deb-python-autobahn/doc/websocket/programming.rst

WebSocket Programming

This guide introduces WebSocket programming with .

You'll see how to create WebSocket server ("creating-websocket-servers") and client applications ("creating-websocket-clients").

Resources:

• Example Code for this Guide: Twisted-based or asyncio-based
• More WebSocket Examples <websocket_examples>

Creating Servers

Using you can create WebSocket servers that will be able to talk to any (compliant) WebSocket client, including browsers.

We'll cover how to define the behavior of your WebSocket server by writing protocol classes and show some boilerplate for actually running a WebSocket server using the behavior defined in the server protocol.

Server Protocols

To create a WebSocket server, you need to write a protocol class to specify the behavior of the server.

For example, here is a protocol class for a WebSocket echo server that will simply echo back any WebSocket message it receives:

class MyServerProtocol(WebSocketServerProtocol):

   def onMessage(self, payload, isBinary):
      ## echo back message verbatim
      self.sendMessage(payload, isBinary)

This is just three lines of code, but we will go through each one carefully, since writing protocol classes like above really is core to WebSocket programming using .

The first thing to note is that you derive your protocol class from a base class provided by . Depending on whether you write a Twisted or a asyncio based application, here are the base classes to derive from:

• autobahn.twisted.websocket.WebSocketServerProtocol
• autobahn.asyncio.websocket.WebSocketServerProtocol

So a Twisted-based echo protocol would import the base protocol from autobahn.twisted.websocket and derive from autobahn.twisted.websocket.WebSocketServerProtocol

Twisted:

from autobahn.twisted.websocket import WebSocketServerProtocol

class MyServerProtocol(WebSocketServerProtocol):

   def onMessage(self, payload, isBinary):
      ## echo back message verbatim
      self.sendMessage(payload, isBinary)

while an asyncio echo protocol would import the base protocol from autobahn.asyncio.websocket and derive from autobahn.asyncio.websocket.WebSocketServerProtocol

asyncio:

from autobahn.asyncio.websocket import WebSocketServerProtocol

class MyServerProtocol(WebSocketServerProtocol):

   def onMessage(self, payload, isBinary):
      ## echo back message verbatim
      self.sendMessage(payload, isBinary)

Note

In this example, only the imports differ between the Twisted and the asyncio variant. The rest of the code is identical. However, in most real world programs you probably won't be able to or don't want to avoid using network framework specific code.

---

Receiving Messages

The second thing to note is that we override a callback onMessage which is called by whenever the callback related event happens.

In case of onMessage, the callback will be called whenever a new WebSocket message was received. There are more WebSocket related callbacks, but for now the onMessage callback is all we need.

When our server receives a WebSocket message, the autobahn.websocket.interfaces.IWebSocketChannel.onMessage will fire with the message payload received.

The payload is always a Python byte string. Since WebSocket is able to transmit text (UTF8) and binary payload, the actual payload type is signaled via the isBinary flag.

When the payload is text (isBinary == False), the bytes received will be an UTF8 encoded string. To process text payloads, the first thing you often will do is decoding the UTF8 payload into a Python string:

s = payload.decode('utf8')

Tip

You don't need to validate the bytes for actually being valid UTF8 - does that already when receiving the message.

When using WebSocket text messages with JSON payload, typical code for receiving and decoding messages into Python objects that works on both Python 2 and 3 would look like this:

import json
obj = json.loads(payload.decode('utf8'))

We are using the Python standard JSON module :pyjson.

The payload (which is of type bytes on Python 3 and str on Python 2) is decoded from UTF8 into a native Python string, and then parsed from JSON into a native Python object.

---

Sending Messages

The third thing to note is that we use methods like sendMessage provided by the base class to perform WebSocket related actions, like sending a WebSocket message.

As there are more methods for performing other actions (like closing the connection), we'll come back to this later, but for now, the sendMessage method is all we need.

autobahn.websocket.interfaces.IWebSocketChannel.sendMessage takes the payload to send in a WebSocket message as Python bytes. Since WebSocket is able to transmit payloads of text (UTF8) and binary type, you need to tell the actual type of the payload bytes. This is done using the isBinary flag.

