Tutorial Part 2: Implement a Chat Server

This tutorial begins where Tutorial 1 left off. We’ll get the room page working so that you can chat with yourself and others in the same room.

Add the room view

We will now create the second view, a room view that lets you see messages posted in a particular chat room.

Create a new file chat/templates/chat/room.html. Your app directory should now look like:

chat/
    __init__.py
    templates/
        chat/
            index.html
            room.html
    urls.py
    views.py

Create the view template for the room view in chat/templates/chat/room.html:

<!-- chat/templates/chat/room.html -->
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8"/>
    <title>Chat Room</title>
</head>
<body>
    <textarea id="chat-log" cols="100" rows="20"></textarea><br/>
    <input id="chat-message-input" type="text" size="100"/><br/>
    <input id="chat-message-submit" type="button" value="Send"/>
</body>
<script>
    var roomName = {{ room_name_json }};

    var chatSocket = new WebSocket(
        'ws://' + window.location.host +
        '/ws/chat/' + roomName + '/');

    chatSocket.onmessage = function(e) {
        var data = JSON.parse(e.data);
        var message = data['message'];
        document.querySelector('#chat-log').value += (message + '\n');
    };

    chatSocket.onclose = function(e) {
        console.error('Chat socket closed unexpectedly');
    };

    document.querySelector('#chat-message-input').focus();
    document.querySelector('#chat-message-input').onkeyup = function(e) {
        if (e.keyCode === 13) {  // enter, return
            document.querySelector('#chat-message-submit').click();
        }
    };

    document.querySelector('#chat-message-submit').onclick = function(e) {
        var messageInputDom = document.querySelector('#chat-message-input');
        var message = messageInputDom.value;
        chatSocket.send(JSON.stringify({
            'message': message
        }));

        messageInputDom.value = '';
    };
</script>
</html>

Create the view function for the room view in chat/views.py. Add the imports of mark_safe and json and add the room view function:

# chat/views.py
from django.shortcuts import render
from django.utils.safestring import mark_safe
import json

def index(request):
    return render(request, 'chat/index.html', {})

def room(request, room_name):
    return render(request, 'chat/room.html', {
        'room_name_json': mark_safe(json.dumps(room_name))
    })

Create the route for the room view in chat/urls.py:

# chat/urls.py
from django.conf.urls import url

from . import views

urlpatterns = [
    url(r'^$', views.index, name='index'),
    url(r'^(?P<room_name>[^/]+)/$', views.room, name='room'),
]

Start the Channels development server:

$ python3 manage.py runserver

Go to http://127.0.0.1:8000/chat/ in your browser and to see the index page.

Type in “lobby” as the room name and press enter. You should be redirected to the room page at http://127.0.0.1:8000/chat/lobby/ which now displays an empty chat log.

Type the message “hello” and press enter. Nothing happens. In particular the message does not appear in the chat log. Why?

The room view is trying to open a WebSocket to the URL ws://127.0.0.1:8000/ws/chat/lobby/ but we haven’t created a consumer that accepts WebSocket connections yet. If you open your browser’s JavaScript console, you should see an error that looks like:

WebSocket connection to 'ws://127.0.0.1:8000/ws/chat/lobby/' failed: Unexpected response code: 500

Write your first consumer

When Django accepts an HTTP request, it consults the root URLconf to lookup a view function, and then calls the view function to handle the request. Similarly, when Channels accepts a WebSocket connection, it consults the root routing configuration to lookup a consumer, and then calls various functions on the consumer to handle events from the connection.

We will write a basic consumer that accepts WebSocket connections on the path /ws/chat/ROOM_NAME/ that takes any message it receives on the WebSocket and echos it back to the same WebSocket.

Note

It is good practice to use a common path prefix like /ws/ to distinguish WebSocket connections from ordinary HTTP connections because it will make deploying Channels to a production environment in certain configurations easier.

In particular for large sites it will be possible to configure a production-grade HTTP server like nginx to route requests based on path to either (1) a production-grade WSGI server like Gunicorn+Django for ordinary HTTP requests or (2) a production-grade ASGI server like Daphne+Channels for WebSocket requests.

Note that for smaller sites you can use a simpler deployment strategy where Daphne serves all requests - HTTP and WebSocket - rather than having a separate WSGI server. In this deployment configuration no common path prefix like is /ws/ is necessary.

Create a new file chat/consumers.py. Your app directory should now look like:

chat/
    __init__.py
    consumers.py
    templates/
        chat/
            index.html
            room.html
    urls.py
    views.py

Put the following code in chat/consumers.py:

# chat/consumers.py
from channels.generic.websocket import WebsocketConsumer
import json

class ChatConsumer(WebsocketConsumer):
    def connect(self):
        self.accept()

    def disconnect(self, close_code):
        pass

    def receive(self, text_data):
        text_data_json = json.loads(text_data)
        message = text_data_json['message']

        self.send(text_data=json.dumps({
            'message': message
        }))

This is a synchronous WebSocket consumer that accepts all connections, receives messages from its client, and echos those messages back to the same client. For now it does not broadcast messages to other clients in the same room.

