Sub mounting a flask application under a URL prefix

Skill - Sub mounting a flask application under a URL prefix

Table of Contents

Skills Required

• Setup python development environment
• Flask python module introduction

Please make sure to have all the skills mentioned above to understand and execute the code mentioned below. Go through the above skills if necessary for reference or revision

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• In this post we will learn how to sub-mount a flask application under a URL prefix

Use cases

Mounting an application under a URL prefix can help in

• dispatching multiple flask applications from a single server
• Placing the flask application behind a reverse proxy like (nginx or IIS) with a URL prefix. URL prefix is required because the reverse proxy can serve multiple applications each with different URL prefix

Sub mounting an application using DispatcherMiddleware class

• DispatcherMiddleware class can be used to dispatch one or more flask applications each with different URL prefix
• Each URL prefix should start with a / (example /app1)
• If no prefixes are matched, the DispatcherMiddleware can be configured to route the requests to a particular flask application or send a “404 not found” error

Example

from flask import Flask
from werkzeug.middleware.dispatcher import DispatcherMiddleware
from werkzeug.exceptions import NotFound

# create a server instance
app = Flask(__name__)

@app.route('/')
def index():
    return "Hello World!!!"

@app.route('/help')
def helpPage():
    return "This is the Help Page"

hostedApp = Flask(__name__)
hostedApp.wsgi_app = DispatcherMiddleware(NotFound(), {
    "/myApp": app
})

# run the server
hostedApp.run(host="0.0.0.0", port=50100, debug=True)

• In the above example, the flask application named app is sub-mounted under a URL prefix “/myApp”
• In case of no matching URL prefix, “404 not found page” will be returned

Combining multiple flask applications

from flask import Flask
from werkzeug.middleware.dispatcher import DispatcherMiddleware
from werkzeug.exceptions import NotFound

app1 = Flask(__name__)

@app1.route('/')
def index1():
    return "App1 Hello World!!!"

@app1.route('/help')
def helpPage1():
    return "App1 Help Page"

app2 = Flask(__name__)

@app2.route('/')
def index2():
    return "App2 Home Page"

@app2.route('/help')
def helpPage2():
    return "App2 Help Page"


app3 = Flask(__name__)

@app3.route('/')
def index3():
    return "App3 Home"

@app3.route('/help')
def helpPage3():
    return "App3 Help Page"

hostedApp = Flask(__name__)
hostedApp.wsgi_app = DispatcherMiddleware(app1, {
    "/abc": app2,
    "/def": app3
})

# run the server
hostedApp.run(host="0.0.0.0", port=50100, debug=True)

• In the above example, 3 different flask applications are dispatched from a single server using DispatcherMiddleware
• DispatcherMiddleware is configure to route the requests to app1 if no URL prefixes are matched
• The requests are sent to app2 if the URL prefix is “abc” and the requests are sent to app3 if the URL prefix is “def”

Video

The video for this post can be seen here

References

• DispatcherMiddleware docs - https://flask.palletsprojects.com/en/2.2.x/patterns/appdispatch/#combining-applications