Tutorial Part 2: Implement a Chat Server
========================================

This tutorial begins where :doc:`Tutorial 1 </tutorial/part_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.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::

    $ 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 ``/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 :doc:`/topics/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.urls import re_path

    from . import consumers

    websocket_urlpatterns = [
        re_path(r'ws/chat/(?P<room_name>\w+)/$', 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 :ref:`scope <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 :doc:`Tutorial 3 </tutorial/part_3>`.

