Introduction

In this guide, we will be setting up a simple Python application using the Flask micro-framework on CentOS 7. The bulk of this article will be about how to set up the uWSGI application server to launch the application and Nginx to act as a front end reverse proxy.

Prerequisites

Before starting on this guide, you should have a non-root user configured on your server. This user needs to have sudo privileges so that it can perform administrative functions. To learn how to set this up, follow our initial server setup guide.

To learn more about uWSGI, our application server and the WSGI specification, you can read the linked section of this guide. Understanding these concepts will make this guide easier to follow.

When you are ready to continue, read on.

Install the Components from the CentOS and EPEL Repositories

Our first step will be to install all of the pieces that we need from the repositories. We will need to add the EPEL repository, which contains some extra packages, in order to install some of the components we need.

You can enable the EPEL repo by typing:

sudo yum install epel-release

Once access to the EPEL repository is configured on our system, we can begin installing the packages we need. We will install pip, the Python package manager, in order to install and manage our Python components. We will also get a compiler and the Python development files needed to build uWSGI. We'll install Nginx now as well.

You can install all of these components by typing:

sudo yum install python-pip python-devel gcc nginx

Create a Python Virtual Environment

Next, we'll set up a virtual environment in order to isolate our Flask application from the other Python files on the system.

Start by installing the virtualenv package using pip:

sudo pip install virtualenv

Now, we can make a parent directory for our Flask project. Move into the directory after you create it:

mkdir ~/myproject
cd ~/myproject

We can create a virtual environment to store our Flask project's Python requirements by typing:

virtualenv myprojectenv

This will install a local copy of Python and pip into a directory called myprojectenv within your project directory.

Before we install applications within the virtual environment, we need to activate it. You can do so by typing:

source myprojectenv/bin/activate

Your prompt will change to indicate that you are now operating within the virtual environment. It will look something like this (myprojectenv)user@host:~/myproject$.

Set Up a Flask Application

Now that you are in your virtual environment, we can install Flask and uWSGI and get started on designing our application:

Install Flask and uWSGI

We can use the local instance of pip to install Flask and uWSGI. Type the following commands to get these two components:

pip install uwsgi flask

Create a Sample App

Now that we have Flask available, we can create a simple application. Flask is a micro-framework. It does not include many of the tools that more full-featured frameworks might, and exists mainly as a module that you can import into your projects to assist you in initializing a web application.

While your application might be more complex, we'll create our Flask app in a single file, which we will call myproject.py:

nano ~/myproject/myproject.py

Within this file, we'll place our application code. Basically, we need to import flask and instantiate a Flask object. We can use this to define the functions that should be run when a specific route is requested. We'll call our Flask application in the code application to replicate the examples you'd find in the WSGI specification:

from flask import Flask
application = Flask(__name__)

@application.route("/")
def hello():
    return "<h1 style='color:blue'>Hello There!</h1>"

if __name__ == "__main__":
    application.run(host='0.0.0.0')

This basically defines what content to present when the root domain is accessed. Save and close the file when you're finished.

You can test your Flask app by typing:

python myproject.py

Visit your server's domain name or IP address followed by the port number specified in the terminal output (most likely :5000) in your web browser. You should see something like this:

When you are finished, hit CTRL-C in your terminal window a few times to stop the Flask development server.

Create the WSGI Entry Point

Next, we'll create a file that will serve as the entry point for our application. This will tell our uWSGI server how to interact with the application.

We will call the file wsgi.py:

nano ~/myproject/wsgi.py

The file is incredibly simple, we can simply import the Flask instance from our application and then run it:

from myproject import application

if __name__ == "__main__":
    application.run()

Save and close the file when you are finished.

Configure uWSGI

Our application is now written and our entry point established. We can now move on to uWSGI.

Testing uWSGI Serving

The first thing we will do is test to make sure that uWSGI can serve our application.

We can do this by simply passing it the name of our entry point. We'll also specify the socket so that it will be started on a publicly available interface and the protocol so that it will use HTTP instead of the uwsgi binary protocol:

uwsgi --socket 0.0.0.0:8000 --protocol=http -w wsgi

If you visit your server's domain name or IP address with :8000 appended to the end in your web browser, you should see a page that looks like this:

When you have confirmed that it's functioning properly, press CTRL-C in your terminal window.

We're now done with our virtual environment, so we can deactivate it:

deactivate

Any operations now will be done to the system's Python environment.

Creating a uWSGI Configuration File

We have tested that uWSGI is able to serve our application, but we want something more robust for long-term usage. We can create a uWSGI configuration file with the options we want.

Let's place that in our project directory and call it myproject.ini:

nano ~/myproject/myproject.ini

Inside, we will start off with the [uwsgi] header so that uWSGI knows to apply the settings. We'll specify the module by referring to our wsgi.py file, minus the extension:

[uwsgi]
module = wsgi

Next, we'll tell uWSGI to start up in master mode and spawn five worker processes to serve actual requests:

[uwsgi]
module = wsgi

master = true
processes = 5

When we were testing, we exposed uWSGI on a network port. However, we're going to be using Nginx to handle actual client connections, which will then pass requests to uWSGI. Since these components are operating on the same computer, a Unix socket is preferred because it is more secure and faster. We'll call the socket myproject.sock and place it in this directory.

