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How to Add Websockets to a Django App without Extra Dependencies

by Jayden WindleFebruary 10th, 2020

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Django 3.0 ships with ASGI support out of the box, adding Websockets to your Django app requires no extra dependencies. We'll go over how to handle Websocket connections, send and receive data, and implement the business logic in a sample ASGI application. We're going to use the Asynchronous Server Gateway Interface (ASGI) to build web services that support long-lived connections, such as Websockets and Server Sent Events. In this post, we'll learn how to use Django to handle these connections.

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featured image - How to Add Websockets to a Django App without Extra Dependencies

Now that Django 3.0 ships with ASGI support out of the box, adding Websockets to your Django app requires no extra dependencies. In this post, you'll learn how to handle Websockets with Django by extending the default ASGI application. We'll go over how to handle Websocket connections, send and receive data, and implement the business logic in a sample ASGI application.

Getting started

To start, you'll need Python >= 3.6 installed on your machine. Django 3.0 is only compatible with Python 3.6 and up because it makes use of the

async

and

await

keywords. Once you've got your Python version setup, create a project directory and

cd

into it. Then, install Django inside of a

virtualenv

and create a new Django app in your project directory:

$ mkdir django_websockets && cd django_websockets
$ python -m venv venv
$ source venv/bin/activate
$ pip install django
$ django-admin startproject websocket_app .

Take a look in the

websocket_app

directory of your Django app. You should see a file called

asgi.py

. Its contents will look something like this:

import os

from django.core.asgi import get_asgi_application

os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'websocket_app.settings')

application = get_asgi_application()

This file provides the default Django ASGI setup, and exposes an ASGI

application

called application which can be run using an ASGI server such as

uvicorn

daphne

. Before we go much further, let's take a look at how ASGI applications are structured.

ASGI app structure

ASGI, or the Asynchronous Server Gateway Interface, is a specification for building asynchronous web services with Python. It's the spiritual successor to WSGI, which has been used by frameworks like Django and Flask for a long time. ASGI lets you use Python's native

async

await

functionality to build web services that support long-lived connections, such as Websockets and Server Sent Events.

An ASGI application is a single

async

function which takes in 3 parameters:

scope

(the context of the current request),

receive

(an

async

function that lets you listen for incoming events), and

send

(an

async

function that lets you send events to the client).

Inside of an ASGI application, you can route requests based on values in the

scope

dictionary. For example, you can check whether the request is an HTTP request or a Websocket request by checking the value of

scope['type']

. To listen for data from the client, you can

await

the

receive

function. When you're ready to send data to the client, you can

await

the

send

function, and pass in any data you want to send to the client. Let's take a look at how this works in a sample application.

Creating an ASGI app

In our

asgi.py

file, we're going to wrap Django's default ASGI application function with our own ASGI application in order to handle Websocket connections ourselves. To do this, we'll need to define an

async

function called

application

, that takes in the 3 ASGI parameters:

scope

receive

, and

send

. Rename the result of the

get_asgi_application

call to

django_application

, because we'll need it process HTTP requests. Inside of our

application

function we'll check the value of

scope['type']

to determine the request type. If the request type is

'http'

, then the request is a normal HTTP request and we should let Django handle it. If the request type is

'websocket'

, then we'll want to handle the logic ourselves. The resulting

asgi.py

file should look something like this:

import os

from django.core.asgi import get_asgi_application

os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'websocket_app.settings')

django_application = get_asgi_application()

async def application(scope, receive, send):
    if scope['type'] == 'http':
        # Let Django handle HTTP requests
        await django_application(scope, receive, send)
    elif scope['type'] == 'websocket':
        # We'll handle Websocket connections here
        pass
    else:
        raise NotImplementedError(f"Unknown scope type {scope['type']}")

