Resources for Developers

Get Started with Celery and Flask

Requirements

Before doing this tutorial you should have setup your environment:

Initialize your application

$ mkdir my-app
$ cd my-app
$ pip install virtualenv
$ virtualenv venv
$ . venv/bin/activate

Application infrastructure

Our goal is to create two applications communicating via Redis using the Celery platform:

  • The Celery app will provide a custom hello task.
  • The Flask app will provide a web server that will send a task to the Celery app and display the answer in a web page.

The Redis connection URL will be send using the REDIS_URL environment variable.

Create a Celery server

Install Celery

pip install celery
pip install redis

Defining a custom task

Create a file named task.py containing:

import celery
import os

app = celery.Celery('scalingo-sample')


app.conf.update(BROKER_URL=os.environ['REDIS_URL'],
                CELERY_RESULT_BACKEND=os.environ['REDIS_URL'])
@app.task
def hello(name):
    return "Hello "+name

Create a Flask server

Install Flask

pip install Flask

Creating a custom web server

Create file named app.py containing:

import os
from flask import Flask
from flask import render_template
from flask import request

app = Flask(__name__)

@app.route("/")
def hello():
    return render_template('index.html')

if __name__ == "__main__":
    port = int(os.environ.get("PORT", 5000))
    app.run(host='0.0.0.0', port=port)

And create a file named templates/index.html containing a basic HTML page:

<!DOCTYPE html>
<html>
<head>
	<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
	<title>Celery Flask Application</title>
</head>
<body>
<h1>Greetings from Scalingo</h1>
</body>
</html>

Communication between Celery and Flask

In order to have some communication between Flask and Celery, we will provide a form that will take user input, send it to Celery, get the Celery response and display it on the Web page.

Modify the app.py file:

import os
from flask import Flask
from flask import render_template
from flask import request

import task

app = Flask(__name__)

@app.route("/")
def hello():
    name = request.args.get('name', 'John doe')
    result = task.hello.delay(name)
    result.wait()
    return render_template('index.html', celery=result)

if __name__ == "__main__":
    port = int(os.environ.get("PORT", 5000))
    app.run(host='0.0.0.0', port=port)

And our template templates/index.html:

<!DOCTYPE html>
<html>
<head>
	<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
	<title>Celery Flask Application</title>
</head>
<body>
  <h1>Greetings from Scalingo</h1>
  
  <h2> Celery returned: {{ celery.result }} </h2>
  

  <form method="GET" action="/">
    <label for="name"> Enter your name : </label>
    <input type="text" name="name"/>
    <input type="submit"/>
  </form>
</body>
</html>

Define how to start your application

Create a Procfile at the root of your project:

worker: celery worker --app=task.app
web: python app.py

Freeze all your dependencies

$ pip freeze > requirements.txt

Commit your application

$ git init
$ echo "venv/" >> .gitignore
$ echo "*.pyc" >> .gitignore
$ git add .
$ git commit -m "Base Celery and flask application"

Create your application on Scalingo

$ scalingo create my-app

Add a Redis addon to your app

$ scalingo --app my-app addons-add redis free

Deploy your app

$ git push scalingo master

Scaling your application

By default Scalingo only launch your web application. You must manually start the worker container:

$ scalingo --app my-app scale web:1 worker:1

Live demo

This application is currently running on Scalingo here.


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