The Hacker Club Flask Template

Introduction

This template provides a starting point for new Hacker Club Flask projects.

Features include:

• Dartmouth CAS authentication pre-configured
• Latest Twitter Bootstrap and jQuery from CDN
• Jinja2 templating pre-configured

Usage

Assuming you already have Python and pip installed, go ahead and install virtualenv if you haven't already:

$ sudo pip install virtualenv

Now, create a new virtualenv (this provides an isolated environment for the app to run in):

$ virtualenv <project_name>
$ cd project_name
$ source bin/activate

The virtualenv is now set up. You can clone this repository inside the virtualenv:

$ git clone https://github.com/DartmouthHackerClub/flask_template.git

I've included a requirements.txt file, which, coupled with pip and virtualenv, provides an automatic method for installing all necessary dependencies:

$ cd flask_template
$ pip install -r requirements.txt

Everything is now set up. You can now launch the app:

$ python app.py

Check out the Flask docs to learn where you can go from here.

Deployment

I recommend using several gunicorn instances behind nginx. This works as follows:

Launch the gunicorn app server

$ gunicorn app:app -b 127.0.0.1:1337 -w 2

Two gunicorn worker processes will be launched and bound to port 1337. The app:app part refers to the app variable (i.e. the Flask object) within app.py.

Notice that gunicorn will only serve requests originating from localhost -- this is ideal because gunicorn is not meant to be accessed directly (except by nginx, which is on localhost).

Configure nginx as a reverse proxy to gunicorn

server {
    server_name app.hacktown.cs.dartmouth.edu;
    root /srv/http/app.hacktown.cs.dartmouth.edu/public;

    location / {
        try_files $uri @gunicorn;
        index index.html index.htm;
    }

    location @gunicorn {
        proxy_pass         http://127.0.0.1:1337/;
        proxy_redirect     off;
        proxy_set_header   Host             $host;
        proxy_set_header   X-Real-IP        $remote_addr;
        proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
    }
} 

This configuration will first try to directly serve a static file if it exists, and if not, proxy the request to gunicorn. In other words, if you put static assets in /srv/http/app.hacktown.cs.dartmouth.edu/public, they will be served efficiently by nginx and not by gunicorn.

Set up process control

Now the app should be available at http://app.hacktown.cs.dartmouth.edu/. However, what happens if the machine is restarted or if gunicorn is terminated abnormally? You will get a 503 error because gunicorn isn't running. We need something that will automatically restart gunicorn if it isn't running for some reason. We need a process control system.

The process control system I use is Supervisor. Once you've got it installed, you can use the following configuration to keep gunicorn running all the time:

[program:app_name]
command = /home/deploy/env/app_name/bin/gunicorn app:app -b 127.0.0.1:1337 -w 2
directory = /home/deploy/env/app_name/app_name
user = deploy

Automate updates with fabric

I've included a fabfile.py which greatly simplifies pushing new code to the staging and production servers. To deploy the latest code to the testing server:

$ fab test deploy

Or to the production server:

$ fab prod deploy