Hence, to send a WebSocket text message, you will usually encode the payload to UTF8:

payload = s.encode('utf8')
self.sendMessage(payload, isBinary = False)

Warning

will NOT validate the bytes of a text payload being sent for actually being valid UTF8. You MUST ensure that you only provide valid UTF8 when sending text messages. If you produce invalid UTF8, a conforming WebSocket peer will close the WebSocket connection due to the protocol violation.

When using WebSocket text messages with JSON payload, typical code for encoding and sending Python objects that works on both Python 2 and 3 would look like this:

import json
payload = json.dumps(obj, ensure_ascii = False).encode('utf8')

We are using the Python standard JSON module :pyjson.

The ensure_ascii == False option allows the JSON serializer to use Unicode strings. We can do this since we are encoding to UTF8 afterwards anyway. And UTF8 can represent the full Unicode character set.

---

Running a Server

Now that we have defined the behavior of our WebSocket server in a protocol class, we need to actually start a server based on that behavior.

Doing so involves two steps:

1. Create a Factory for producing instances of our protocol class
2. Create a TCP listening server using the former Factory

Here is one way of doing that when using Twisted

Twisted:

if __name__ == '__main__':

   import sys

   from twisted.python import log
   from twisted.internet import reactor
   log.startLogging(sys.stdout)

   from autobahn.twisted.websocket import WebSocketServerFactory
   factory = WebSocketServerFactory()
   factory.protocol = MyServerProtocol

   reactor.listenTCP(9000, factory)
   reactor.run()

What we are doing here is

1. Setup Twisted logging
2. Create a autobahn.twisted.websocket.WebSocketServerFactory and set our MyServerProtocol on the factory (the highlighted lines)
3. Start a server using the factory, listening on TCP port 9000

Similar, here is the asyncio way

asyncio:

if __name__ == '__main__':

   try:
      import asyncio
   except ImportError:
      ## Trollius >= 0.3 was renamed
      import trollius as asyncio

   from autobahn.asyncio.websocket import WebSocketServerFactory
   factory = WebSocketServerFactory()
   factory.protocol = MyServerProtocol

   loop = asyncio.get_event_loop()
   coro = loop.create_server(factory, '127.0.0.1', 9000)
   server = loop.run_until_complete(coro)

   try:
      loop.run_forever()
   except KeyboardInterrupt:
      pass
   finally:
      server.close()
      loop.close()

What we are doing here is

1. Import asyncio, or the Trollius backport
2. Create a autobahn.asyncio.websocket.WebSocketServerFactory and set our MyServerProtocol on the factory (the highlighted lines)
3. Start a server using the factory, listening on TCP port 9000

Note

As can be seen, the boilerplate to create and run a server differ from Twisted, but the core code of creating a factory and setting our protocol (the highlighted lines) is identical (other than the differing import for the WebSocket factory).

You can find complete code for above examples here:

• WebSocket Echo (Twisted-based)
• WebSocket Echo (Asyncio-based)

Connection Lifecycle

As we have seen above, will fire callbacks on your protocol class whenever the event related to the respective callback occurs.

It is in these callbacks that you will implement application specific code.

The core WebSocket interface autobahn.websocket.interfaces.IWebSocketChannel provides the following callbacks:

• autobahn.websocket.interfaces.IWebSocketChannel.onConnect
• autobahn.websocket.interfaces.IWebSocketChannel.onOpen
• autobahn.websocket.interfaces.IWebSocketChannel.onMessage
• autobahn.websocket.interfaces.IWebSocketChannel.onClose

We have already seen the callback for receiving-messages. This callback will usually fire many times during the lifetime of a WebSocket connection.

In contrast, the other three callbacks above each only fires once for a given connection.

Opening Handshake

Whenever a new client connects to the server, a new protocol instance will be created and the autobahn.websocket.interfaces.IWebSocketChannel.onConnect callback fires as soon as the WebSocket opening handshake is begun by the client.

