Long ago, the first time I read “The Pragmatic Programmer”, I read some advice that really
stuck with me.
“Don’t Use Manual Procedures”.
This in the chapter on Ubiquitous Automation. To summarize, they want you to automate all the things.
The trouble was that I hadn’t much of an idea how to actually go
about automating any of the things. At the time, I was still trapped in a world
where every program was a bulky Java program with a CLI, and I was under the
impression that the command line interface was nothing but a dying remnant of times past.
My journey to some semblance of automation is filled with pitfalls and embarassing tales
of my own incompetence, and I hope to share these tales with you.
One of the pitfalls of interacting with a command-line system is that every complicated
interaction takes an equally complicated command to invoke - and for commands that I might
have to run again and again, I would have to remember the precise invocation each time.
Most of the time this worked out, although with no small amount of checking Google or
StackOverflow every single time.
One persistent thing about fantasy fiction is you always see acolytes with a wand,
but no spellbook. This is because they are stupid. Wizards carry a spellbook.
So I would start writing down my commands, so that I wouldn’t forget them.
Instead of writing these commands down on post-it notes or in a notebook,
I’d write them to my home directory.
$ cat work ssh [email protected] -P 8888 -p=hunter2
At first, I would just cat this file, and then copy the command back into the command line.
$ cat work ssh [email protected] -P 8888 -p=hunter2 $ ssh cl[email protected] -P 8888 -p=hunter2 Welcome to Work Computer. Blorf.
I told you that I would share embarassing tales of my incompetence.
I soon realized that, without really having intended to, I was writing bash scripts -
I just wasn’t executing them sanely. If I just told the system that I wanted
to execute this file like a program, it should work sanely enough.
$ chmod u+x work $ ./work Welcome to Work Computer. Blorf.
This alone is good enough to get this script into working order, but we’re not doing a lot
to help the system figure out exactly how to run this mystery script that we have passed it.
Let’s imagine we create a file:
This is valid Python 3, but not valid Python 2. If we save this file as ‘hello’,
then we know very little about how to run it. And if we try to run it, the computer
will guess, and it will guess wrong.
$ ./hello idfk, man, that doesn't look right to me
If we name the file hello.py, we give ourselves a handy clue as to what sort of file
it is, and then we can execute it with the Python interpreter.
# python hello.py that's python 3, man, I'm python 2. Whateva. # python3 hello.py hello
Okay, that worked - but it’s a lot of extra stress for a program invocation.
One Unix innovation from the seventies can clear up this problem: the hashbang.
At the beginning of our script, we can declare exactly which interpreter we want
#!python3 print("hello") #!bash echo "hello"
The only rule, here, is that the interpreter has to be visible from the system’s $PATH.
Which is great if you’re running the script locally - but if you’re running the script
remotely without a pseudo-terminal session set, you might not have a $PATH, which would
cause your hashbang declaration to fail.
The workaround? The hashbang can include the full path to the interpreter that you
intend to use for the program.
#!/bin/python3 print("hello") #!/bin/bash echo "hello"
You don’t see this exciting technology in a lot of modern programming, because it’s
not spectacularly portable - the location and even the name of the interpreter you’re
using to execute a python program is often wildly different from system to system.
Bash is always at /bin/bash, though, so that base is covered, which is why every bash
program under the sun opens with #!/bin/bash.
So, before long, my home directories everywhere started to fill up with helpful little
bash files with names like
If you’ve programmed in Django, you might remember that just about every command
that you can issue to your Django environment starts by directly invoking the program
I might start the Django development server like this:
$ cd ~/code/django_project $ ./manage.py runserver 0:8000
Which is easy enough, but I had to start that Django development server dozens of times a day!
All it took was a slight adjustment to my
And suddenly, regardless of my current working directory, I could spin up the server
with a simple
$ dj runserver 0:8000
This was a new and heady feeling. This alias tool would serve me well.
As I was building more and more subtle automation steps into my programming environments,
it started to become unwieldy to have a separate file for each instruction I wanted
Enter the Make tool.
Now, this is not at all what the make tool is for - Make is a build tool! But it served my
purposes quite well to lay out all of a project’s automations in a make file, like:
run: manage.py runserver 0:8000 shell: manage.py shell test: nosetests /home/vagrant/code/things lint: pyflakes /home/vagrant/code/things --no-bitching-about-long-lines
Then, I could run my coterie of assorted tasks without having to interact with a group
of disconnected shell files, like this:
$ make lint
Ultimately, this wasn’t much more stable or convenient than shell files, and it introduced
a strange Make dependency on a bunch of projects for which a Make dependency didn’t make
any real sense.
