Deploying Stuff With Git

Dec 12, 2011 / By Yanick Champoux

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Somebody at $work asked me how I use Git to deploy stuff, probably working under the false hypothesis that wisdom is in any way, shape or form affiliated with yours truly. The fool…

Yet, it is true that I had my share of tinkering with Git, and that I might have gleaned from my experience. So I sent him a couple of links leading to more wisdom-certified sources. They are listed at the end of this blog entry; if you have more good articles/blog posts that you’d like to add to the pile, please don’t be shy and mention them in the comments.

But I can’t just stop there, really. So, if you allow me, I’ll dust off the good ol’ soap-box, hop on it, and share my thoughts on the subject. As usual, don’t forget to take an adequatly-sized grain of salt to go with your popcorn.

Deploying Stuff With Git?

The first reaction of most people is probably to add that question mark to the title. Using a VCS for deployment, is it really a good idea? A fair question, for sure, and one that my experience allows me to answer rather succinctly:

Hell yeah.

Okay, maybe I should flesh that out a little more. Git is not only a VCS (Version Control System), it’s a DVCS. The ‘D’ is for distributed, and that means that one of the core raison d’ĂȘtre of Git is to bandy your code around with a maximum of ease. That sounds like a lot like what deployment is about, no?

Also, the fact that each clone of a Git repository contains the whole history of the codebase makes rollbacks and flips to different versions much easier. Once a deployment machine is synched with the master repository, it is independent from the mothership. The network can go down, the master repository can go up in flames, and you’ll still be able to jump back and forth to any version you want. And thanks to the frankly amazing efficiency with which Git compresses its information, it’s a feature with come with a much smaller memory footprint you would imagine. For example, the repo of the whole history of the Mozilla project clocks 450M. Considering that the checkout of the latest version is 350M, it means that for an overhead of not even 150% you get everything single commit that was done for the last ten years. Unless you are routinely deploying the whole KDE codebase on venerable Commodore 64 machines, the trade-off between additional memory and conveniency is so overwhelming it’s not even fair.

All of that being said, I should perhaps stick a disclaimer here:

most of the applications I deal with are Perl-based. As with any interpreter language, what is deployed is uncompiled, plain text source code. And pure text stuff is what Git (and most VCS) excels at. With compiled applications (C, C++, java, etc), the trade-offs are going to be different. Do I still think that Git can still be a viable deployment tool in those cases? Most probably. But remember: it’s not because I’m extolling you the virtues of my hammer that we should forget that there is more than nails out there.

Good. Now that this is out, let’s see what hammering tips I can find for you…

Create a Deployment User

If you are deploying an application, create a user for it. On any Unix-like platform, creating a user is only a adduser away. It makes security matters so much easier on all fronts: for the ssh key generation, for sudo permissions, etc. It also makes things easier to find — it’s all under the ~app account. And, as a bonus, you can stuff the ‘.bashrc‘ of that user with application-centric commands and settings.

Have a Branch for Releases

Unless the repo is not used for development at all, consider using a release branch for the states of the repo that you want to deploy. And even then, having that release branch is probably a good idea to make things as clear and obvious as possible. Because the last thing anyone want is to have the latest experiment from a crazed developer seep out on all deployment machine. As one of those crazed developers, trust me on that one.

Decoupling Checkout and Local Repo

By default, Git stashes the repository information in a ‘.git‘ subdirectory at the root of the clone. Sometimes, because of the nature of what you’re deploying or just for good ol’ OCD’s sake, you might want to have the repo and the checkout at two different places. That’s no problem, thanks to the GIT_DIR (aka repo subdir) and GIT_WORK_TREE (aka checkout) environment variables.

For example, let’s assume that we are deploying an application called Galuga, that our master repository is on the machine Enkidu and that our deployment machines are Gilgamesh and Siduri. Let’s further assume that we heeded the previous recommendation, and that on the deployment machines we have a galuga user, with all the proper ssh key sharing between them and the master user on Enkidu. So, if want the repo information to be in ‘~/repo‘ and the checkout to be in ‘~/galuga‘, we can add the following lines in the ‘.bashrc‘ of our deployment drones:

# in the .bashrc of galuga@[Siduri,Gilgamesh]
export GIT_DIR=~/repo
export GIT_WORK_TREE=~/checkout

And then from Enkidu:

$ ssh galuga@gilgamesh 
    'git clone ssh://yanick@enkidu/home/yanick/work/Galuga $GIT_DIR'
Cloning into /home/galuga/repo...

$ ssh galuga@gilgamesh 'ls ~/repo'
branches
config
description
HEAD
...

$ ssh galuga@gilgamesh 'ls ~/checkout'
Changes
galuga.conf
lib
Makefile.PL
...

Bonus point, since our user only has one repo to worry about, and that the information is contained in the .bashrc, we don’t even have to be in the git checkout directory to use any of the git commands:

$ ssh galuga@gilgamesh 'git status'
# On branch master
nothing to commit (working directory clean)

Don’t Push Directly to the Local Branches

Of course, we all know what we should never edit deployed code directly. Any change should be made in a development environment, pushed to the master repo, and then pushed to the deployment boxes.

We should never edit deployed code directly. But we will. Now, don’t give me that outraged look. We don’t want to do that. We know that it’s generally a bad idea to do that. But sometimes, when dealing with a honest-to-God emergency, it’s the right thing to do. So it’s better to keep a distinction of the branches as they are being pushed from the master repo, and as they are on the deployment machine, even if we don’t expect them to be ever different (and unless we have a very good reason for that, they shouldn’t).