For a WebSocket server protocol, onConnect() will fire with autobahn.websocket.protocol.ConnectionRequest providing information on the client wishing to connect via WebSocket.

class MyServerProtocol(WebSocketServerProtocol):

   def onConnect(self, request):
      print("Client connecting: {}".format(request.peer))

On the other hand, for a WebSocket client protocol, onConnect() will fire with autobahn.websocket.protocol.ConnectionResponse providing information on the WebSocket connection that was accepted by the server.

class MyClientProtocol(WebSocketClientProtocol):

   def onConnect(self, response):
      print("Connected to Server: {}".format(response.peer))

In this callback you can do things like

• checking or setting cookies or other HTTP headers
• verifying the client IP address
• checking the origin of the WebSocket request
• negotiate WebSocket subprotocols

For example, a WebSocket client might offer to speak several WebSocket subprotocols. The server can inspect the offered protocols in onConnect() via the supplied instance of autobahn.websocket.protocol.ConnectionRequest. When the server accepts the client, it'll chose one of the offered subprotocols. The client can then inspect the selected subprotocol in it's onConnect() callback in the supplied instance of autobahn.websocket.protocol.ConnectionResponse.

Connection Open

The autobahn.websocket.interfaces.IWebSocketChannel.onOpen callback fires when the WebSocket opening handshake has been successfully completed. You now can send and receive messages over the connection.

class MyProtocol(WebSocketProtocol):

   def onOpen(self):
      print("WebSocket connection open.")

Closing a Connection

The core WebSocket interface autobahn.websocket.interfaces.IWebSocketChannel provides the following methods:

• autobahn.websocket.interfaces.IWebSocketChannel.sendMessage
• autobahn.websocket.interfaces.IWebSocketChannel.sendClose

We've already seen one of above in sending-messages.

The autobahn.websocket.interfaces.IWebSocketChannel.sendClose will initiate a WebSocket closing handshake. After starting to close a WebSocket connection, no messages can be sent. Eventually, the autobahn.websocket.interfaces.IWebSocketChannel.onClose callback will fire.

After a WebSocket connection has been closed, the protocol instance will get recycled. Should the client reconnect, a new protocol instance will be created and a new WebSocket opening handshake performed.

Connection Close

When the WebSocket connection has closed, the autobahn.websocket.interfaces.IWebSocketChannel.onClose callback fires.

class MyProtocol(WebSocketProtocol):

   def onClose(self, wasClean, code, reason):
      print("WebSocket connection closed: {}".format(reason))

When the connection has closed, no messages will be received anymore and you cannot send messages also. The protocol instance won't be reused. It'll be garbage collected. When the client reconnects, a completely new protocol instance will be created.

Creating Clients

Note

Creating WebSocket clients using works very similar to creating WebSocket servers. Hence you should have read through creating-websocket-servers first.

As with servers, the behavior of your WebSocket client is defined by writing a protocol class.

Client Protocols

To create a WebSocket client, you need to write a protocol class to specify the behavior of the client.

For example, here is a protocol class for a WebSocket client that will send a WebSocket text message as soon as it is connected and log any WebSocket messages it receives:

class MyClientProtocol(WebSocketClientProtocol):

   def onOpen(self):
      self.sendMessage(u"Hello, world!".encode('utf8'))

   def onMessage(self, payload, isBinary):
      if isBinary:
         print("Binary message received: {0} bytes".format(len(payload)))
      else:
         print("Text message received: {0}".format(payload.decode('utf8')))

Similar to WebSocket servers, you derive your WebSocket client protocol class from a base class provided by . Depending on whether you write a Twisted or a asyncio based application, here are the base classes to derive from:

• autobahn.twisted.websocket.WebSocketClientProtocol
• autobahn.asyncio.websocket.WebSocketClientProtocol

So a Twisted-based protocol would import the base protocol from autobahn.twisted.websocket and derive from autobahn.twisted.websocket.WebSocketClientProtocol

