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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. (These days people sometimes use the words "infrastructure as code" to describe automation of deployments, and provisioning.)

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

REPO_URL = 'https://github.com/hjwp/book-example.git'  (1)

def deploy():
    site_folder = f'/home/{env.user}/sites/{env.host}'  (2)
    run(f'mkdir -p {site_folder}') (3) (4)
    with cd(site_folder):  (5)
        _get_latest_source()
        _update_virtualenv()
        _create_or_update_dotenv()
        _update_static_files()
        _update_database()
1 You’ll want to update the REPO_URL variable with the URL of your own Git repo on its code-sharing site.
2 env.user will contain the username you’re using to log in to the server; env.host will be the address of the server we’ve specified at the command line (e.g., superlists.ottg.eu).[1]
3 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.
4 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.
5 cd is a fabric context manager that says "run all the following statements inside this working directory".[2]

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.

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():
    if exists('.git'):  (1)
        run('git fetch')  (2)
    else:
        run(f'git clone {REPO_URL} .')  (3)
    current_commit = local("git log -n 1 --format=%H", capture=True)  (4)
    run(f'git reset --hard {current_commit}')  (5)
1 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 our site folder.
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).
3 Alternatively we use git clone with the repo URL to bring down a fresh source tree.
4 Fabric’s local command runs a command on your local machine—​it’s just a wrapper around subprocess.call 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. Another common gotcha!).
5 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.

Updating the Virtualenv

Next we create or update the virtualenv:

deploy_tools/fabfile.py (ch09l004)
def _update_virtualenv():
    if not exists('virtualenv/bin/pip'):  (1)
        run(f'python3.6 -m venv virtualenv')
    run('./virtualenv/bin/pip install -r requirements.txt')  (2)
1 We look inside the virtualenv folder for the pip executable as a way of checking whether it already exists.
2 Then we use pip install -r like we did earlier.

Creating a New .env File if Necessary

Our deploy script can also save us some of the manual work creating a .env script:

deploy_tools/fabfile.py (ch09l005)
def _create_or_update_dotenv():
    append('.env', 'DJANGO_DEBUG_FALSE=y')  (1)
    append('.env', f'SITENAME={env.host}')
    current_contents = run('cat .env')  (2)
    if 'DJANGO_SECRET_KEY' not in current_contents:  (2)
        new_secret = ''.join(random.SystemRandom().choices(  (3)
            'abcdefghijklmnopqrstuvwxyz0123456789', k=50
        ))
        append('.env', f'DJANGO_SECRET_KEY={new_secret}')
1 The append command conditionally adds a line to a file, if that line isn’t already there.
2 For the secret key we first manually check whether there’s already an entry in the file…​
3 And if not, we use our little one-liner from earlier to generate a new one (we can’t rely on the append's conditional logic here because our new key and any potential existing one won’t be the same).

Updating Static Files

Updating static files is a single command:

deploy_tools/fabfile.py (ch09l006)
def _update_static_files():
    run('./virtualenv/bin/python manage.py collectstatic --noinput')  (1)
1 We use the virtualenv version of Python 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():
    run('./virtualenv/bin/python manage.py migrate --noinput')  (1)
1 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

Before we try, we need to make sure our latest commits are up on GitHub, or we won’t be able to sync the server with our local commits.

$ git push

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

$ cd deploy_tools
$ fab deploy:[email protected]
[[email protected]] Executing task 'deploy'
[[email protected]] run: mkdir -p
/home/elspeth/sites/superlists-staging.ottg.eu
[[email protected]] run: git fetch
[[email protected]] out: remote: Counting objects: [...]
[[email protected]] out: remote: Compressing objects: [...]
[localhost] local: git log -n 1 --format=%H
[[email protected]] run: git reset --hard
[...]
[[email protected]] out: HEAD is now at [...]
[[email protected]] out:
[[email protected]] run: ./virtualenv/bin/pip install -r
requirements.txt
[[email protected]] out: Requirement already satisfied:
django==1.11 in ./virtualenv/lib/python3.6/site-packages (from -r
requirements.txt (line 1))
[[email protected]] out: Requirement already satisfied:
gunicorn==19.7.1 in ./virtualenv/lib/python3.6/site-packages (from -r
requirements.txt (line 2))
[[email protected]] out: Requirement already satisfied: pytz
in ./virtualenv/lib/python3.6/site-packages (from django==1.11->-r
requirements.txt (line 1))
[[email protected]] out:
[[email protected]] run: ./virtualenv/bin/python manage.py
collectstatic --noinput
[[email protected]] out:
[[email protected]] out: 0 static files copied to
'/home/elspeth/sites/superlists-staging.ottg.eu/static', 15 unmodified.
[[email protected]] out:
[[email protected]] run: ./virtualenv/bin/python manage.py
migrate --noinput
[[email protected]] out: Operations to perform:
[[email protected]] out:   Apply all migrations: auth,
contenttypes, lists, sessions
[[email protected]] out: Running migrations:
[[email protected]] out:   No migrations to apply.
[[email protected]] out:

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 command goes through happily, even though the directory already exist. Next git pull pulls down the couple of commits we just made. 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 needing to apply anything.

