Over the past two years, Carl and I have been working on-again off-again on the problem of dependency resolution. Fabien Franzen implemented the first partial solution for Merb, with thor merb:gem:install .

When we started working on Rails, we knew we wanted to finally crack the nut, making it possible for Rails itself to have some of its own dependencies, and solving some persistent, long-term problems with gem plugins.

Earlier this year, we released bundler, our first real attempt to solve this problem. Among other things, it shipped with the first true dependency resolver for Rubygems. It also modeled the entire dependency process on top of Rubygems, allowing us to support git repositories in the same dependency resolution process as Rubygems themselves.

Over the next few months, we refined the bundler quite a bit. We're proud of the fact that individuals, web shops, and deployment companies have adopted bundler, and Gemfile has become a standard way of expressing gem dependencies for an app.

During this time, we've also received quite a bit of feedback. As we approach the release of Bundler 1.0, which we hope to ship along with Rails 3.0 final, we took the opportunity to take a look at all the feedback people have sent so far.

Having done so, we're proud to announce Bundler 0.9 with radically improved workflows that fit our needs and the needs of those who have contributed feedback and patches.

The Cache

Before Bundler 0.9, Bundler installed gems and git repositories to a local application cache. When we looked at the deployment workflow, we found that while people liked the ability to cache their .gem files in their application, they had a lot of trouble at deployment time.

Essentially, people (and we!) wanted the ability to expand the gems to a central cache that they could reuse across deployments. People tried various symlinking approaches, but it became clear to us Bundler 1.0 needed to support this workflow natively.

On a related note, people who developed a number of applications on a single machine wanted to be able to reuse a single system cache across their application, and not need to connect to remotes so often when they already had all the gems they needed on their systems.

Finally, Rails itself grated against the default Bundler workflow. Most people installing a new Rails app run gem install rails , then rails my_app , and then want to go into their application and start working. But with Bundler 0.5 to 0.8, they needed to hit the remotes again even though, by definition, the system gem cache already had all the gems they needed.

Runtime

The fully-packaged application works fantastically for deployment, enabling a repeatable, reliable development to staging to production workflow. However, the need to explicitly bundle the application after added a new dependency in the very early stages of an application's life cycle feels more like a compile step than a lean mean agile machine.

In Bundler 0.9, we're adding the ability to run your application against gems already on your system, with the same reliability that you've grown to love from our dependency resolver.

Once you've gotten your application working, you can lock down the dependencies, so coworkers or production will use exactly the same environment that worked for you.

This allows you to use your common system gem repository across multiple apps, with a fully resolved virtual environment for each application based on the app's dependencies specified in the Gemfile.

Locking

A lot of people (including me) love the idea of storing the .gem files in the application to create a single deployable unit with no network dependencies.

Storing the .gem files in the repository also provides a record of the fully resolved dependencies for your application, so that a new released gem between testing and deployment cannot change the environment. In Bundler 0.9, we provide a mechanism for you to save that record without having to also store the .gem files.

This won't give you a single, dependency-less deployment package, but it will save you from unexpected environment changes.

New Commands

When we started Bundler, we had just one command: gem bundle . Over the following months, the command took on a life of its own, with a slew of flags representing both options and verbs. Bundler 1.0 will have a small, streamlined list of commands, each with its own set of flags:

bundle pack

Take all .gem files needed for the application and place them into the local application. By default, bundler places them in vendor/gems .

bundle lock

Resolve all dependencies in the Gemfile, and store a record of that process. In the future, use that record, preventing any changes in the environment or in the remote sources from changing the gems used by the application.

bundle install

Install the bundle specified by the Gemfile to the cache. By default, Bundler installs the gems (and git repositories) into system gems. You can override this to install into another location, or into your local application, just like in Bundler 0.8 and before.

After cloning a new repository for the first time, you will probably want to run this command.

If you pack ed gems, Bundler will use the gems from your application. If you pack ed and lock ed gems, Bundler will install the gems immediately without updating the remote sources.

bundle check

Check to see whether your cache has all the required dependencies. If it does not, Bundler will print out the list of missing gems.

bundle exec ...

Run the specified command using the local environment. For instance, if you have Rails 2.3 and 3.0 installed, if you run bundle exec rails . in a Rails 2.3 application, Bundler will run the rails command using Rails 2.3 (and other gems in the current environment).

Environments

In Bundler 0.9, as in Bundler 0.8 and before, the Gemfile specifies an app-specific environment. New in Bundler 0.9, you may store the .gem files and unpacked gems in a central location, as well as in the application itself.

Because Bundler ships with a dependency resolver, you do not need to specify named environments and switch between them. Instead, the environments are virtual, based on the gems you specify as application dependencies in the Gemfile, and you still get all the benefits of a shared, system-wide cache.