Now we need to create a function to handle Websocket connections. Create a file called

websocket.py

in the same folder as your

asgi.py

file, and define an ASGI application function called

websocket_application

that takes in the 3 ASGI parameters. Next, we'll import

websocket_application

in our

asgi.py

file, and call it inside of our

application

function to handle Websocket requests, passing in the

scope

receive

, and

send

parameters. It should look something like this:

# asgi.py
import os

from django.core.asgi import get_asgi_application
from websocket_app.websocket import websocket_application

os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'websocket_app.settings')

django_application = get_asgi_application()

async def application(scope, receive, send):
    if scope['type'] == 'http':
        await django_application(scope, receive, send)
    elif scope['type'] == 'websocket':
        await websocket_application(scope, receive, send)
    else:
        raise NotImplementedError(f"Unknown scope type {scope['type']}")

# websocket.py
async def websocket_application(scope, receive, send):
    pass

Next, let's implement some logic for our Websocket application. We're going to listen for all Websocket connections, and when the client sends the string

"ping"

, we'll respond with the string

"pong!"

Inside of the

websocket_application

function, we're going to define an indefinite loop that will handle Websocket requests until the connection is closed. Inside that loop, we'll wait for any new events that the server receives from the client. Then we'll act on the contents of the event, and send the response to the client.

To start, let's handle connections. When a new Websocket client connects to the server, we'll receive a

'websocket.connect'

event. In order to allow this connection, we'll send a

'websocket.accept'

event in response. This will complete the Websocket handshake and establish a persistent connection with the client.

We'll also need to handle disconnection events when a client terminates their connection to the server. To do that, we'll listen for a

'websocket.disconnect'

event. When a client disconnects, we'll break out of our indefinite loop.

Finally, we need to handle requests from the client. To do that, we'll listen for a

'websocket.receive'

event. When we receive a

'websocket.receive'

event from the client, we'll check and see if the value of

event['text']

'ping'

. If it is, we'll send a

'websocket.send'

event, with a text value of

'pong!'

After setting up the Websocket logic, our

websocket.py

file should look something like this:

# websocket.py
async def websocket_application(scope, receive, send):
    while True:
        event = await receive()

        if event['type'] == 'websocket.connect':
            await send({
                'type': 'websocket.accept'
            })
        
        if event['type'] == 'websocket.disconnect':
            break
        
        if event['type'] == 'websocket.receive':
            if event['text'] == 'ping':
                await send({
                    'type': 'websocket.send',
                    'text': 'pong!'
                })

Testing it out

Now our ASGI application is set up to handle Websocket connections and we've implemented our Websocket server logic, let's test it out. Right now, the Django development server doesn't use the

asgi.py

file, so you won't be able to test your connections using

./manage.py runserver

. Instead, you'll need to run the app with an ASGI server such as

uvicorn

. Let's install it:

$ pip install uvicorn

Once

uvicorn

is installed, we can run our ASGI application using the following command:

$ uvicorn websocket_app.asgi:application
INFO:     Started server process [25557]
INFO:     Waiting for application startup.
INFO:     ASGI 'lifespan' protocol appears unsupported.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

To test the Websocket connection, open up your browser's development tools in a new tab. In the console, create a new

Websocket

instance called

ws

pointed to

ws://localhost:8000/

. Then attach an

onmessage

handler to

ws

that logs

event.data

to the console. Finally, call

ws.send('ping')

to send the message to the server. You should see the value

"pong!"

logged to the console.

> ws = new WebSocket('ws://localhost:8000/')
  WebSocket {url: "ws://localhost:8000/", readyState: 0, bufferedAmount: 0, onopen: null, onerror: null, …}
> ws.onmessage = event => console.log(event.data)
  event => console.log(event.data)
> ws.send("ping")
  undefined
  pong!

Congrats! Now you know how to add Websocket support to your Django application using ASGI. Go build something awesome with it 😎

👋 Hi, I'm Jayden. I love building apps and teaching others how to build apps. For more posts about building apps with Django, React, and GraphQL, follow me on Twitter or subscribe to my newsletter at jaydenwindle.com.