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.
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:[email protected]
[[email protected]] Executing task 'deploy'
[[email protected]] run: mkdir -p
/home/elspeth/sites/superlists.ottg.eu
[[email protected]] run: git clone
https://github.com/hjwp/book-example.git .
[[email protected]] out: Cloning into '.'...
[...]
[[email protected]] out: Receiving objects: 100% [...]
[...]
[[email protected]] out: Resolving deltas: 100% [...]
[[email protected]] out: Checking connectivity... done.
[[email protected]] out:
[localhost] local: git log -n 1 --format=%H
[[email protected]] run: git reset --hard [...]
[[email protected]] out: HEAD is now at [...]
[[email protected]] out:
[[email protected]] run: python3.6 -m venv virtualenv
[[email protected]] run: ./virtualenv/bin/pip install -r
requirements.txt
[[email protected]] out: Collecting django==1.11 [...]
[[email protected]] out:   Using cached [...]
[[email protected]] out: Collecting gunicorn==19.7.1 [...]
[[email protected]] out:   Using cached [...]
[[email protected]] out: Collecting pytz [...]
[[email protected]] out:   Using cached [...]
[[email protected]] out: Installing collected packages: pytz, django,
gunicorn
[[email protected]] out: Successfully installed django-1.11
gunicorn-19.7.1 pytz-2017.3

[[email protected]] run: echo 'DJANGO_DEBUG_FALSE=y' >> "$(echo .env)"
[[email protected]] run: echo 'SITENAME=superlists.ottg.eu' >> "$(echo
.env)"
[[email protected]] run: echo
'DJANGO_SECRET_KEY=[...]
[[email protected]] run: ./virtualenv/bin/python manage.py
collectstatic --noinput
[[email protected]] out: Copying
'/home/elspeth/sites/superlists.ottg.eu/lists/static/base.css'
[...]
[[email protected]] out: 15 static files copied to
'/home/elspeth/sites/superlists.ottg.eu/static'.
[[email protected]] out:

[[email protected]] run: ./virtualenv/bin/python manage.py migrate
[...]
[[email protected]] out: Operations to perform:
[[email protected]] out:   Apply all migrations: auth, contenttypes,
lists, sessions
[[email protected]] out: Running migrations:
[[email protected]] out:   Applying contenttypes.0001_initial... OK
[[email protected]] out:   Applying
contenttypes.0002_remove_content_type_name... OK
[[email protected]] out:   Applying auth.0001_initial... OK
[[email protected]] out:   Applying
auth.0002_alter_permission_name_max_length... OK
[...]
[[email protected]] out:   Applying lists.0004_item_list... OK
[[email protected]] out:   Applying sessions.0001_initial... OK
[[email protected]] out:


Done.
Disconnecting from [email protected] 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.

Provisioning: 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.

Now let’s use a little Unix command-line magic!

[email protected]:$ cat ./deploy_tools/nginx.template.conf \
    | sed "s/DOMAIN/superlists.ottg.eu/g" \
    | sudo tee /etc/nginx/sites-available/superlists.ottg.eu

sed ("stream editor") takes a stream of text and performs edits on it. In this case we ask it to substitute the string DOMAIN for the address of our site, with the s/replaceme/withthis/g syntax.[3] We pipe (|) that to another sed process to set our unique SECRET_KEY, and then we pipe the output once more output to a root-user process (sudo), which uses tee to write its input to a file, in this case the Nginx sites-available virtualhost config file.

For bonus points, why not build an even bigger Bash "one-liner" that includes the python random.choices command to generate the secret key? Answers on a postcard!

Next we activate that file with a symlink:

[email protected]:$ sudo ln -s /etc/nginx/sites-available/superlists.ottg.eu \
    /etc/nginx/sites-enabled/superlists.ottg.eu

And we write the Systemd service, with another, slightly simpler sed:

[email protected]: cat ./deploy_tools/gunicorn-systemd.template.service \
    | sed "s/DOMAIN/superlists.ottg.eu/g" \
    | sudo tee /etc/systemd/system/gunicorn-superlists.ottg.eu.service

Finally we start both services:

[email protected]:$ sudo systemctl daemon-reload
[email protected]:$ sudo systemctl reload nginx
[email protected]:$ sudo systemctl enable gunicorn-superlists.ottg.eu
[email protected]:$ sudo systemctl start gunicorn-superlists.ottg.eu

And we take a look at our site: Brrp, brrp, brrp…​it worked!. It works—​hooray!

A screenshot of the production site, working
Figure 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:

Automating Provisioning with Ansible

For some ideas on how you might go about automating the provisioning step, and an alternative to Fabric called Ansible, go check out [appendix3].

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 [appendix3]), 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. You may be wondering why we didn’t just use run to do the cd. It’s because Fabric doesn’t store any state from one command to the next—​each run command runs in a separate shell session on the server.
3. You might have seen nerdy people using this strange s/change-this/to-this/ notation on the internet. Now you know why!

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