HTTP/2 provides a facility for performing ‘flow control’, enabling both ends of a HTTP/2 connection to influence the rate at which data is received. When used correctly flow control can be a powerful tool for maximising the efficiency of a connection. However, when used poorly, flow control leads to severe inefficiency and can adversely affect the throughput of the connection.
By default hyper does its best to manage the flow control window for you, trying to avoid severe inefficiencies. In general, though, the user has a much better idea of how to manage the flow control window than hyper will: you know your use case better than hyper possibly can.
For that reason, hyper provides a facility for using pluggable window managers. A window manageris an object that is in control of resizing the flow control window. This object gets informed about every frame received on the connection, and can make decisions about when to increase the size of the receive window. This object can take advantage of knowledge from layers above hyper, in the user’s code, as well as knowledge from hyper‘s layer.
To implement one of these objects, you will want to subclass the BaseFlowControlManager class and implement the increase_window_size() method. As a simple example, we can implement a very stupid flow control manager that always resizes the window in response to incoming data like this:
class StupidFlowControlManager(BaseFlowControlManager):
def increase_window_size(self, frame_size):
return frame_size
The class can then be plugged straight into a connection object:
HTTP20Connection('http2bin.org', window_manager=StupidFlowControlManager)
Note that we don’t plug an instance of the class in, we plug the class itself in. We do this because the connection object will spawn instances of the class in order to manage the flow control windows of streams in addition to managing the window of the connection itself.