Fabric is a tool for Python similar to Makefiles but with the ability to execute commands on a remote server. In combination with a properly set up Python package (Larger Applications) and a good concept for configurations (Configuration Handling) it is very easy to deploy Flask applications to external servers.
Before we get started, here a quick checklist of things we have to ensure upfront:
A fabfile is what controls what Fabric executes. It is named fabfile.py and executed by the fab command. All the functions defined in that file will show up as fab subcommands. They are executed on one or more hosts. These hosts can be defined either in the fabfile or on the command line. In this case we will add them to the fabfile.
This is a basic first example that has the ability to upload the current sourcecode to the server and install it into a pre-existing virtual environment:
from fabric.api import * # the user to use for the remote commands env.user = 'appuser' # the servers where the commands are executed env.hosts = ['server1.example.com', 'server2.example.com'] def pack(): # create a new source distribution as tarball local('python setup.py sdist --formats=gztar', capture=False) def deploy(): # figure out the release name and version dist = local('python setup.py --fullname', capture=True).strip() # upload the source tarball to the temporary folder on the server put('dist/%s.tar.gz' % dist, '/tmp/yourapplication.tar.gz') # create a place where we can unzip the tarball, then enter # that directory and unzip it run('mkdir /tmp/yourapplication') with cd('/tmp/yourapplication'): run('tar xzf /tmp/yourapplication.tar.gz') # now setup the package with our virtual environment's # python interpreter run('/var/www/yourapplication/env/bin/python setup.py install') # now that all is set up, delete the folder again run('rm -rf /tmp/yourapplication /tmp/yourapplication.tar.gz') # and finally touch the .wsgi file so that mod_wsgi triggers # a reload of the application run('touch /var/www/yourapplication.wsgi')
The example above is well documented and should be straightforward. Here a recap of the most common commands fabric provides:
Now how do you execute that fabfile? You use the fab command. To deploy the current version of the code on the remote server you would use this command:
$ fab pack deploy
However this requires that our server already has the /var/www/yourapplication folder created and /var/www/yourapplication/env to be a virtual environment. Furthermore are we not creating the configuration or .wsgi file on the server. So how do we bootstrap a new server into our infrastructure?
This now depends on the number of servers we want to set up. If we just have one application server (which the majority of applications will have), creating a command in the fabfile for this is overkill. But obviously you can do that. In that case you would probably call it setup or bootstrap and then pass the servername explicitly on the command line:
$ fab -H newserver.example.com bootstrap
To setup a new server you would roughly do these steps:
Create the directory structure in /var/www:
$ mkdir /var/www/yourapplication $ cd /var/www/yourapplication $ virtualenv --distribute env
Upload a new application.wsgi file to the server and the configuration file for the application (eg: application.cfg)
Create a new Apache config for yourapplication and activate it. Make sure to activate watching for changes of the .wsgi file so that we can automatically reload the application by touching it. (See mod_wsgi (Apache) for more information)
So now the question is, where do the application.wsgi and application.cfg files come from?
The WSGI file has to import the application and also to set an environment variable so that the application knows where to look for the config. This is a short example that does exactly that:
import os os.environ['YOURAPPLICATION_CONFIG'] = '/var/www/yourapplication/application.cfg' from yourapplication import app
The application itself then has to initialize itself like this to look for the config at that environment variable:
app = Flask(__name__) app.config.from_object('yourapplication.default_config') app.config.from_envvar('YOURAPPLICATION_CONFIG')
This approach is explained in detail in the Configuration Handling section of the documentation.
Now as mentioned above, the application will find the correct configuration file by looking up the YOURAPPLICATION_CONFIG environment variable. So we have to put the configuration in a place where the application will able to find it. Configuration files have the unfriendly quality of being different on all computers, so you do not version them usually.
A popular approach is to store configuration files for different servers in a separate version control repository and check them out on all servers. Then symlink the file that is active for the server into the location where it’s expected (eg: /var/www/yourapplication).
Either way, in our case here we only expect one or two servers and we can upload them ahead of time by hand.
Now we can do our first deployment. We have set up the servers so that they have their virtual environments and activated apache configs. Now we can pack up the application and deploy it:
$ fab pack deploy
Fabric will now connect to all servers and run the commands as written down in the fabfile. First it will execute pack so that we have our tarball ready and then it will execute deploy and upload the source code to all servers and install it there. Thanks to the setup.py file we will automatically pull in the required libraries into our virtual environment.
From that point onwards there is so much that can be done to make deployment actually fun:
Working with Fabric is fun and you will notice that it’s quite magical to type fab deploy and see your application being deployed automatically to one or more remote servers.