code: public void setSystemEventHandler(final SystemEventHandler systemEventHandler) {
      if (systemEventSubscription != null) {
        systemEventSubscription.unsubscribe();
      }
      if (systemEventHandler != null) {
        systemEventSubscription = eventBus().get()
            .filter(evt -> evt.type().equals(EventType.SYSTEM))
            .subscribe(new Subscriber<CouchbaseEvent>() {
              @Override
              public void onCompleted() { /* Ignoring on purpose. */}
  
              @Override
              public void onError(Throwable e) { /* Ignoring on purpose. */ }
  
              @Override
              public void onNext(CouchbaseEvent evt) {
                systemEventHandler.onEvent(evt);
              }
            });
      }
    }
docs: Set/Override the control event handler.