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Simple web application deployment with "git push"

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README.md

GIDDYUP!

Does deploying your application seem like a bit too much of a chore? Do you wish it wasn't so hard? Well, with giddyup it can become just a tiny bit easier.

If you've ever used, or seen, Heroku's git-based deployment model, you know how simple app deployment can be. While we don't try to emulate all of Heroku's excellent infrastructure, giddyup does handle a small corner of that -- the ability to deploy your app with a simple 'git push'.

Installation

At this stage, giddyup is only available directly as a git repo. There are no versioned releases, nor are there distribution packages available. My recommendation is to create a clone of the main giddyup repo somewhere (I suggest /usr/local/lib/giddyup if you've got root, and probably ~/giddyup otherwise) and then to update to newer giddyup functionality, just git pull in that clone. You then have a fixed location to point your hook symlinks and inclusions of functions.sh.

Eventually, when giddyup stabilises there will be distribution packages; they'll likely put everything into /usr/lib/giddyup, at which time you can just delete your git repo at /usr/local/lib/giddyup and symlink that location /usr/lib/giddyup, and nothing else will need to change.

Symlink the giddyup script to somewhere in your PATH; this will allow you to trivially setup new deployment destinations. Do not copy the giddyup script; it relies on being in the same directory as update-hook in order to be able to find the hook script.

Setting up new deployments

  1. Run the giddyup script, passing it the location of the "root" of your new deployment. The script will create the basic directory structure and symlink the update hook into the right place.

  2. Set the git config variables in the config for the repo you created in step 1 (see the section "Configuration" for the available variables)

    e.g. git config -f /home/appuser/appname/repo/config giddyup.environment production

  3. Add the necessary hooks to your application's local git repo to effect proper deployment.

  4. Add the newly created git repo as a remote in your local working copy, then push to that remote to make your initial deploy:

    e.g. git remote add deploy appuser@example.com:appname/repo; git push deploy master:master

  5. Configure your webserver to pass requests to the appserver, and test that everything is working properly.

Deployment tree structure

Giddyup creates a tree of directories underneath the specified "root" directory, that contains (hopefully) everything related to your application. The structure is as follows:

  • repo: This is the repository to which git pushes are made, and within which the Giddyup hook script is placed. It must be a "bare" git repo (because otherwise Git gets ridiculously confused and annoyed) and it is the parent of this directory which is considered to be the "root" of the entire application. (Note: this doesn't actually have to be named repo; but it's best to keep a consistent convention, for the sake of sanity).

  • shared: All of your application's data which should persist between releases should be kept in here -- log files (if you want to keep the same log files between releases), definitely your customers' uploaded assets -- all that sort of thing goes in here, and should be symlinked from your release. See the section "Shared data", below.

  • releases: This is where each individual release goes. Each deployment of your application gets a directory of it's own in here, named after the current date/time at the time of deployment, in the format YYYYMMDD-HHMMSS.

  • current: A symlink to the currently running version of your application.

Shared data

Anything you need to be shared across deployments should live under the shared directory, and symlinks placed in your releases to point to the relevant data in shared. You share things in your deployments by calling the share helper function within your hook scripts. This function will take a relative path (relative to the root of your repo) and create a symlink into the same path within shared.

For example, you might have a hook script that uses Bundler to setup your local gems. Since you don't want to have to dick around reinstalling all your gems with every release, you want to share that bundle between releases. Your hook script might look something like this:

. /path/to/giddyup/functions.sh
share vendor/bundle
cd "${RELEASE}"
bundle install --deployment

This will create a symlink from $ROOT/releases/<timestamp>/vendor/bundle to $ROOT/shared/vendor/bundle, then run bundle in the new release to ensure that all your required gems are available.

If a file or directory already exists in releases/<timestamp> that is supposed to be symlinked, we'll remove it before creating the symlink. The directory structure in shared will always mirror that of your releases when you use share; this improves simplicity and comprehensibility.

If a symlink destination doesn't exist already within shared, then we'll copy the source content in your release (if it exists) into shared. Leading directory components will be created in shared automatically, as well.

Hooks

Because everyone's application is slightly different, Giddyup itself doesn't actually do very much itself -- instead, it delegates a lot of things back to code that you write, as hooks. To understand hooks, it's best to understand how Giddyup works:

  1. You push your updated code to the repo controlled by giddyup
  2. The update hook, provided by Giddyup, starts to run
  3. Giddyup makes a copy of the code in your deployment repo
  4. Giddyup runs the stop hook
  5. Giddyup changes the "current" symlink for the deployment to point at the newly configured code
  6. Giddyup runs the start hook
  7. Old releases are tidied up, if required (see the giddyup.keepreleases config variable)

To put it another way, the following hooks are available:

  • stop: Run after the new release is in place, but while the system's idea of the "current" release still points to the currently running code. You'll probably want to do whatever's required to stop your appserver from running in this hook, run bundler, and perhaps put up a maintenance page.

  • start: Run after the "current" symlink has been changed to point to the new code. In here you'd probably want to do database migrations, start your appserver, and take down your maintenance page.