Note

Channels also supports writing asynchronous consumers for greater performance. However any asynchronous consumer must be careful to avoid directly performing blocking operations, such as accessing a Django model. See the Consumers reference for more information about writing asynchronous consumers.

We need to create a routing configuration for the chat app that has a route to the consumer. Create a new file chat/routing.py. Your app directory should now look like:

chat/
    __init__.py
    consumers.py
    routing.py
    templates/
        chat/
            index.html
            room.html
    urls.py
    views.py

Put the following code in chat/routing.py:

# chat/routing.py
from django.conf.urls import url

from . import consumers

websocket_urlpatterns = [
    url(r'^ws/chat/(?P<room_name>[^/]+)/$', consumers.ChatConsumer),
]

The next step is to point the root routing configuration at the chat.routing module. In mysite/routing.py, import AuthMiddlewareStack, URLRouter, and chat.routing; and insert a 'websocket' key in the ProtocolTypeRouter list in the following format:

# mysite/routing.py
from channels.auth import AuthMiddlewareStack
from channels.routing import ProtocolTypeRouter, URLRouter
import chat.routing

application = ProtocolTypeRouter({
    # (http->django views is added by default)
    'websocket': AuthMiddlewareStack(
        URLRouter(
            chat.routing.websocket_urlpatterns
        )
    ),
})

This root routing configuration specifies that when a connection is made to the Channels development server, the ProtocolTypeRouter will first inspect the type of connection. If it is a WebSocket connection (ws:// or wss://), the connection will be given to the AuthMiddlewareStack.

The AuthMiddlewareStack will populate the connection’s scope with a reference to the currently authenticated user, similar to how Django’s AuthenticationMiddleware populates the request object of a view function with the currently authenticated user. (Scopes will be discussed later in this tutorial.) Then the connection will be given to the URLRouter.

The URLRouter will examine the HTTP path of the connection to route it to a particular consumer, based on the provided url patterns.

Let’s verify that the consumer for the /ws/chat/ROOM_NAME/ path works. Run migrations to apply database changes (Django’s session framework needs the database) and then start the Channels development server:

$ python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions
Running migrations:
  Applying contenttypes.0001_initial... OK
  Applying auth.0001_initial... OK
  Applying admin.0001_initial... OK
  Applying admin.0002_logentry_remove_auto_add... OK
  Applying contenttypes.0002_remove_content_type_name... OK
  Applying auth.0002_alter_permission_name_max_length... OK
  Applying auth.0003_alter_user_email_max_length... OK
  Applying auth.0004_alter_user_username_opts... OK
  Applying auth.0005_alter_user_last_login_null... OK
  Applying auth.0006_require_contenttypes_0002... OK
  Applying auth.0007_alter_validators_add_error_messages... OK
  Applying auth.0008_alter_user_username_max_length... OK
  Applying auth.0009_alter_user_last_name_max_length... OK
  Applying sessions.0001_initial... OK
$ python3 manage.py runserver

Go to the room page at http://127.0.0.1:8000/chat/lobby/ which now displays an empty chat log.

Type the message “hello” and press enter. You should now see “hello” echoed in the chat log.

However if you open a second browser tab to the same room page at http://127.0.0.1:8000/chat/lobby/ and type in a message, the message will not appear in the first tab. For that to work, we need to have multiple instances of the same ChatConsumer be able to talk to each other. Channels provides a channel layer abstraction that enables this kind of communication between consumers.

Go to the terminal where you ran the runserver command and press Control-C to stop the server.

Enable a channel layer

A channel layer is a kind of communication system. It allows multiple consumer instances to talk with each other, and with other parts of Django.

A channel layer provides the following abstractions:

  • A channel is a mailbox where messages can be sent to. Each channel has a name. Anyone who has the name of a channel can send a message to the channel.
  • A group is a group of related channels. A group has a name. Anyone who has the name of a group can add/remove a channel to the group by name and send a message to all channels in the group. It is not possible to enumerate what channels are in a particular group.

Every consumer instance has an automatically generated unique channel name, and so can be communicated with via a channel layer.

In our chat application we want to have multiple instances of ChatConsumer in the same room communicate with each other. To do that we will have each ChatConsumer add its channel to a group whose name is based on the room name. That will allow ChatConsumers to transmit messages to all other ChatConsumers in the same room.

