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 lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Chat Room</title>
    <link rel="icon" href="https://fav.mhciael.com/💬">
</head>

<body>
    <form>
        <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">
        {{ room_name|json_script:"room-name" }}
    </form>
    <script>
        const roomName = JSON.parse(document.getElementById('room-name').textContent);

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

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

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

        document.querySelector('#chat-message-input').focus();


        document.querySelector('form').addEventListener('submit', (e) => {
            e.preventDefault();
            const messageInputDom = document.querySelector('#chat-message-input');
            const message = messageInputDom.value;
            chatSocket.send(JSON.stringify({
                'message': message
            }));
            messageInputDom.value = '';
        });
    </script>
</body>

</html>

Create the view function for the room view in chat/views.py:

# chat/views.py
from django.shortcuts import render


def index(request):
    return render(request, "chat/index.html")


def room(request, room_name):
    return render(request, "chat/room.html", {"room_name": room_name})

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

# chat/urls.py
from django.urls import path

from . import views

urlpatterns = [
    path("", views.index, name="index"),
    path("<str:room_name>/", views.room, name="room"),
]

Start the Channels development server:

uv run python manage.py runserver

Go to http://127.0.0.1:8000/chat/ 🔗 in your browser 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.

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
import json

from channels.generic.websocket import WebsocketConsumer


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.

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.urls import re_path

from . import consumers

websocket_urlpatterns = [
    re_path(r"ws/chat/(?P<room_name>\w+)/$", consumers.ChatConsumer.as_asgi()),
]

We call the as_asgi() classmethod in order to get an ASGI application that will instantiate an instance of our consumer for each user-connection. This is similar to Django’s as_view(), which plays the same role for per-request Django view instances.

(Note we use re_path() due to limitations in URLRouter.)

The next step is to point the main ASGI configuration at the chat.routing module. Replace the contents of locallibrary_config/asgi.py with the code below. The highlighted lines are what changed since you edited this file in Part 1:

• Imports: AuthMiddlewareStack and AllowedHostsOriginValidator are new, and URLRouter joins ProtocolTypeRouter on the existing channels.routing line.
• django_asgi_app: get_asgi_application() is now called before chat.routing is imported, and its result is saved in django_asgi_app. The comment above it explains why the order matters.
• from chat.routing import websocket_urlpatterns: this import is new. It deliberately sits below the get_asgi_application() call rather than at the top of the file with the other imports.
• "http": now uses the saved django_asgi_app instead of calling get_asgi_application() again.
• "websocket": this key is new and replaces the # Just HTTP for now comment.

# locallibrary_config/asgi.py
import os

from channels.auth import AuthMiddlewareStack
from channels.routing import ProtocolTypeRouter, URLRouter
from channels.security.websocket import AllowedHostsOriginValidator
from django.core.asgi import get_asgi_application

os.environ.setdefault("DJANGO_SETTINGS_MODULE", "locallibrary_config.settings")
# Initialize Django ASGI application early to ensure the AppRegistry
# is populated before importing code that may import ORM models.
django_asgi_app = get_asgi_application()

from chat.routing import websocket_urlpatterns

application = ProtocolTypeRouter(
    {
        "http": django_asgi_app,
        "websocket": AllowedHostsOriginValidator(
            AuthMiddlewareStack(URLRouter(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. Django’s session framework needs the database, which you already migrated while building catalog. Run migrate anyway to be sure, and then start the Channels development server:

uv run python manage.py migrate

You should see:

Operations to perform:
  Apply all migrations: admin, auth, catalog, contenttypes, sessions
Running migrations:
  No migrations to apply.

Then start the server:

uv run python 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 in a separate terminal (press Control-C to stop it):

docker run --rm -p 6379:6379 redis:7

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

uv add channels-redis

Commit the updated pyproject.toml and uv.lock again.

Before we can use a channel layer, we must configure it. Edit the locallibrary_config/settings.py file and add a CHANNEL_LAYERS setting to the bottom, below the ASGI_APPLICATION line you added in Part 1. It should look like:

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

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

uv run python manage.py shell

17 objects imported automatically (use -v 2 for details).

>>> 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'}

(The number of objects the shell imports automatically depends on your models; that’s the same auto-import you’ve seen in the shell before.)

Type Control-D (or exit()) 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
import json

from asgiref.sync import async_to_sync
from channels.generic.websocket import WebsocketConsumer


class ChatConsumer(WebsocketConsumer):
    def connect(self):
        self.room_name = self.scope["url_route"]["kwargs"]["room_name"]
        self.room_group_name = f"chat_{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 = f"chat_{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 alphanumerics, hyphens, underscores, or 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 and are limited to a maximum length of 100 in the default backend. 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 or exceeds the length limit.

• 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. This translation is done by replacing . with _, thus in this example, chat.message calls the chat_message method.

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:

uv run python 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.