flask

Lightweight and limited components

Features included:

Jinja 2 template engine ( Building HTML ) - similar to DJANGO
Werkzeug ( HTTP and routing )
Development server and debugger
Unit testing support

Installing the flask dependency

python -m pip install flask

Setting up two environment variables :

Which module can my find my app(Unix) :

export FLASK_APP=flashcards.py
What environment ( dev enables debugger )

export FLASK_ENV=development
Start Command

flask run

How to add some images / css / javascript

subfolder static/

Model Template View Pattern

View - Functions already defined by @app.route functions
Jinja Templates
- Display data to users
- Generating HTML
- Calling templates from views(func)
- Passing Data from view to template
- Jinja variables
Data Model ( DB side )
- model layer

Jinja Templates

import the render_template and pass the template name

return render_template("welcome.html")
/templates folder is where the scan happens
{{message}} : Jinja variable in the html for eval
Value passed from the view(function)

return render_template("welcome.html" , message="")

Model Layer [ DB]

Refer to the model.py module

Logic Handling

View(Function) Logic
- Take Arg from Url
  
  @app.route('/card/int:index')
- Return HTTP Error
  
  import abort -- abort(404)
- Serving Data as REST API
Template Logic - Jinja For and IF statements

refer to card.html

Creating Links to Cards

Rest API ( Data )

refer to @app.route('/api/**) in flashcards.py refer how jsonify is used on List obejcts

Deploying to Production ( best practice )

Not to Use flask server Use popular pythin httpserver : gunicorn
Run behind a Reverse Proxy (nginx)

Setup on Ubuntu

apt install the following packages - nginx - python3 , python3-pip

git clone
sudo apt install gunicorn3 ( at system level and not pip )
pip3 install -r requirements.txt ( use Venv )
gunicorn3 flashcards:app

This makes the server listen on port 8000 , but we want it to be on 80 on public ip

Use nginx ( webserver runs on port 80 ) ( prevents DDOS ) Now run in Daemon mode after logout as well

gunicorn3 -D flashcards:app

Nginx configuration

Remove the default home page of the webserver at :

cd /etc/nginx/sites-available
sudo rm default

Create a new Site ( refer to e.g config from gunicorn.org)

vi default

 server {
 listen 80;
 server_name example.org;
 access_log  /var/log/nginx/example.log;

 location / {
     proxy_pass http://127.0.0.1:8000;
     proxy_set_header Host $host;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 }
}

Restart the nginx service

sudo service nginx restart

Prometheus Flask Exporter

In your venv install the prometheus-flask-exporter

pip install prometheus-flask-exporter

satadrubasu/flask