Twisted:

from autobahn.twisted.websocket import WebSocketClientProtocol

class MyClientProtocol(WebSocketClientProtocol):

   def onOpen(self):
      self.sendMessage(u"Hello, world!".encode('utf8'))

   def onMessage(self, payload, isBinary):
      if isBinary:
         print("Binary message received: {0} bytes".format(len(payload)))
      else:
         print("Text message received: {0}".format(payload.decode('utf8')))

while an asyncio-based protocol would import the base protocol from autobahn.asyncio.websocket and derive from autobahn.asyncio.websocket.WebSocketClientProtocol

asyncio:

from autobahn.asyncio.websocket import WebSocketClientProtocol

class MyClientProtocol(WebSocketClientProtocol):

   def onOpen(self):
      self.sendMessage(u"Hello, world!".encode('utf8'))

   def onMessage(self, payload, isBinary):
      if isBinary:
         print("Binary message received: {0} bytes".format(len(payload)))
      else:
         print("Text message received: {0}".format(payload.decode('utf8')))

Note

In this example, only the imports differs between the Twisted and the asyncio variant. The rest of the code is identical. However, in most real world programs you probably won't be able to or don't want to avoid using network framework specific code.

---

Receiving and sending WebSocket messages as well as connection lifecycle in clients works exactly the same as with servers. Please see

• receiving-messages
• sending-messages
• connection-lifecycle

Running a Client

Now that we have defined the behavior of our WebSocket client in a protocol class, we need to actually start a client based on that behavior.

Doing so involves two steps:

1. Create a Factory for producing instances of our protocol class
2. Create a TCP connecting client using the former Factory

Here is one way of doing that when using Twisted

Twisted:

if __name__ == '__main__':

   import sys

   from twisted.python import log
   from twisted.internet import reactor
   log.startLogging(sys.stdout)

   from autobahn.twisted.websocket import WebSocketClientFactory
   factory = WebSocketClientFactory()
   factory.protocol = MyClientProtocol

   reactor.connectTCP("127.0.0.1", 9000, factory)
   reactor.run()

What we are doing here is

1. Setup Twisted logging
2. Create a autobahn.twisted.websocket.WebSocketClientFactory and set our MyClientProtocol on the factory (the highlighted lines)
3. Start a client using the factory, connecting to localhost 127.0.0.1 on TCP port 9000

Similar, here is the asyncio way

asyncio:

if __name__ == '__main__':

   try:
      import asyncio
   except ImportError:
      ## Trollius >= 0.3 was renamed
      import trollius as asyncio

   from autobahn.asyncio.websocket import WebSocketClientFactory
   factory = WebSocketClientFactory()
   factory.protocol = MyClientProtocol

   loop = asyncio.get_event_loop()
   coro = loop.create_connection(factory, '127.0.0.1', 9000)
   loop.run_until_complete(coro)
   loop.run_forever()
   loop.close()

What we are doing here is

1. Import asyncio, or the Trollius backport
2. Create a autobahn.asyncio.websocket.WebSocketClientFactory and set our MyClientProtocol on the factory (the highlighted lines)
3. Start a client using the factory, connecting to localhost 127.0.0.1 on TCP port 9000

Note

As can be seen, the boilerplate to create and run a client differ from Twisted, but the core code of creating a factory and setting our protocol (the highlighted lines) is identical (other than the differing import for the WebSocket factory).

You can find complete code for above examples here:

• WebSocket Echo (Twisted-based)
• WebSocket Echo (Asyncio-based)

Upgrading

From < 0.7.0

Starting with release 0.7.0, now supports both Twisted and asyncio as the underlying network library. This required renaming some modules.

Hence, code for < 0.7.0

from autobahn.websocket import WebSocketServerProtocol

should be modified for >= 0.7.0 for (using Twisted)

from autobahn.twisted.websocket import WebSocketServerProtocol

or (using asyncio)

from autobahn.asyncio.websocket import WebSocketServerProtocol

Two more small changes:

1. The method WebSocketProtocol.sendMessage had parameter binary renamed to isBinary (for consistency with onMessage)
2. The ConnectionRequest object no longer provides peerstr, but only peer, and the latter is a plain, descriptive string (this was needed since we now support both Twisted and asyncio, and also non-TCP transports)