Chapter 11. Automating Deployment with Fabric

Automate, automate, automate.

Cay Horstman

Automating deployment is critical for our staging tests to mean anything. By making sure the deployment procedure is repeatable, we give ourselves assurances that everything will go well when we deploy to production.

Fabric is a tool which lets you automate commands that you want to run on servers. “fabric3” is the Python 3 fork:

$ pip install fabric3

Tip

It’s safe to ignore any errors that say “failed building wheel” during the fabric3 installation, as long as it says “Successfully installed…” at the end.

The usual setup is to have a file called fabfile.py, which will contain one or more functions that can later be invoked from a command-line tool called fab, like this:

fab function_name:host=SERVER_ADDRESS

That will call function_name, passing in a connection to the server at SERVER_ADDRESS. There are lots of other options for specifying usernames and passwords, which you can find out about using fab --help.

Breakdown of a Fabric Script for Our Deployment

The best way to see how it works is with an example. Here’s one I made earlier, automating all the deployment steps we’ve been going through. The main function is called deploy; that’s the one we’ll invoke from the command line. It then calls out to several helper functions, which we’ll build together one by one, explaining as we go.

deploy_tools/fabfile.py (ch09l001)

from fabric.contrib.files import append, exists, sed
from fabric.api import env, local, run
import random

REPO_URL = 'https://github.com/hjwp/book-example.git'  

def deploy():
    site_folder = f'/home/{env.user}/sites/{env.host}'  
    source_folder = site_folder + '/source'
    _create_directory_structure_if_necessary(site_folder)
    _get_latest_source(source_folder)
    _update_settings(source_folder, env.host)  
    _update_virtualenv(source_folder)
    _update_static_files(source_folder)
    _update_database(source_folder)

You’ll want to update the REPO_URL variable with the URL of your own Git repo on its code-sharing site.

env.host will contain the address of the server we’ve specified at the command line (e.g., superlists.ottg.eu).

env.user will contain the username you’re using to log in to the server.

Hopefully all of those helper functions have fairly self-descriptive names. Because any function in a fabfile can theoretically be invoked from the command line, I’ve used the convention of a leading underscore to indicate that they’re not meant to be part of the “public API” of the fabfile. Let’s take a look at each one, in chronological order.

Creating the Directory Structure

Here’s how we build our directory structure, in a way that doesn’t fall down if it already exists:

deploy_tools/fabfile.py (ch09l002)

def _create_directory_structure_if_necessary(site_folder):
    for subfolder in ('database', 'static', 'virtualenv', 'source'):
        run(f'mkdir -p {site_folder}/{subfolder}')  

run is the most common Fabric command. It says “run this shell command on the server”. The run commands in this chapter will replicate many of the commands we did manually in the last two.

mkdir -p is a useful flavour of mkdir, which is better in two ways: it can create directories several levels deep, and it only creates them if necessary. So, mkdir -p /tmp/foo/bar will create the directory bar but also its parent directory foo if it needs to. It also won’t complain if bar already exists.¹

Pulling Down Our Source Code with Git

Next we want to download the latest version of our source code to the server, like we did with git pull in the previous chapters:

deploy_tools/fabfile.py (ch09l003)

def _get_latest_source(source_folder):
    if exists(source_folder + '/.git'):  
        run(f'cd {source_folder} && git fetch')  
    else:
        run(f'git clone {REPO_URL} {source_folder}')  
    current_commit = local("git log -n 1 --format=%H", capture=True)  
    run(f'cd {source_folder} && git reset --hard {current_commit}')  

exists checks whether a directory or file already exists on the server. We look for the .git hidden folder to check whether the repo has already been cloned in that folder.

Many commands start with a cd in order to set the current working directory. Fabric doesn’t have any state, so it doesn’t remember what directory you’re in from one run to the next.²

git fetch inside an existing repository pulls down all the latest commits from the web (it’s like git pull, but without immediately updating the live source tree).

Alternatively we use git clone with the repo URL to bring down a fresh source tree.

Fabric’s local command runs a command on your local machine—it’s just a wrapper around subprocess.Popen really, but it’s quite convenient. Here we capture the output from that git log invocation to get the ID of the current commit that’s on your local PC. That means the server will end up with whatever code is currently checked out on your machine (as long as you’ve pushed it up to the server).

We reset --hard to that commit, which will blow away any current changes in the server’s code directory.

The end result of this is that we either do a git clone if it’s a fresh deploy, or we do a git fetch + git reset --hard if a previous version of the code is already there; the equivalent of the git pull we used when we did it manually, but with the reset --hard to force overwriting any local changes.

