public synchronized void update(ApplicationId key, Collection<String> newHosts) {
        verifyHosts(key, newHosts);
        Collection<String> currentHosts = getHosts(key);
        log.log(Level.FINE, () -> "Setting hosts for key '" + key + "', " +
                                  "newHosts: " + newHosts + ", " +
                                  "currentHosts: " + currentHosts);
        Collection<String> removedHosts = findRemovedHosts(newHosts, currentHosts);
        removeHosts(removedHosts);
        addHosts(key, newHosts);
    }
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