Running hooks

Giddyup always runs hooks from the newly deployed version of your codebase (so that if you happened to push a deploy with a dodgy stop hook, you can recover from it by pushing a fixed version). It looks for hooks in the location specified by the giddyup.hookdir git config variable (see "Configuration", below).

It is looking for files or directories that match the name of the hook (start, stop, etc). If there is a file named after the hook, and it is executable, then that file is executed. If, on the other hand, there is a directory named after the hook, then all executable files in that directory are executed, in the lexical order of the names of the files.

If you really, really need to be able to run hook files that aren't already executable (I don't know why; git stores executable bits just fine) then you can set the autochmodhooks config variable; this will have the effect of making every file in your hook directories (if you're using them) a hook script. So don't combine this config option with a README...

Each hook script is run as a separate process, and as such cannot effect the environment or working directory of giddyup itself or any other hook script.

Hook environment

The environment of the hook is very minimal; only the following environment variables will be set:

  • PATH -- the same as the path of the Giddyup script itself.
  • APP_ENV -- the environment specified by giddyup.environment (see "Configuration", below).
  • ROOT -- the directory that is the "root" of the entire deployment; the directory which the deployment git repository (and everything else) lives
  • RELEASE -- the canonical directory that contains the data in this release of the application.
  • NEWREV -- The SHA of the commit being deployed.
  • OLDREV -- The SHA of the commit being replaced.

The working directory of all hooks is the root of the deployment tree. During the 'stop' hook, the current symlink will point to the previous running release of the application, while during the start hook the current symlink will point to the new release you're currently deploying.

Hook script helpers

To help you make your hook scripts easier to write, there are some shell functions available to help you on your way. To use them, merely add:

. /path/to/giddyup/functions.sh

at the top of your hook script, and then call away to your heart's content.

Error handling

At present, the error handling in giddyup is a bit primitive. In general, if any part of the giddyup script fails, the whole process will abort. This mostly won't hurt your running app, because giddyup does as much as it can before starting or stopping anything. However, if your hook scripts bomb out, or one of the things that runs between stopping and starting the app fails, then things could be left in a bit of a limbo state.

Improvements to this part of the system, particularly around intelligent rollback strategies, is planned. Patches welcome.

Configuration

Gidduyp is controlled by git repository configuration variables; these can either be set using git config in the deployment repository, or by directly editing the config file in the deployment repository.

Available configuration variables are given below.

giddyup.environment

(OPTIONAL; default: none)

Many web application frameworks have a concept of "environments"; that is, different sets of configuration parameters (like database credentials) that vary between instances of the application running with the same source code.

Giddyup supports this paradigm with the use of the giddyup.environment variable. If set, the value will be passed through to the APP_ENV environment variable in your hook scripts; this allows you to do slightly different things (like reload a different appserver, or assist in running your database migrations against a different database). Since this environment is specified per-repository, you can run several environments (say, uat, staging, and production) with the same in-repo deployment hooks, just by pushing to different remote repositories (which have different settings for giddyup.environment, of course).

giddyup.hookdir

(OPTIONAL; default config/hooks)

The directory within which Giddyup will look to find hook scripts, relative to the root of your working copy. The intent is that the hooks relating to the deployment of your application are best kept with your application.

giddyup.keepreleases

(OPTIONAL; default: 5)

Giddyup will keep a few older deployed releases available, in case you need to make an emergency rollback, or examine exactly what was recently deployed. This configuration parameter determines how many releases to keep. Set this to 1 to not keep any previous releases, and set it to 0 if you want to keep every release ever (not a good idea unless you've got one of those fancy new infinite-capacity hard drives).

giddyup.debug

(OPTIONAL; default: false)

If you want to have far, far too much gory detail about what giddyup is doing, you can set this to true.

giddyup.autochmodhooks

(OPTIONAL; default: false)

Sometimes the hook files found in giddyup.hookdir (e.g. in config/hooks/) are not marked as executable in the repository. Setting giddyup.autochmodhooks to true will cause giddyup to set the executable bit itself before running the hook script.

Frequently Answered Questions

What? Is that all?

Yes, the main giddyup update hook script is quite minimal. The aim here is to provide the necessary scaffolding upon which customised deployment processes can be implemented.

To put it another way: yes, Giddyup is small and dumb. You're supposed to provide the smarts in hooks. If you have a general-purpose hook you'd like to share with others, please feel free to send it to me and I'll include it in the examples.

I need moah hooks!

Weeeeell... maybe you do, maybe you don't. Remember that you can execute as many different hook scripts as you like, and in the order you specify. For example, you might think you need a "pre-stop" hook, to do things that might take a while, before the application is stopped (to minimise downtime). You don't actually need a separate hook in giddyup; all you need to do is move the hook script that actually stops your appservers to after the hook script that does whatever time-consuming task you have in mind.

What's with the name?!?

Take the phrase "git deployment", chop off most of "deployment" to produce something like "gitde", say it as a single word a few times until the 't' kinda disappears and the 'e' lengthens so it sounds like "gidee", then shout "yeehaw!" like an old time cowboy. Waving of Stetsons is optional, but strongly encouraged.

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