code: public void locateGroup(String groupAddress, Handler<AsyncResult<String>> doneHandler) {
      Set<String> registry = vertx.sharedData().getSet(groupAddress);
      synchronized (registry) {
        if (!registry.isEmpty()) {
          locateNode(groupAddress, new HashSet<>(registry), doneHandler);
        }
      }
    }
docs: Locates the local address of the nearest group node if one exists.
  
  @param groupAddress The full address of the group to search.
  @param doneHandler A handler to be called once the address has been located.