Django is a powerful Python framework for web development. It is fast, secure, and reliable. Channels allow Django projects to handle HTTP along with asynchronous protocols like WebSockets, MQTT, chatbots, and more.
Channels:
Channels preserve the synchronous behavior of Django and add a layer of asynchronous protocols allowing users to write the views that are entirely synchronous, asynchronous, or a mixture of both. Channels basically allow the application to support “long-running connections”. It replaces Django’s default WSGI with its ASGI.
ASGI:
ASGI (Asynchronous Server Gateway Interface) provides an interface between async Python web servers and applications while it supports all the features provided by WSGI.
Consumers:
A consumer is a basic unit of Channels. It is an event-driven class that supports both async and sync applications. Consumers can run longer and hence they support web sockets that need persistent connection.
In this post, we will set up a basic example of channels. We will build a calculator app that will allow the user to send multiple expressions to the server and receive the result through a single persistent connection.
Environment Setup:
- It is always a good idea to create a virtual environment for the python apps in order to avoid version conflicts. Run the following commands in the terminal to get started
easy-install pip
python3 -m pip install virtualenv
virtualenv venv
source venv/bin/activate
- Now install Django and Channels:
pip install django
pip install channels
# On windows, try an unofficial wheel of 'Twisted' in case of dependency errors
LiveCalculator App:
Now start a Django project and create an app named ‘liveCalculator’
django-admin startproject sampleProject
cd sampleProject
python3 manage.py startapp liveCalculator
In sampleProject/settings.py, register channels and liveCalculator.
settings.py:
INSTALLED_APPS = [
'channels',
'liveCalculator',
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
]
In sampleProject/asgi.py, add the http protocol.
asgi.py:
Python3
import os
import django
from channels.http import AsgiHandler
from channels.routing import ProtocolTypeRouter
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'sampleProject.settings')
django.setup()
application = ProtocolTypeRouter({
"http": AsgiHandler(),
})
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Now we need to register this asgi into our application. Add this line in sampleProject/settings.py :
ASGI_APPLICATION = "sampleProject.asgi.application"
Create a new folder liveCalculator/templates/liveCalculator and create a new file index.html inside it. It will be the starting page of our app. Add the following code in index.html:
index.html:
HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Live Calculator</title>
</head>
<body>
<textarea name="ta" id="results" cols="30" rows="10">
</textarea><br>
Enter the expression: <input type="text" id="exp">
<input type="button" id="submit" value="Get Results">
<script>
socket.onmessage = (e) => {
result = JSON.parse(e.data).result;
document.getElementById("results").value += "Server: " + result + "\n";
}
socket.onclose = (e) => {
console.log("Socket closed!");
}
document.querySelector('#exp').onkeyup = function (e) {
if (e.keyCode === 13) { // enter, return
document.querySelector('#submit ').click();
}
};
document.querySelector("#submit").onclick = (e) => {
inputfield = document.querySelector("#exp")
exp = inputfield.value
socket.send(JSON.stringify(
{
expression: exp
}
))
document.querySelector("#results").value += "You: " + exp + "\n";
inputfield.value = "";
}
</script>
</body>
</html>
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The above code will render a text area and an input box where the user can enter the expression. It will create a socket connection that we will make later and append the received result in the text area. When the user inputs the expression, it will send the expression through a socket connection.
Now create a view to render this page in liveCalculator/views.py :
liveCalculator/views.py:
Python3
from django.shortcuts import render
def index(request):
return render(request, 'liveCalculator/index.html', {})
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Next, we need to create a route for this view. Add a new file urls.py in liveCalculator directory and add the following code:
liveCalculator/urls.py:
Python3
from django.conf.urls import url
from . import views
urlpatterns = [
url(r'^$', views.index, name="index"),
]
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Register this route in sampleProject/urls.py :
sampleProject/urls.py:
Python3
from django.contrib import admin
from django.urls import path
from django.conf.urls import include, url
urlpatterns = [
path('admin/', admin.site.urls),
url(r'^', include('liveCalculator.urls'))
]
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Now we need to create the consumer for our web socket connection. We will use the generic WebsocketConsumer class to implement its event-driven methods. Create a new file consumers.py in liveCalculator folder and add the following code:
consumers.py:
Python3
import json
from channels.generic.websocket import WebsocketConsumer
class Calculator(WebsocketConsumer):
def connect(self):
self.accept()
def disconnect(self, close_code):
self.close()
def receive(self, text_data):
text_data_json = json.loads(text_data)
expression = text_data_json['expression']
try:
result = eval(expression)
except Exception as e:
result = "Invalid Expression"
self.send(text_data=json.dumps({
'result': result
}))
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The WebsocketConsumer class supports these user-defined methods:
- connect(): We can write the business logic of what should happen when the client sends a connection request.
- disconnect(): We can write the business logic of what should happen when the client sends a disconnection request.
- receive(): We can write the business logic of what should happen when the client sends a message.
It also supports these built-in methods:
- accept(): It will accept the incoming connection.
- close(): It will close the current connection.
- send(): It will send the specified message to the client.
We have simply used the above methods in our Calculator class to accept the connection, evaluate the expression when a message a received, and send it to the client.
Next, we also need to define the routing method for this consumer. Create a new file routing.py in the same folder and add the following code to it:
routing.py:
Python3
from django.urls import re_path
from . import consumers
websocket_urlpatterns = [
re_path(r'ws/livec/$', consumers.Calculator.as_asgi()),
]
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Note that we have used as_asgi() method on our Calculator class to use it for our application. This will enable the socket on ws://<IP:Port>/ws/livec. Now register routing.py into asgi.py by declaring the WebSocket protocol.
asgi.py:
Python3
import os
from channels.auth import AuthMiddlewareStack
from channels.routing import ProtocolTypeRouter, URLRouter
from django.core.asgi import get_asgi_application
import liveCalculator.routing
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "sampleProject.settings")
application = ProtocolTypeRouter({
"http": get_asgi_application(),
"websocket": AuthMiddlewareStack(
URLRouter(
liveCalculator.routing.websocket_urlpatterns
)
),
})
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We are almost done with our first Channels application. Save all the files and run the following commands in the terminal:
python3 manage.py makemigrations
python3 manage.py migrate
python3 manage.py runserver
Now open http://localhost:8000 on your browser, and you should see the output like this:
See the log of the server. Note that we have created the connection only once, and we can send the message multiple times without creating a new connection.