Note

For this script to work, you need to have done a git push of your current local commit, so that the server can pull it down and reset to it. If you see an error saying Could not parse object, try doing a git push.

Updating settings.py

Next we update our settings file, to set the ALLOWED_HOSTS and DEBUG variables, and to create a new SECRET_KEY:

deploy_tools/fabfile.py (ch09l004)

def _update_settings(source_folder, site_name):
    settings_path = source_folder + '/superlists/settings.py'
    sed(settings_path, "DEBUG = True", "DEBUG = False")  
    sed(settings_path,
        'ALLOWED_HOSTS =.+$',
        f'ALLOWED_HOSTS = ["{site_name}"]'  
    )
    secret_key_file = source_folder + '/superlists/secret_key.py'
    if not exists(secret_key_file):  
        chars = 'abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)'
        key = ''.join(random.SystemRandom().choice(chars) for _ in range(50))
        append(secret_key_file, f'SECRET_KEY = "{key}"')
    append(settings_path, '\nfrom .secret_key import SECRET_KEY')  

The Fabric sed command does a string substitution in a file; here it’s changing DEBUG from True to False.

And here it is adjusting ALLOWED_HOSTS, using a regex to match the right line.

Django uses SECRET_KEY for some of its crypto—things like cookies and CSRF protection. It’s good practice to make sure the secret key on the server is different from the one in your source code repo, because that code might be visible to strangers. This section will generate a new key to import into settings, if there isn’t one there already (once you have a secret key, it should stay the same between deploys). Find out more in the Django docs.

append just adds a line to the end of a file. (It’s clever enough not to bother if the line is already there, but not clever enough to automatically add a newline if the file doesn’t end in one. Hence the back-n.)

I’m using a relative import (from .secret_key instead of from secret_key) to be absolutely sure we’re importing the local module, rather than one from somewhere else on sys.path. I’ll talk a bit more about relative imports in the next chapter.

Note

Hacking the settings file like this is one way of changing configuration on the server. Another common pattern is to use environment variables. We’ll see that in Chapter 21. See which one you like best.

Updating the Virtualenv

Next we create or update the virtualenv:

deploy_tools/fabfile.py (ch09l005)

def _update_virtualenv(source_folder):
    virtualenv_folder = source_folder + '/../virtualenv'
    if not exists(virtualenv_folder + '/bin/pip'):  
        run(f'python3.6 -m venv {virtualenv_folder}')
    run(f'{virtualenv_folder}/bin/pip install -r {source_folder}/requirements.txt')  

We look inside the virtualenv folder for the pip executable as a way of checking whether it already exists.

Then we use pip install -r like we did earlier.

Updating static files is a single command:

deploy_tools/fabfile.py (ch09l006)

def _update_static_files(source_folder):
    run(
        f'cd {source_folder}'  
        ' && ../virtualenv/bin/python manage.py collectstatic --noinput'  
    )

You can split long strings across multiple lines like this in Python; they concatenate to a single string. It’s a common source of bugs when what you actually wanted was a list of strings, but you forgot a comma!

We use the virtualenv binaries folder whenever we need to run a Django manage.py command, to make sure we get the virtualenv version of Django, not the system one.

Migrating the Database If Necessary

Finally, we update the database with manage.py migrate:

deploy_tools/fabfile.py (ch09l007)

def _update_database(source_folder):
    run(
        f'cd {source_folder}'
        ' && ../virtualenv/bin/python manage.py migrate --noinput'
    )

The --noinput removes any interactive yes/no confirmations that Fabric would find hard to deal with.

And we’re done! Lots of new things to take in, I imagine, but I hope you can see how this is all replicating the work we did manually earlier, with a bit of logic to make it work both for brand new deployments and for existing ones that just need updating. If you like words with Latin roots, you might describe it as idempotent, which means it has the same effect whether you run it once or multiple times.

