Cooking with Flask...

Joshua McDill -

The following is a little web environment I built to test out load balancing between Flask web apps. I need to clean up the VM hosting this and didn't want to lose the activity, so here it is in a nutshell for all to enjoy and replicate as needed. I'm not going into a lot of setup details as by the time you read this (possibly even now as I write it) the precise steps would have changed. Always consult your latest documentation.

The recipe is as follows:

Steps (high-level)

  • Install Apache
  • Install Balance module
  • Configure httpd configs for Balance (see example below)
  • Install Python3
  • Install Flask
  • Code you web apps
  • Fire your web apps and see if it works!

Below are a couple of snippets from my HTTPD config file showing how the Balancer is configured and used...in this case with two nodes:


Note: I'm replacing angle brackets with regular brackets to because of how the code is rendering in my blog engine. Copy-pasters beware!


[VirtualHost *:80]
        DocumentRoot /var/www/html
        ServerName warybyte.com
        [Proxy balancer://mycluster]
                BalancerMember http://127.0.0.1:8090
                BalancerMember http://127.0.0.1:8091
        #       Require ip 127.0.0.1
        [/Proxy]
        # call hello app
        ProxyPass /hello http://127.0.0.1:8090/
        ProxyPassReverse /hello https://127.0.0.1:8090/
        
        # call howdy app
        ProxyPass /howdy http://127.0.0.1:8091
        ProxyPassReverse /howdy http://127.0.0.1:8091/
        
        # balance main over both
        ProxyPass / balancer://mycluster/
        ProxyPassReverse / balancer://mycluster/
[/VirtualHost]

Below is one of the Flask apps...a very simple 'hello world' like app, nothing fancy, but lets me know what app I'm looking at when I'm testing the balancer


$ cat backend1.py 
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello world!'
    
$ cat backend2.py 
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Howdy world!'

Below is the script I wrote to fire all my Flask apps together...nifty when you have a bunch or want to start them via Cron.


$ cat fireflasks.sh
#$/bin/bash
# clean up existing apps
killall flask;
echo "Proceeding with webapp start!";

# python app1
export FLASK_APP=backend1.py; 
flask run --port=8090 >/dev/null 2>&1 &
ps -elf | grep "8090";

# python app2
export FLASK_APP=backend2.py; 
flask run --port=8091 >/dev/null 2>&1 &
ps -elf | grep "8091"