We'll also have to change the permissions on the socket. We'll be giving the Nginx group ownership of the uWSGI process later on, so we need to make sure the group owner of the socket can read information from it and write to it. We will also clean up the socket when the process stops by adding the "vacuum" option:

[uwsgi]
module = wsgi

master = true
processes = 5

socket = myproject.sock
chmod-socket = 660
vacuum = true

The last thing we need to do is set the die-on-term option. This is needed because the Upstart init system and uWSGI have different ideas on what different process signals should mean. Setting this aligns the two system components, implementing the expected behavior:

[uwsgi]
module = wsgi

master = true
processes = 5

socket = myproject.sock
chmod-socket = 660
vacuum = true

die-on-term = true

You may have noticed that we did not specify a protocol like we did from the command line. That is because by default, uWSGI speaks using the uwsgi protocol, a fast binary protocol designed to communicate with other servers. Nginx can speak this protocol natively, so it's better to use this than to force communication by HTTP.

When you are finished, save and close the file.

Create a Systemd Unit File

The next piece we need to take care of is the Systemd service unit file. Creating a Systemd unit file will allow CentOS's init system to automatically start uWSGI and serve our Flask application whenever the server boots.

Create a unit file ending in .service within the /etc/systemd/system directory to begin:

sudo nano /etc/systemd/system/myproject.service

Inside, we'll start with the [Unit] section, which is used to specify metadata and dependencies. We'll put a description of our service here and tell the init system to only start this after the networking target has been reached:

[Unit]
Description=uWSGI instance to serve myproject
After=network.target

Next, we'll open up the [Service] section. We'll specify the user and group that we want the process to run under. We will give our regular user account ownership of the process since it owns all of the relevant files. We'll give the Nginx user group ownership so that it can communicate easily with the uWSGI processes.

We'll then map out the working directory and set the PATH environmental variable so that the init system knows where our the executables for the process are located (within our virtual environmment). We'll then specify the commanded to start the service. Systemd requires that we give the full path to the uWSGI executable, which is installed within our virtual environment. We will pass the name of the .ini configuration file we created in our project directory:

[Unit]
Description=uWSGI instance to serve myproject
After=network.target

[Service]
User=user
Group=nginx
WorkingDirectory=/home/user/myproject
Environment="PATH=/home/user/myproject/myprojectenv/bin"
ExecStart=/home/user/myproject/myprojectenv/bin/uwsgi --ini myproject.ini

Finally, we'll add an [Install] section. This will tell Systemd what to link this service to if we enable it to start at boot. We want this service to start when the regular multi-user system is up and running:

[Unit]
Description=uWSGI instance to serve myproject
After=network.target

[Service]
User=user
Group=nginx
WorkingDirectory=/home/user/myproject
Environment="PATH=/home/user/myproject/myprojectenv/bin"
ExecStart=/home/user/myproject/myprojectenv/bin/uwsgi --ini myproject.ini

[Install]
WantedBy=multi-user.target

With that, our Systemd service file is complete. Save and close it now.

We can now start the uWSGI service we created and enable it so that it starts at boot:

sudo systemctl start myproject
sudo systemctl enable myproject

Configuring Nginx to Proxy Requests

Our uWSGI application server should now be up and running, waiting for requests on the socket file in the project directory. We need to configure Nginx to pass web requests to that socket using the uwsgi protocol.

Begin by opening up Nginx's default configuration file:

sudo nano /etc/nginx/nginx.conf

Open up a server block just above the other server {} block that is already in the file:

http {
    . . .

    include /etc/nginx/conf.d/*.conf;

    server {
    }

    server {
        listen 80 default_server;

        . . .

We will put all of the configuration for our Flask application inside of this new block. We will start by specifying that this block should listen on the default port 80 and that it should respond to our server's domain name or IP address:

server {
    listen 80;
    server_name server_domain_or_IP;
}

The only other thing that we need to add is a location block that matches every request. Within this block, we'll include the uwsgi_params file that specifies some general uWSGI parameters that need to be set. We'll then pass the requests to the socket we defined using the uwsgi_pass directive:

server {
    listen 80;
    server_name server_domain_or_IP;

    location / {
        include uwsgi_params;
        uwsgi_pass unix:/home/user/myproject/myproject.sock;
    }
}

That's actually all we need to serve our application. Save and close the file when you're finished.

The nginx user must have access to our application directory in order to access the socket file there. By default, CentOS locks down each user's home directory very restrictively, so we will add the nginx user to our user's group so that we can then open up the minimum permissions necessary to grant access.

You can add the nginx user to your user group with the following command. Substitute your own username for the user in the command:

sudo usermod -a -G user nginx

Now, we can give our user group execute permissions on our home directory. This will allow the Nginx process to enter and access content within:

chmod 710 /home/user

With the permissions set up, we can test our Nginx configuration file for syntax errors:

sudo nginx -t

If this returns without indicating any issues, we can start and enable the Nginx process so that it starts automatically at boot:

sudo systemctl start nginx
sudo systemctl enable nginx

You should now be able to go to your server's domain name or IP address in your web browser and see your application:

Conclusion

In this guide, we've created a simple Flask application within a Python virtual environment. We create a WSGI entry point so that any WSGI-capable application server can interface with it, and then configured the uWSGI app server to provide this function. Afterwards, we created Systemd service unit file to automatically launch the application server on boot. We created an Nginx server block that passes web client traffic to the application server, relaying external requests.

Flask is a very simple, but extremely flexible framework meant to provide your applications with functionality without being too restrictive about structure and design. You can use the general stack described in this guide to serve the flask applications that you design.