Getting Started¶
This document explains how to get started using wsproto to connect to WebSocket servers as well as how to write your own.
We assume some level of familiarity with writing Python and networking code. If you’re not familiar with these we highly recommend you read up on these first. It may also be helpful to study Sans-I/O, which describes the ideas behind writing a network protocol library that doesn’t do any network I/O.
Connections¶
The main class you’ll be working with is the
WSConnection
object. This object
represents a connection to a WebSocket client or server and contains all the
state needed to communicate with the entity at the other end. Whether you’re
connecting to a server or receiving a connection from a client, this is the
object you’ll use.
wsproto provides two layers of abstractions. You need to write code that interfaces with both of these layers. The following diagram illustrates how your code is like a sandwich around wsproto.
Application |
<APPLICATION GLUE> |
wsproto |
<NETWORK GLUE> |
Network Layer |
wsproto does not do perform any network I/O, so <NETWORK GLUE>
represents the code you need to write to glue wsproto to the actual
network layer, i.e. code that can send and receive data over the
network. The WSConnection
class provides two methods for this purpose. When data has been
received on a network socket, you feed this data into wsproto by
calling receive_data
. When wsproto sends
events the send
will
return the bytes that need to be sent over the network. Your code is
responsible for actually sending that data over the network.
Note
If the connection drops, a standard Python socket.recv()
will return
zero. You should call receive_data(None)
to update the internal
wsproto state to indicate that the connection has been closed.
Internally, wsproto process the raw network data you feed into it and turns it
into higher level representations of WebSocket events. In <APPLICATION
GLUE>
, you need to write code to process these events. The
WSConnection
class contains a
generator method events
that
yields WebSocket events. To send a message, you call the send
method.
Connecting to a WebSocket server¶
Begin by instantiating a connection object in the client mode and then
create a Request
instance to
send. The Request must specify host
and target
arguments. If
the WebSocket server is located at http://example.com/foo
, then you
would instantiate the connection as follows:
ws = WSConnection(ConnectionType.CLIENT)
ws.send(Request(host="example.com", target='foo'))
Now you need to provide the network glue. For the sake of example, we will use standard Python sockets here, but wsproto can be integrated with any network layer:
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("example.com", 80))
To read from the network:
data = sock.recv(4096)
ws.receive_data(data)
You also need to send data returned by the send method:
data = ws.send(Message(data=b"Hello"))
sock.send(data)
A standard Python socket will block on the call to sock.recv()
, so you
will probably need to use a non-blocking socket or some form of concurrency like
threading, greenlets, asyncio, etc.
You also need to provide the application glue. To send a WebSocket message:
ws.send(Message(data="Hello world!"))
And to receive WebSocket events:
for event in ws.events():
if isinstance(event, AcceptConnection):
print('Connection established')
elif isinstance(event, RejectConnection):
print('Connection rejected')
elif isinstance(event, CloseConnection):
print('Connection closed: code={} reason={}'.format(
event.code, event.reason
))
sock.send(ws.send(event.response()))
elif isinstance(event, Ping):
print('Received Ping frame with payload {}'.format(event.payload))
sock.send(ws.send(event.response()))
elif isinstance(event, TextMessage):
print('Received TEXT data: {}'.format(event.data))
if event.message_finished:
print('Message finished.')
elif isinstance(event, BytesMessage):
print('Received BINARY data: {}'.format(event.data))
if event.message_finished:
print('BINARY Message finished.')
else:
print('Unknown event: {!r}'.format(event))
The method events()
returns a generator which will yield events for all of
the data currently in the wsproto internal buffer and then exit. Therefore,
you should iterate over this generator after receiving new network data.
For a more complete example, see synchronous_client.py.
WebSocket Servers¶
A WebSocket server is similar to a client except that it uses a different constant:
ws = WSConnection(ConnectionType.SERVER)
A server also needs to explicitly send an AcceptConnection
after it receives a
Request
event:
for event in ws.events():
if isinstance(event, Request):
print('Accepting connection request')
sock.send(ws.send(AcceptConnection()))
elif isinstance(event, CloseConnection):
print('Connection closed: code={} reason={}'.format(
event.code, event.reason
))
sock.send(ws.send(event.response()))
elif isinstance(event, Ping):
print('Received Ping frame with payload {}'.format(event.payload))
sock.send(ws.send(event.response()))
elif isinstance(event, TextMessage):
print('Received TEXT data: {}'.format(event.data))
if event.message_finished:
print('TEXT Message finished.')
elif isinstance(event, BinaryMessage):
print('Received BINARY data: {}'.format(event.data))
if event.message_finished:
print('BINARY Message finished.')
else:
print('Unknown event: {!r}'.format(event))
Alternatively a server can explicitly reject the connection by sending
RejectConnection
after
receiving a Request
event.
For a more complete example, see synchronous_server.py.
Protocol Errors¶
Protocol errors relating to either incorrect data or incorrect state
changes are raised when the connection receives data or when events
are sent. A LocalProtocolError
is raised if the local actions
are in error whereas a RemoteProtocolError
is raised if the remote
actions are in error.
Closing¶
WebSockets are closed with a handshake that requires each endpoint to
send one frame and receive one frame. Sending a
CloseConnection
instance
sets the state to LOCAL_CLOSING
. When a close frame is received,
it yields a CloseConnection
event, sets the state to
REMOTE_CLOSING
and requires a reply to be sent, this reply
should be a CloseConnection
event. To aid with this the
CloseConnection
class has a response()
method to create the
appropriate reply. For example,
if isinstance(event, CloseConnection):
sock.send(ws.send(event.response()))
When the reply has been received by the initiator, it will also yield
a CloseConnection
event.
Regardless of which endpoint initiates the closing handshake, the
server is responsible for tearing down the underlying connection. When
the server receives a CloseConnection
event, it should send
pending wsproto data (if any) and then it can start tearing down the
underlying connection.
Note
Both client and server connections must remember to reply to
CloseConnection
events initiated by the remote party.
Ping Pong¶
The WSConnection
class supports
sending WebSocket ping and pong frames via sending Ping
and Pong
. When a
Ping
frame is received it requires a reply, this reply should be
a Pong
event. To aid with this the Ping
class has a
response()
method to create the
appropriate reply. For example,
if isinstance(event, Ping):
sock.send(ws.send(event.response()))
Note
Both client and server connections must remember to reply to
Ping
events initiated by the remote party.