Rails Applications and Gems: Solving the Dependency Problem

There's a post today on the Relevance blog about Frozen Gems Generator. I tried posting a comment there, but it seems to have not gone through, so I'll blog my solution here instead.

Chad Woolley at Pivotal Labs created GemInstaller to solve the problem of specifying exactly what gems you want your Rails app (or other Ruby code) to use. I've dealt with this issue a lot over the course of building Rails apps, and while at first blush I didn't think this was a good solution, I'm now really like it, and use it on most of my projects (basically all the projects I control or can :)

So, why is it better than other solutions, or at least the other solutions I've seen? First, let me give a quick synopsis: it is a simple gem that allows you to create a geminstaller.yml file that specifies the version(s) of gems your app wants. This can be an explicit version, or can use things like >= version, etc. It can then automatically install the gems for you on app launch, on deploy, or just at the command line. The benefits of this solution for a Rails app include:

• Solves the arhictecture/platform-specific gems problem. I haven't seen any of the other solutions do this, or do it well. Most just punt on it, others require a convoluted process or hacking up your other code. Because geminstaller simply relies on the gems being installed on your system, it will use the proper version for whatever system it is running on. This also ties into the next point...


• No polluting your source control with gems. This speeds up your source control, as well as your deploys. Further, for architecture specific gems, you now don't have to have every version of each gem in your source control for each platform you need (which is quite likely at least two: your dev boxes (e.g. Macs) and your deployment boxes (Linux), but could be even more).


• Easier, single location, statement of what gems your app requires. By using the geminstaller.yml file, you have a single place to go see what gems and which versions of those gems your app uses. This is much better than trying to look through your vendor directory, and determine what version of a gem you might have.


• Great for bootstrapping your development environment. Sure, frozen gems usually solve this too (except for the architecture specific ones!). You can just run geminstaller after pulling down the code and it'll go install all the specific version gems you need.


• Allows for multiple config files, so that you can build common ones you use across projects, etc. Or even cooler, your plugins or whatever can provide a file to specify what they need and you can integrate that into your config!


• Easy to install and use. In Rails 2 environments, you can simply drop the few lines needed to use it into its own file in config/initializers. In Rails 1.x, you add these lines to your environment.rb.


• You determine what level of function you want geminstaller doing in your app: e.g. do you want it automatically installing missing gems or just warning you? Should it put them on the load path so you are guaranteed the proper version loads, or do you want to just use it to bootstrap and live dangerously otherwise ;-)


• Makes it easier to experiment with new versions of gems. Since you'll have to install the gem anyway (or most solutions need that to freeze them in, but not all), you can experiment by simply changing the version number in your geminstaller.yml file. To undo it, just change the number back. No need to copy the gem into vendor or a private gem repo, etc. Easy.


• GemInstaller can tell you what gems you have on your system, but are not in your config file, as a way to see what you might need.

Check out the GemInstaller page for more details. I highly recommend this, and thanks Chad for creating it.