But it did introduce me to the next step in my journey: Instead of using scripts, try
Instead of a Make dependency, why not a dependency that also exists within the Python
universe? Enter Fabric, an excellent Python command runner. If you’re more familiar
with Ruby than Python, you might remember Fabric’s evil counterpart, Capistrano.
Fabric allowed me to structure my test runs like this:
from fabric.api import run def go(): """ Documentation documentation documentation. """ run('manage.py runserver 0:8000') def test(): run('nosetests /home/vagrant/code/things') def lint(): run('pyflakes /home/vagrant/code/things --no-bitching-about-long-lines')
Which I could then call with a:
$ fab go
There we go- all of our utility scripts in one convenient spot.
This was pretty nice,
and I started using it for everything. Even the thorny problem
of deploying things to remote servers - which Fabric supports extremely well.
Just tell it to SSH into a remote server, or a whole bunch of remote servers, at once,
and fabric was able to run my scripts across all of them.
I started using Fabric to set up servers. New servers would need PostgreSQL and nginx,
and it was nice to have scripts to set those things up for me rather than having
to handle all of their fiddly installation steps by hand.
Fabric is great, but there are a few things it doesn’t do particularly well.
While debugging the Fabric tasks, I’d run
the tasks on the same server again and again and again, and each time it would
start from the beginning and run the whole script. If there was a slow step,
it could add hours to my debugging process.
Which meant that I would just comment out large chunks of my scripts as I worked
on them, testing out the “head” of the script while I left the tail commented
out, so that I could push further and further into my automations.
There were lots of operations that would pass the first time they ran, and then
fail all subsequent times, like “create a database” or “create a directory” -
these operations would fail the second time around because the database or
directory already existed the second time I ran them.
The problem? Most Fabric operations lacked a quality called “idempotence” -
a five dollar word that means “running the operation more than once
always produces the same result.”
Another problem was that of large configuration files. I’d find myself
often either writing ugly Bash operations to perform minor surgery on
large configuration files, or using string tools to build the configuration
files in Python and then drop them into place.
One last problem? Credentials. It was never quite clear exactly where
credentials needed to be located in order to have systems working properly.
I eventually settled on environment variables, but it meant that every
new system that I’d set up, I’d immediately need to commit three dozen
variables immediately to a
This still worked pretty well, but we could do better. Fortunately,
there was a framework out there that could run operations with these
properties, and still keep us firmly within the realm of Python. Ansible.
Ansible provides a YAML format that can be used to describe a working
system, and then it will build that system over an SSH connection. That’s
pretty nice - once I got over some of the
inherent disgust of ‘coding’ in YAML!
Ansible also contains, amongst other things, a full-featured
templating system for creating configuration files, and a password
store with two-way encryption to stash important configuration credentials.
Ansible solves a lot of problems.
But one problem that Ansible doesn’t solve? It’s not a very good task runner.
In fact, Ansible commands can be downright arcane to run.
Running a playbook on a set of servers might look like this:
ansible-playbook -i inventory.yml mongo -u root --vault-password-file ~/.vaultpw mongo.yml
I don’t know about you, but that’s not the sort of thing
that I can easily commit to muscle memory.
So once again we have need of a task runner. We could go back to Fabric, but
the maintainers of Fabric have essentially abandoned it in favor of an
extremely ambitious version 2.0 that is showing some definite signs
of second-system effect - the
most obvious symptom being that it’s been in development for 4 years and
has yet to see the light of day.
So Fabric is out of the question for now - but while Fabric 2.0 is struggling
towards life, a half-completed chunk of Fabric 2.0 has emerged. And while
limited in functionality, it’s … much better than Fabric.
It’s called Invoke and it just provides the
“task runner” half of Fabric without providing any of the “ssh” or “deployment”
bits. But that’s all we want! It’s perfect!
So we can wrap our ansible deployment like this:
from invoke import task, run @task def provision_mongo(ctx): ctx.run('ansible-playbook -i inventory.yml mongo -u root --vault-password-file ~/.vaultpw mongo.yml')
And then run it like this:
$ inv provision_mongo
And we can include that with the utility scripts that we use to run the
rest of the application.
@task def go(ctx): ctx.run('manage.py runserver 0:8000') @task def test(ctx): ctx.run('nosetests /home/vagrant/code/things')
Invoke has many more features, but covering them all would be beyond the scope
of this already over-long blog post.
Clearly, my journey towards repeatable, useful project automation still
has a long way to go, but I’ve definitely found some very useful tools
in the intersection between
We get all of the composability of Python, all of the practicality
of Ansible, and all of the convenience of a task runner.
One final gripe: Invoke supports Python 3 beautifully, but Ansible is still
tied to the Python Dark Ages of Python 2, so in order to run both Invoke and
Python together, we must accept an inferior Python.
gulp. Am I going to have to settle for