Troops -- a software deployment tool
Troops is a reaction to Cfengine/BCFG2/Chef/Puppet. It is an explicit attempt to build something different.
It is yet to be seen whether it's a viable approach.
See also Fabric (run commands over SSH, with actions/hosts/roles), Kokki (which is a pretty close port of Chef to Python) and Poni (which is something different, but it's as of yet unclear what).
Things Troops does not intend to achieve:
- centralization in any form
- bloat, required services such as message queues, databases, search engines
- crypto that is not SSH/GnuPG
- "uploading" code/data/cookbooks, that want to live in version control anyway
- slowness caused by latency and lack of parallelism, every node should act independently; think "pull you can push to"
- supporting every possible platform; especially, non-Linux servers seem just pointless at this time
- Ruby; that's been done enough already, and Homey just don't play that
- abstractions; adding a "recipe" will take as little code as feasible
- (weaker anti-goal): complete declarativeness; not doable without a custom language, and I'd rather not build that
- running shell commands over SSH; Troops will install the code required on the destination machine, giving you a powerful language to get things done
- doing things the same way as other deployment tools do it; the philosophy is "better or bust", not cloning features
- cloud friendly; especially Debian/Ubuntu-style preseeding with EC2-style userdata, because that's what I use
- based on editing files & using version control; not Poni-style command line object manipulation
- support "green/blue deployment", roll out changes to a fraction of nodes first
- low learning curve (given sufficient background knowledge): "cookbooks" are just Python packages, code is just code you could run locally without Troops (TODO: reality check)
- (non-goal but strong bias:) nodes/sandboxes are discardable; it's easier to build a new one from scratch than to spend time handling uninstall etc cleanup
- a base design that is flexible, for example handling both push and pull deployments with the same logic
Your first deployment
Let's say you want to ensure all your machines always have
installed. Here's how you'd write that:
from pydebianadmin import apt apt.install('tmux')
That's it! Running that bit of code will now install
pydebianadmin is just a completely generic utility library,
and isn't tied to Troops in any way. That is, Troops deployments are
just regular old Python. Now, how to get that to run on the desired
machines, we'll get back to that later.
Conditional deployment using roles
Now, you might have a bunch of machines where nobody ever logs in.
tmux only when the machine has the role
import troops from pydebianadmin import apt if troops.have_role('interactive'): apt.install('tmux')
Now, when does a machine have that role? Exactly when you tell it to! For example, if you want to use hostnames:
import troops if troops.hostname().startswith('shellbox-'): troops.roles.add('interactive')
troops.hostname() is a convenience wrapper around
socket.gethostname() that tries to get the fully qualified name.
You can also list out the machines that have a certain role:
import troops troops.define_role( 'interactive', hosts=[ 'shellbox-1.example.com', 'shellbox-2.example.com', ], )
Organizing things into functions can help keep code organized. Any
functions defined with the decorator
@deployable() will be
deployable can take a
roles parameter to limit where the
function is called:
import troops from pydebianadmin import apt @troops.deployable(roles=['interactive']) def tmux(): apt.install('tmux')
This is a trick section. Troops never deploys anything remotely. Instead, we tell the remote machine what to do, and then the remote machine does it "locally". Which, if you think of it, is exactly what e.g. Fabric does, over SSH; Troops just shifts the focus from "run this shell command" to "run this Python". And we let you use whatever libraries are needed, so it feels more like actual programming than writing a shell script inside Python.
To do this, we ship the deployment script (and auxiliary files, if any) to the remote node, install the required dependencies, and just run it.
Instead of reinventing the wheel and writing our own shipping mechanism, we use Git for synchronizing changes. That also nicely solves the problem of concurrent edits for us.
To make things more concrete: say you have a virtual machine with a
base Ubuntu install, and a user who can log in with SSH keys, with
sudo access. Let's call it
shellbox-1 (TODO details, but it's just the
ubuntu admin Troops on
ssh ubuntu@shellbox-1 sudo gpasswd -a ubuntu troops
Set up a Git repository for the instructions on what to deploy (on your local machine):
mkdir deploy cd deploy git init
Tell it what to deploy; add a file
import troops from pydebianadmin import apt troops.define_role( 'interactive', hosts=[ 'shellbox-1.example.com', 'shellbox-2.example.com', ], ) @troops.deployable(roles=['interactive']) def tmux(): apt.install('tmux')
Next, list the dependencies the above code needs, in
requirements.txt, in Pip format
(Troops itself is installed automatically; you can list a versioned
dependency to ensure availability of certain utility functions):
You can also use
requirements.txt to install specific versions or
forks of the dependencies, with the
-e option. This is most useful
when you need to fix a bug in a dependency.
