.entrySet().stream()
                                 .filter(entry -> entry.getValue().equals(key))
                                 .map(Map.Entry::getKey)
                                 .collect(Collectors.toSet());
    }
A host registry with a mapping between hosts (hostname as a String) and some type T
TODO: Maybe we should have a Host type, but using String for now.

@author Ulf Lilleengen
configserver/src/main/java/com/yahoo/vespa/config/server/host/HostRegistry.java