docs: def set(self, key, value):
          """Set a value at key and return a Future.
  
          :rtype: Iterable[tornado.concurrent.Future]
          """
          value_encoded = encode(value)
  
          if key in self.data and self.data[key] == value_encoded:
              return []
  
          self.data[key] = value_encoded
          return self.trigger_callbacks(key)