Next up, commit the above files to Git:
git add deploy.py requirements.txt git commit -m 'Install tmux on interactive boxes.'
You can now run the deploy locally:
troops deploy --repository=.
Or ship them over to the remote machine and trigger a deploy there:
git push ubuntu@shellbox-1:/var/lib/troops/repo/main.git master
This will trigger a run, and the output of that will be shown to you. This lets you do push-style deploys fairly nicely.
The pull deployment model
If you have lots of machines, push deployments get
frustrating. Thankfully, Troops handles pull deployments just as
nicely. To set this up, you need to host a Git repository somewhere.
For this example, we'll call that
If you are using SSH for connecting to the deploy repository, you need
to authorize the Troops installation on
shellbox-1; the SSH public
key is in
shellbox-1 where to fetch from, run:
ssh ubuntu@shellbox-1 git --git-dir=/var/lib/troops/repo/main.git \ remote add NAME email@example.com:deploy
NAME with whatever you consider descriptive.
Now, we can tell Troops to try to fetch from all the defined remotes (when we said no centralization, we meant it), and any remote master branch with fast-forwardable  changes will be deployed. The local branch will be updated once the deploy succeeds, and only then; this makes Troops retry failed deploys until they work.
- ssh ubuntu@shellbox-1 troops pull
|[ff]||fast-forwarding is a Git term that means there is no divergence in the two branches, only the remote branch has new commits; this guarantees there will be automated merging, and especially no merge conflicts|
Using Python C extensions
If you want to use Python C extensions that are not in the standard
library, and that depend on specific C libraries to build, you need to
make sure the right platform packages are installed before Pip can
install the package. Hence,
requirements.txt is not sufficient.
lxml is a C extension that needs
libxslt1-dev to compile. To do this, you need to write something
import pip from pydebianadmin import apt apt.install('libxml2-dev', 'libxslt1-dev') pip.main(['install', '-e', 'lxml']) import lxml # now it should work
Coordinating multiple nodes
Let's say you're using one of those clouds that gives your nodes static-but-random IP addresses. How do we tell node A how to reach node B?
Or, even more critically, on initial deploy B creates a private/public key pair. Or even just a plain old username/password. How do we transmit this data to A, with maximum trust: no eavesdroppers, no forgery.
First off, we need to understand and accept that we will not have this information until B is up and running. Hence, we either need to delay deploying A until we have all the information, or we need to deploy it in some sort of degraded mode, where it will not depend on being able to reach B. Or in case there's one A and lots of B1, B2, etc machines, just not add Bn into A's configuration until Bn is up & ready.
This is fairly easy to solve for architectures where there's a coordinating node that can communicate with both A and B. But Troops is designed to be minimal, and there's no need to build such a centralization design smell into Troops itself. Instead, we will provide you enough rope to to implement it yourself.
Obviously, Troops itself got deployed somehow. This tends to take one of two forms: 1. an "image", or a "seeded install" that is given read-only information to consume, or 2. an SSH connection is used to customize a "base image". Either way, it's easy to arrange a shell accessible with an given SSH public key.
We can use that SSH trust relationship to directly handle the exchange; a management node C can SSH to both B and A, and transmit the necessary information. For the fully automated case, this would need to be automated and triggered somehow by the creation of B (or A). For now, let's look at the simpler manual way.
Let's say the deploy code for B has:
import subprocess subprocess.call([ 'ssh-keygen', '-N', '', '-f', '/srv/foo/ssh', ])
Now, after deploying B, on our comfy local machine we trigger all the deploys from, we can run:
scp user@B:/srv/foo/ssh.pub B.pub scp B.pub user@A:/srv/foo/keys/B.pub
And then trigger the Troops on A to notice that the key is there:
ssh user@A troops deploy
(TODO: the above is a rambly, edit for clarity)