Trying It Out

Let’s try it out on our existing staging site, and see it working to update a deployment that already exists:

$ cd deploy_tools
$ fab deploy:host=elspeth@superlists-staging.ottg.eu

[superlists-staging.ottg.eu] Executing task 'deploy'
[superlists-staging.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists-stagin
[superlists-staging.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists-stagin
[superlists-staging.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists-stagin
[superlists-staging.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists-stagin
[superlists-staging.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists-stagin
[superlists-staging.ottg.eu] run: cd /home/elspeth/sites/superlists-staging.ottg
[localhost] local: git log -n 1 --format=%H
[superlists-staging.ottg.eu] run: cd /home/elspeth/sites/superlists-staging.ottg
[superlists-staging.ottg.eu] out: HEAD is now at 85a6c87 Add a fabfile for autom
[superlists-staging.ottg.eu] out:

[superlists-staging.ottg.eu] run: sed -i.bak -r -e 's/DEBUG = True/DEBUG = False
[superlists-staging.ottg.eu] run: echo 'ALLOWED_HOSTS = ["superlists-staging.ott
[superlists-staging.ottg.eu] run: echo 'SECRET_KEY = '\\''4p2u8fi6)bltep(6nd_3tt
[superlists-staging.ottg.eu] run: echo 'from .secret_key import SECRET_KEY' >> "

[superlists-staging.ottg.eu] run: /home/elspeth/sites/superlists-staging.ottg.eu
[superlists-staging.ottg.eu] out: Requirement already satisfied (use --upgrade t
[superlists-staging.ottg.eu] out: Requirement already satisfied (use --upgrade t
[superlists-staging.ottg.eu] out: Cleaning up...
[superlists-staging.ottg.eu] out:

[superlists-staging.ottg.eu] run: cd /home/elspeth/sites/superlists-staging.ottg
[superlists-staging.ottg.eu] out:
[superlists-staging.ottg.eu] out: 0 static files copied, 11 unmodified.
[superlists-staging.ottg.eu] out:

[superlists-staging.ottg.eu] run: cd /home/elspeth/sites/superlists-staging.ottg
[superlists-staging.ottg.eu] out: Creating tables ...
[superlists-staging.ottg.eu] out: Installing custom SQL ...
[superlists-staging.ottg.eu] out: Installing indexes ...
[superlists-staging.ottg.eu] out: Installed 0 object(s) from 0 fixture(s)
[superlists-staging.ottg.eu] out:
Done.
Disconnecting from superlists-staging.ottg.eu... done.

Awesome. I love making computers spew out pages and pages of output like that (in fact I find it hard to stop myself from making little ’70s computer <brrp, brrrp, brrrp> noises like Mother in Alien). If we look through it we can see it is doing our bidding: the mkdir -p commands go through happily, even though the directories already exist. Next git pull pulls down the couple of commits we just made. The sed and echo >> modify our ’settings.py’. Then pip install -r requirements.txt completes happily, noting that the existing virtualenv already has all the packages we need. collectstatic also notices that the static files are all already there, and finally the migrate completes without a hitch.

Fabric Configuration

If you are using an SSH key to log in, are storing it in the default location, and are using the same username on the server as locally, then Fabric should “just work”. If you aren’t, there are several tweaks you may need to apply in order to get the fab command to do your bidding. They revolve around the username, the location of the SSH key to use, or the password.

You can pass these in to Fabric at the command line. Check out:

$ fab --help

Or see the Fabric documentation for more info.

Deploying to Live

So, let’s try using it for our live site!

$ fab deploy:host=elspeth@superlists.ottg.eu

$ fab deploy --host=superlists.ottg.eu
[superlists.ottg.eu] Executing task 'deploy'
[superlists.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists.ottg.eu
[superlists.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists.ottg.eu/databa
[superlists.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists.ottg.eu/static
[superlists.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists.ottg.eu/virtua
[superlists.ottg.eu] run: mkdir -p /home/elspeth/sites/superlists.ottg.eu/source
[superlists.ottg.eu] run: git clone https://github.com/hjwp/book-example.git /ho
[superlists.ottg.eu] out: Cloning into '/home/elspeth/sites/superlists.ottg.eu/s
[superlists.ottg.eu] out: remote: Counting objects: 3128, done.
[superlists.ottg.eu] out: Receiving objects:   0% (1/3128)
[...]
[superlists.ottg.eu] out: Receiving objects: 100% (3128/3128), 2.60 MiB | 829 Ki
[superlists.ottg.eu] out: Resolving deltas: 100% (1545/1545), done.
[superlists.ottg.eu] out:

[localhost] local: git log -n 1 --format=%H
[superlists.ottg.eu] run: cd /home/elspeth/sites/superlists.ottg.eu/source && gi
[superlists.ottg.eu] out: HEAD is now at 6c8615b use a secret key file
[superlists.ottg.eu] out:
[superlists.ottg.eu] run: sed -i.bak -r -e 's/DEBUG = True/DEBUG = False/g' "$(e
[superlists.ottg.eu] run: echo 'ALLOWED_HOSTS = ["superlists.ottg.eu"]' >> "$(ec
[superlists.ottg.eu] run: echo 'SECRET_KEY = '\\''mqu(ffwid5vleol%ke^jil*x1mkj-4
[superlists.ottg.eu] run: echo 'from .secret_key import SECRET_KEY' >> "$(echo /
[superlists.ottg.eu] run: python3.6 -m venv /home/elspeth/sites/superl
[superlists.ottg.eu] out: Using interpreter /usr/bin/python3.6
[superlists.ottg.eu] out: Using base prefix '/usr'
[superlists.ottg.eu] out: New python executable in /home/elspeth/sites/superlist
[superlists.ottg.eu] out: Also creating executable in /home/elspeth/sites/superl
[superlists.ottg.eu] out: Installing Setuptools............................done.

[superlists.ottg.eu] out: Installing Pip...................................done.
[superlists.ottg.eu] out:

[superlists.ottg.eu] run: /home/elspeth/sites/superlists.ottg.eu/source/../virtu
[superlists.ottg.eu] out: Downloading/unpacking Django==1.11 (from -r /home/el
[superlists.ottg.eu] out:   Downloading Django-1.11.tar.gz (8.0MB):
[...]
[superlists.ottg.eu] out:   Downloading Django-1.11.tar.gz (8.0MB): 100%  8.0M
[superlists.ottg.eu] out:   Running setup.py egg_info for package Django
[superlists.ottg.eu] out:
[superlists.ottg.eu] out:     warning: no previously-included files matching '__
[superlists.ottg.eu] out:     warning: no previously-included files matching '*.
[superlists.ottg.eu] out: Downloading/unpacking gunicorn==17.5 (from -r /home/el
[superlists.ottg.eu] out:   Downloading gunicorn-17.5.tar.gz (367kB): 100%  367k
[...]

[superlists.ottg.eu] out:   Downloading gunicorn-17.5.tar.gz (367kB): 367kB down
[superlists.ottg.eu] out:   Running setup.py egg_info for package gunicorn
[superlists.ottg.eu] out:
[superlists.ottg.eu] out: Installing collected packages: Django, gunicorn
[superlists.ottg.eu] out:   Running setup.py install for Django
[superlists.ottg.eu] out:     changing mode of build/scripts-3.3/django-admin.py
[superlists.ottg.eu] out:
[superlists.ottg.eu] out:     warning: no previously-included files matching '__
[superlists.ottg.eu] out:     warning: no previously-included files matching '*.
[superlists.ottg.eu] out:     changing mode of /home/elspeth/sites/superlists.ot
[superlists.ottg.eu] out:   Running setup.py install for gunicorn
[superlists.ottg.eu] out:
[superlists.ottg.eu] out:     Installing gunicorn_paster script to /home/elspeth
[superlists.ottg.eu] out:     Installing gunicorn script to /home/elspeth/sites/
[superlists.ottg.eu] out:     Installing gunicorn_django script to /home/elspeth
[superlists.ottg.eu] out: Successfully installed Django gunicorn
[superlists.ottg.eu] out: Cleaning up...
[superlists.ottg.eu] out:

[superlists.ottg.eu] run: cd /home/elspeth/sites/superlists.ottg.eu/source && ..
[superlists.ottg.eu] out: Copying '/home/elspeth/sites/superlists.ottg.eu/source
[superlists.ottg.eu] out: Copying '/home/elspeth/sites/superlists.ottg.eu/source
[...]
[superlists.ottg.eu] out: Copying '/home/elspeth/sites/superlists.ottg.eu/source
[superlists.ottg.eu] out:
[superlists.ottg.eu] out: 11 static files copied.
[superlists.ottg.eu] out:
[superlists.ottg.eu] run: cd /home/elspeth/sites/superlists.ottg.eu/source && ..
[superlists.ottg.eu] out: Creating tables ...
[superlists.ottg.eu] out: Creating table auth_permission
[...]
[superlists.ottg.eu] out: Creating table lists_item
[superlists.ottg.eu] out: Installing custom SQL ...
[superlists.ottg.eu] out: Installing indexes ...
[superlists.ottg.eu] out: Installed 0 object(s) from 0 fixture(s)
[superlists.ottg.eu] out:
Done.
Disconnecting from superlists.ottg.eu... done.

Brrp brrp brpp. You can see the script follows a slightly different path, doing a git clone to bring down a brand new repo instead of a git pull. It also needs to set up a new virtualenv from scratch, including a fresh install of pip and Django. The collectstatic actually creates new files this time, and the migrate seems to have worked too.