We will use a channel layer that uses Redis as its backing store. To start a Redis server on port 6379, run the following command:

$ docker run -p 6379:6379 -d redis:2.8

We need to install channels_redis so that Channels knows how to interface with Redis. Run the following command:

$ pip3 install channels_redis

Before we can use a channel layer, we must configure it. Edit the mysite/settings.py file and add a CHANNEL_LAYERS setting to the bottom. It should look like:

# mysite/settings.py
# Channels
ASGI_APPLICATION = 'mysite.routing.application'
CHANNEL_LAYERS = {
    'default': {
        'BACKEND': 'channels_redis.core.RedisChannelLayer',
        'CONFIG': {
            "hosts": [('127.0.0.1', 6379)],
        },
    },
}

Note

It is possible to have multiple channel layers configured. However most projects will just use a single 'default' channel layer.

Let’s make sure that the channel layer can communicate with Redis. Open a Django shell and run the following commands:

$ python3 manage.py shell
>>> import channels.layers
>>> channel_layer = channels.layers.get_channel_layer()
>>> from asgiref.sync import async_to_sync
>>> async_to_sync(channel_layer.send)('test_channel', {'type': 'hello'})
>>> async_to_sync(channel_layer.receive)('test_channel')
{'type': 'hello'}

Type Control-D to exit the Django shell.

Now that we have a channel layer, let’s use it in ChatConsumer. Put the following code in chat/consumers.py, replacing the old code:

# chat/consumers.py
from asgiref.sync import async_to_sync
from channels.generic.websocket import WebsocketConsumer
import json

class ChatConsumer(WebsocketConsumer):
    def connect(self):
        self.room_name = self.scope['url_route']['kwargs']['room_name']
        self.room_group_name = 'chat_%s' % self.room_name

        # Join room group
        async_to_sync(self.channel_layer.group_add)(
            self.room_group_name,
            self.channel_name
        )

        self.accept()

    def disconnect(self, close_code):
        # Leave room group
        async_to_sync(self.channel_layer.group_discard)(
            self.room_group_name,
            self.channel_name
        )

    # Receive message from WebSocket
    def receive(self, text_data):
        text_data_json = json.loads(text_data)
        message = text_data_json['message']

        # Send message to room group
        async_to_sync(self.channel_layer.group_send)(
            self.room_group_name,
            {
                'type': 'chat_message',
                'message': message
            }
        )

    # Receive message from room group
    def chat_message(self, event):
        message = event['message']

        # Send message to WebSocket
        self.send(text_data=json.dumps({
            'message': message
        }))

When a user posts a message, a JavaScript function will transmit the message over WebSocket to a ChatConsumer. The ChatConsumer will receive that message and forward it to the group corresponding to the room name. Every ChatConsumer in the same group (and thus in the same room) will then receive the message from the group and forward it over WebSocket back to JavaScript, where it will be appended to the chat log.

Several parts of the new ChatConsumer code deserve further explanation:

  • self.scope[‘url_route’][‘kwargs’][‘room_name’]
    • Obtains the 'room_name' parameter from the URL route in chat/routing.py that opened the WebSocket connection to the consumer.
    • Every consumer has a scope that contains information about its connection, including in particular any positional or keyword arguments from the URL route and the currently authenticated user if any.
  • self.room_group_name = ‘chat_%s’ % self.room_name
    • Constructs a Channels group name directly from the user-specified room name, without any quoting or escaping.
    • Group names may only contain letters, digits, hyphens, and periods. Therefore this example code will fail on room names that have other characters.
  • async_to_sync(self.channel_layer.group_add)(…)
    • Joins a group.
    • The async_to_sync(…) wrapper is required because ChatConsumer is a synchronous WebsocketConsumer but it is calling an asynchronous channel layer method. (All channel layer methods are asynchronous.)
    • Group names are restricted to ASCII alphanumerics, hyphens, and periods only. Since this code constructs a group name directly from the room name, it will fail if the room name contains any characters that aren’t valid in a group name.
  • self.accept()
    • Accepts the WebSocket connection.
    • If you do not call accept() within the connect() method then the connection will be rejected and closed. You might want to reject a connection for example because the requesting user is not authorized to perform the requested action.
    • It is recommended that accept() be called as the last action in connect() if you choose to accept the connection.
  • async_to_sync(self.channel_layer.group_discard)(…)
    • Leaves a group.
  • async_to_sync(self.channel_layer.group_send)
    • Sends an event to a group.
    • An event has a special 'type' key corresponding to the name of the method that should be invoked on consumers that receive the event.

Let’s verify that the new consumer for the /ws/chat/ROOM_NAME/ path works. To start the Channels development server, run the following command:

$ python3 manage.py runserver

Open a browser tab to the room page at http://127.0.0.1:8000/chat/lobby/. Open a second browser tab to the same room page.

In the second browser tab, type the message “hello” and press enter. You should now see “hello” echoed in the chat log in both the second browser tab and in the first browser tab.

You now have a basic fully-functional chat server!

This tutorial continues in Tutorial 3.