How to do that? Simple, we push to the remote tracking branch instead of the local copy. If we use same example as before, we didn’t specify any name for the origin of the clone, so its name is going to be the default origin:

$ ssh galuga@gilgamesh 'git branch -a'
* releases
  master
  remotes/master
  remotes/releases

Before pushing, we need the master repo to know about its minions:

$ alias pgit='perl -MGit::Repository 
    -e"$git = Git::Repository->new;"'

$ pgit -E'$git->run( "remote", "add", "minion/$_", 
    "ssh://galuga@$_/home/galuga/repo" ) for @ARGV'
    gilgamesh siduri

$  git remote
minion/gilgamesh
minion/siduri

The Perlish way of setting the minions is there because I am thinking ahead when we’ll have hundreds of minions to register. If we only deal with one or two deployment boxes, then

$ git remote add minion/gilgamesh ssh://galuga@gilgamesh/home/galuga/repo
$ git remote add minion/siduri    ssh://galuga@siduri/home/galuga/repo

will do just as well, although it won’t look so l33t.

Anyway, now that we know about our minions, pushing the new changes of the branch ‘releases’ to them is a matter of doing:

$ git push minion/gilgamesh releases:remotes/origin/releases

or, if we have hundreds of deployment boxes:

$ git remote | grep '^minion/' | 
    pgit -n -e'$git->run( 
        "push",  $_, "releases:remotes/origin/releases" 
        )'

You Want to Push Directly to the Local Branch?

You are sure? Really? Okay. It’s your funeral. Just remember that by default Git will prevent you to push directly to the branch being checked out:

$ git push minion/gilgamesh releases
Total 0 (delta 0), reused 0 (delta 0)
remote: error: refusing to update checked out branch: refs/heads/releases
remote: error: By default, updating the current branch in a non-bare repository
remote: error: is denied, because it will make the index and work tree inconsistent
remote: error: with what you pushed, and will require 'git reset --hard' to match
remote: error: the work tree to HEAD.
remote: error:
remote: error: You can set 'receive.denyCurrentBranch' configuration variable to
remote: error: 'ignore' or 'warn' in the remote repository to allow pushing into
remote: error: its current branch; however, this is not recommended unless you
remote: error: arranged to update its work tree to match what you pushed in some
remote: error: other way.
remote: error:
remote: error: To squelch this message and still keep the default behaviour, set
remote: error: 'receive.denyCurrentBranch' configuration variable to 'refuse'.
To ssh://galuga@enkidu/home/galuga/repo
! [remote rejected] releases -> releases (branch is currently checked out)
error: failed to push some refs to 'ssh://galuga@enkidu/home/galuga/repo'

So if you know what you are doing, you’ll need to change the config of the
deployment box to disable the check:

$ ssh galuga@gilgamesh 'git config receive.denyCurrentBranch ignore'

$ git push minion/gilgamesh releases
Total 0 (delta 0), reused 0 (delta 0)
To ssh://galuga@gilgamesh/home/galuga/repo
da7f7a3..d7db465  releases -> releases

But, again, baaaaad idea.

Using Local Branch for Local Configuration

There is one case where it makes sense to have local branches that aren’t just copies of the master repo: when there are local configurations involved.

There are many ways to deal with local configurations. They can all be present in the checkout, and the application can be smart enough to chose the right one. Or we can have, in the master repo, one branch for each deployment machine that contains its configuration delta and is constantly rebased off the main release branch. Or we can push the onus of that local configuration on the local ‘releases’ branch, that will have to be rebased off the pushed ‘releases’ branch. Assuming no merge conflict arise, all we have to do at deploy time is not just to jump on a commit of the origin’s branch, but rebase off it:

# on Gilgamesh
$ cd $GIT_WORK_TREE
# if we just want v1.23 off origin/releases verbatim:
$ git reset --hard v1.23
# but if we want to keep our delta:
$ git rebase v1.23

Deployment Method #1: Deployment Machines Pulling Changes

The most obvious way to deploy is to treat the deployment boxes as just regular git clones, on which we’ll just happen to checkout stuff from the ‘releases’ branch. For example:

# on gilgamesh, the deployment box

# retrieve the latest changes
$ git fetch origin
remote: Counting objects: 7, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 4 (delta 2), reused 0 (delta 0)
Unpacking objects: 100% (4/4), done.
From ssh://enkidu/home/yanick/work/Galuga
d7db465..ee30419  releases -> origin/releases
* [new tag]         v1.23     -> v1.23

# just curious, what's the changes?
git diff --name-only v1.23
Makefile.PL
MANIFEST
...

# looks good, deploy v1.23
$ git reset --hard v1.23

That’s the best way to go if we don’t need to have all the deployment boxes synchronized. Or if the deployment should be human-supervised (or, God forbid, tweaked).

Deployment Method #2: Master Pushing, Minions Reacting

If we have a farm of hundreds of deployment boxes, we might prefer to issue a single command from our master machine and let the deployment boxes scramble and do their magic themselves. To do that, we can play with the post-receive hook.

For example, for the simple case where we want the deployment machine to always be at the bleeding edge:

#!/bin/sh

# no really, that's it
git reset --hard remotes/origin/releases

The push will emit a warning, but it’ll do what we want.

Of course, that’s only the beginning. If you have a twisted mind, you could begin to realise that this provides a crude API to send commands to the deployment boxes. For example, with this post-receive,

#!/bin/bash

RESTART=`git branch command/restart`

if [ -z "$RESTART" ]; then
        ~/checkout/service start
        git branch -D command/restart
fi

restarting the server on gilgamesh is just as easy as

$ git push minion/gilgamesh releases:command/restart

Pure evil? Nyaaah… Maybe. But potentially handy too.

References

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