Nginx and Gunicorn Config Using sed

What else do we need to do to get our live site into production? We refer to our provisioning notes, which tell us to use the template files to create our Nginx virtual host and the Systemd service. How about a little Unix command-line magic?

elspeth@server:$ sed "s/SITENAME/superlists.ottg.eu/g" \
    source/deploy_tools/nginx.template.conf \
    | sudo tee /etc/nginx/sites-available/superlists.ottg.eu

sed (“stream editor”) takes a stream of text and performs edits on it. It’s no accident that the Fabric string substitution command has the same name. In this case we ask it to substitute the string SITENAME for the address of our site, with the s/replaceme/withthis/g syntax.3 We pipe (|) the output of that to a root-user process (sudo), which uses tee to write what’s piped to it to a file, in this case the Nginx sites-available virtualhost config file.

Next we activate that file with a symlink:

elspeth@server:$ sudo ln -s ../sites-available/superlists.ottg.eu \
    /etc/nginx/sites-enabled/superlists.ottg.eu

And we write the Systemd service, with another sed:

elspeth@server: sed "s/SITENAME/superlists.ottg.eu/g" \
    source/deploy_tools/gunicorn-systemd.template.service \
    | sudo tee /etc/systemd/system/gunicorn-superlists.ottg.eu.service

Finally we start both services:

elspeth@server:$ sudo systemctl daemon-reload
elspeth@server:$ sudo systemctl reload nginx
elspeth@server:$ sudo systemctl enable gunicorn-superlists.ottg.eu
elspeth@server:$ sudo systemctl start gunicorn-superlists.ottg.eu

And we take a look at our site: Figure 11-1. It works—hooray!

Figure 11-1. Brrp, brrp, brrp…it worked!

It’s done a good job. Good fabfile, have a biscuit. You have earned the privilege of being added to the repo:

$ git add deploy_tools/fabfile.py
$ git commit -m "Add a fabfile for automated deploys"

Git Tag the Release

One final bit of admin. In order to preserve a historical marker, we’ll use Git tags to mark the state of the codebase that reflects what’s currently live on the server:

$ git tag LIVE
$ export TAG=$(date +DEPLOYED-%F/%H%M)  # this generates a timestamp
$ echo $TAG # should show "DEPLOYED-" and then the timestamp
$ git tag $TAG
$ git push origin LIVE $TAG # pushes the tags up

Now it’s easy, at any time, to check what the difference is between our current codebase and what’s live on the servers. This will come in useful in a few chapters, when we look at database migrations. Have a look at the tag in the history:

$ git log --graph --oneline --decorate
[...]

Anyway, you now have a live website! Tell all your friends! Tell your mum, if no one else is interested! And, in the next chapter, it’s back to coding again.

Further Reading

There’s no such thing as the One True Way in deployment, and I’m no grizzled expert in any case. I’ve tried to set you off on a reasonably sane path, but there’s plenty of things you could do differently, and lots, lots more to learn besides. Here are some resources I used for inspiration:

• Solid Python Deployments for Everybody by Hynek Schlawack

• Git-based fabric deployments are awesome by Dan Bravender

• The deployment chapter of Two Scoops of Django by Dan Greenfeld and Audrey Roy

• The 12-factor App by the Heroku team

For some ideas on how you might go about automating the provisioning step, and an alternative to Fabric called Ansible, go check out Appendix C.

Automated Deployments

Fabric

Fabric lets you run commands on servers from inside Python scripts. This is a great tool for automating server admin tasks.

Idempotency

If your deployment script is deploying to existing servers, you need to design them so that they work against a fresh installation and against a server that’s already configured.

Keep config files under source control

Make sure your only copy of a config file isn’t on the server! They are critical to your application, and should be under version control like anything else.

Automating provisioning

Ultimately, everything should be automated, and that includes spinning up brand new servers and ensuring they have all the right software installed. This will involve interacting with the API of your hosting provider.

Configuration management tools

Fabric is very flexible, but its logic is still based on scripting. More advanced tools take a more “declarative” approach, and can make your life even easier. Ansible and Vagrant are two worth checking out (see Appendix C), but there are many more (Chef, Puppet, Salt, Juju…).

1 If you’re wondering why we’re building up paths manually with f-strings instead of the os.path.join command we saw earlier, it’s because path.join will use backslashes if you run the script from Windows, but we definitely want forward slashes on the server. That’s a common gotcha!

² There is a Fabric “cd” command, but I figured it was one thing too many to add in this chapter.

³ You might have seen nerdy people using this strange s/change-this/to-this/ notation on the internet. Now you know why!