docs: public Collection<ClientEventRegistration> getActiveRegistrations(final String uuid) {
          //This method should not be called from registrationExecutor
          assert (!Thread.currentThread().getName().contains("eventRegistration"));
  
          Future<Collection<ClientEventRegistration>> future = registrationExecutor.submit(
                  new Callable<Collection<ClientEventRegistration>>() {
                      @Override
                      public Collection<ClientEventRegistration> call() {
                          ClientRegistrationKey key = new ClientRegistrationKey(uuid);
                          Map<Connection, ClientEventRegistration> registrationMap = registrations.get(key);
                          if (registrationMap == null) {
                              return Collections.EMPTY_LIST;
                          }
                          LinkedList<ClientEventRegistration> activeRegistrations = new LinkedList<ClientEventRegistration>();
                          for (ClientEventRegistration registration : registrationMap.values()) {
                              activeRegistrations.add(registration);
                          }
                          return activeRegistrations;
                      }
                  });
          try {
              return future.get();
          } catch (Exception e) {
              throw ExceptionUtil.rethrow(e);
          }
      }