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Flask-CDN

Flask-CDN allows you to easily serve all your Flask application’s static assets from a CDN (like Amazon Cloudfront), without having to modify your templates.

How it works

Flask-CDN replaces the URLs that Flask’s flask.url_for function would insert into your templates, with URLs that point to your CDN. This makes setting up an origin pull CDN extremely easy.

Internally, every time url_for is called in one of your application’s templates, flask_cdn.url_for is instead invoked. If the endpoint provided is deemed to refer to static assets, then the CDN URL for the asset specified in the filename argument is instead returned. Otherwise, flask_cdn.url_for passes the call on to flask.url_for.

Installation

If you use pip then installation is simply:

$ pip install flask-cdn

or, if you want the latest github version:

$ pip install git+git://github.com/libwilliam/flask-cdn.git

You can also install Flask-CDN via Easy Install:

$ easy_install flask-cdn

Dependencies

There are no additional dependencies besides Flask itself. Note: Flask-CDN currently only supports applications that use the jinja2 templating system.

Using Flask-CDN

Flask-CDN is incredibly simple to use. In order to start serving your Flask application’s assets from Amazon CDN, the first thing to do is let Flask-CDN know about your flask.Flask application object.

from flask import Flask
from flask_cdn import CDN

app = Flask(__name__)
app.config['CDN_DOMAIN'] = 'mycdnname.cloudfront.net'
CDN(app)

In many cases, however, one cannot expect a Flask instance to be ready at import time, and a common pattern is to return a Flask instance from within a function only after other configuration details have been taken care of. In these cases, Flask-CDN provides a simple function, flask_cdn.CDN.init_app, which takes your application as an argument.

from flask import Flask
from flask_cdn import CDN

cdn = CDN()

def start_app():
    app = Flask(__name__)
    cdn.init_app(app)
    return app

In terms of getting your application to use external CDN URLs when referring to your application’s static assets, passing your Flask object to the CDN object is all that needs to be done. Once your app is running, any templates that contained relative static asset locations, will instead be pulled from your CDN.

Static Asset URLs

URLs generated by Flask-CDN will look like the following:

/static/foo/style.css becomes https://mycdnname.cloudfront.net/static/foo/style.css, assuming that mycdnname.cloudfront.net is the domain of your CDN, and you have chosen to have assets served over HTTPS.

Flask-CDN Options

Within your Flask application’s settings you can provide the following settings to control the behaviour of Flask-CDN. None of the settings are required.

Option	Description	Default
CDN_DEBUG	Activate Debug mode to return relative url rather than CDN enabled ones.	app.debug
CDN_DOMAIN	Set the base domain for your CDN here.	None
CDN_HTTPS	Specifies whether or not to serve your assets over HTTPS. If not specified the asset will be served by the same method the request comes in as.	None
CDN_TIMESTAMP	Specifies whether or not to add a timestamp to the generated urls.	True
CDN_VERSION	The version string to add to the generated urls. Useful when the timestamps of your files are different across servers or if you just want a more stable cache key.	None
CDN_ENDPOINTS	The list of endpoints that will be rewritten to use the CDN. Endpoints will be checked for an exact match and also if it ends with a period followed by the endpoint name.	['static']

Serve Static Assets with CloudFront

CloudFront is a very simple way to seamlessly serve your static assets with it’s CDN. When a request comes into CloudFront, if the asset is not on the CDN or has expired, then CloudFront can get the asset from an “origin server”. This type of setup is called an origin pull CDN.

To setup a new CloudFront “Distribution”:

• Signup for an AWS Account
• Open the Cloudfront Management Console
• Select Create Distribution
• Leave Download selected as the delivery method and select Continue
• In the Origin Domain Name field enter the domain name for your application
• Change Forward Query Strings to Yes
• Keep the other default values as-is and select Create Distribution

It will now take a few minutes for AWS to create the CloudFront distribution.

• Set CDN_DOMAIN in your Flask app to the newly created ‘Domain Name’ of your Cloudfront CDN.

Then you are done! Next time someone visits your site Cloudfront will cache and serve everything under the /static/ directory.