# Advanced Usage

This document explains some of the more advanced usage concepts with wsproto. This is assume you are familiar with wsproto and I/O in Python.

## Back-pressure

Back-pressure is an important concept to understand when implementing a client/server protocol. This section briefly explains the issue and then explains how to handle back-pressure when using wsproto.

Imagine that you have a WebSocket server that reads messages from the client, does some processing, and then sends a response. What happens if the client sends messages faster than the server can process them? If the incoming messages are buffered in memory, then the server will slowly use more and more memory, until the OS eventually kills it. This scenario is directly applicable to wsproto, because every time you call `receive_data(some_byte_string_of_data)`, it appends that data to an internal buffer.

The slow endpoint needs a way to signal the fast endpoint to stop sending messages until the slow endpoint can catch up. This signaling is called “back-pressure”. As a Sans-IO library, wsproto is not responsible for network concerns like back-pressure, so that responsibility belongs to your network glue code.

Fortunately, TCP has the ability to signal backpressure, and the operating system will do that for you automatically—if you follow a few rules! The OS buffers all incoming and outgoing network data. Standard Python socket methods, such as `send(...)` and `recv()`, copy data to and from those OS buffers. For example, if the peer is sending data too quickly, then the OS receive buffer will start to get full, and the OS will signal the peer to stop transmitting. When `recv()` is called, the OS will copy data from its internal buffer into your process, free up space in its own buffer, and then signal to the peer to start transmitting again.

Therefore, you need to follow these two rules to implement back-pressure over TCP:

1. Do not receive from the socket faster than your code can process the messages. Your processing code may need to signal the receiving code when its ready to receive more data.

2. Do not store out-going messages in an unbounded collection. Ideally, out-going messages should be sent to the OS as soon as possible. If you need to buffer messages in memory, the buffer should be bounded so that it can not grow indefinitely.

## Post handshake connection

A WebSocket connection starts with a handshake, which is an agreement to use the WebSocket protocol, and on which sub-protocol and extensions to use. It can be advantageous to perform this handshake outside of wsproto, for example in a dual stack setup whereby the HTTP handling is completed seperately. In this case the `Connection` class can be used directly.

    connection = Connection(extensions)  # Agreed extensions
    sock.send(connection.send(Message(data=b"Hi")))

    connection.receive_data(sock.recv(4096))

    for event in connection.events():
        # As with WSConnection, only without any handshake events

## HTTP/2

WebSockets over HTTP/2 have a distinct difference to HTTP/1 in that only a single HTTP/2 stream is dedicated to the WebSocket rather than the entire connection (as in HTTP/1). This requires the HTTP/2 connection to be managed before the WebSocket connection with Hyper-h2 being recommended for HTTP/2.

Although wsproto doesn’t manage the HTTP/2 connection it can still be used for the WebSocket stream. The HTTP/2 connection will need to handshake the WebSocket stream, with the key being agreement on the extensions used. Once the extensions have been agreed the `Connection` class can be used to manage the WebSocket connection, noting that data to be sent or received will need to be parsed by the HTTP/2 connection first. In practice for a server this looks like,

    from wsproto.connection import Connection, ConnectionType
    from wsproto.extensions import PerMessageDeflate
    from wsproto.handshake import server_extensions_handshake

    # WebSocket request has been received
    request_extensions: List[str]
    supported_extensions = [PerMessageDeflate()]
    accepts = server_extensions_handshake(request_extensions, supported_extensions)
    if accepts:
        response_headers.append({"sec-websocket-extensions": accepts})
    # Send the response headers
    connection = Connection(ConnectionType.SERVER, supported_extensions)

and for a client

    from wsproto.connection import Connection, ConnectionType
    from wsproto.extensions import PerMessageDeflate
    from wsproto.handshake import client_extensions_handshake

    # WebSocket response has been received
    accepted_extensions: List[str]
    proposed_extensions = [PerMessageDeflate()]
    extensions = client_extensions_handshake(accepted_extensions, proposed_extensions)
    connection = Connection(ConnectionType.CLIENT, supported_extensions)

any data received on the stream should be passed to the `connection` via the `receive_bytes` method and bytes returned from the `connection.send` method should be wrapped in a HTTP/2 data frame and sent.