code: private void doMultiMapKeys(final Message<JsonObject> message) {
      final String name = message.body().getString("name");
      if (name == null) {
        message.reply(new JsonObject().putString("status", "error").putString("message", "No name specified."));
        return;
      }
  
      context.execute(new Action<Set<Object>>() {
        @Override
        public Set<Object> perform() {
          return data.getMultiMap(formatKey(name)).keySet();
        }
      }, new Handler<AsyncResult<Set<Object>>>() {
        @Override
        public void handle(AsyncResult<Set<Object>> result) {
          if (result.failed()) {
            message.reply(new JsonObject().putString("status", "error").putString("message", result.cause().getMessage()));
          } else {
            message.reply(new JsonObject().putString("status", "ok").putArray("result", new JsonArray(result.result().toArray(new Object[result.result().size()]))));
          }
        }
      });
    }
docs: Handles a cluster multi-map keys command.