Why Can't I Just Specify Only = Dependencies?

Q: I understand the value of locking my gems down to specific versions, but why can't I just specify = versions for all my dependencies in the Gemfile and forget about the Gemfile.lock?

A: Many of your gems will have their own dependencies, and they are unlikely to specify = dependencies. Moreover, it is probably unwise for gems to lock down all of *their* dependencies so strictly. The Gemfile.lock allows you to specify the versions of the dependencies that your application needs in the Gemfile, while remembering all of the exact versions of third-party code that your application used when it last worked correctly.

By specifying looser dependencies in your Gemfile (such as nokogiri ~> 1.4.2), you gain the ability to run bundle update nokogiri, and let bundler handle updating **only** nokogiri and its dependencies to the latest version that still satisfied the ~> 1.4.2 version requirement. This also allows you to say "I want to use the current version of nokogiri" (gem 'nokogiri' in your Gemfile) without having to look up the exact version number, while still getting the benefits of ensuring that your application always runs with exactly the same versions of all third-party code.

Why Can't I Just Submodule Everything?

Q: I don't understand why I need bundler to manage my gems in this manner. Why can't I just get the gems I need and stick them in submodules, then put each of the submodules on the load path?

A: Unfortunately, that solution requires that you manually resolve all of the dependencies in your application, including dependencies of dependencies. And even once you do that successfully, you would need to redo that work if you wanted to update a particular gem. For instance, if you wanted to update the rails gem, you would need to find all of the gems that depended on dependencies of Rails (rack, erubis, i18n, tzinfo, etc.), and find new versions that satisfy the new versions of Rails' requirements.

Frankly, this is the sort of problem that computers are good at, and which you, a developer, should not need to spend time doing.

More concerningly, if you made a mistake in the manual dependency resolution process, you would not get any feedback about conflicts between different dependencies, resulting in subtle runtime errors. For instance, if you accidentally stuck the wrong version of rack in a submodule, it would likely break at runtime, when Rails or another dependency tried to rely on a method that was not present.

Bottom line: even though it might seem simpler at first glance, it is decidedly significantly more complex.

Why Is Bundler Downloading Gems From --without Groups?

Q: I ran bundle install --without production and bundler is still downloading the gems in the :production group. Why?

A: Bundler's Gemfile.lock has to contain exact versions of all dependencies in your Gemfile, regardless of any options you pass in. If it did not, deploying your application to production might change all your dependencies, eliminating the benefit of Bundler. You could no longer be sure that your application uses the same gems in production that you used to develop and test with. Additionally, adding a dependency in production might result in an application that is impossible to deploy.

For instance, imagine you have a production-only gem (let's call it rack-debugging) that depends on rack =1.1. If we did not evaluate the production group when you ran bundle install --without production, you would deploy your application, only to receive an error that rack-debugging conflicted with rails (which depends on actionpack, which depends on rack ~> 1.2.1).

Another example: imagine a simple Rack application that has gem 'rack' in the Gemfile. Again, imagine that you put rack-debugging in the :production group. If we did not evaluate the :production group when you installed via bundle install --without production, your app would use rack 1.2.1 in development, and you would learn, at deployment time, that rack-debugging conflicts with the version of Rack that you tested with.

In contrast, by evaluating the gems in **all** groups when you call bundle install, regardless of the groups you actually want to use in that environment, we will discover the rack-debugger requirement, and install rack 1.1, which is also compatible with the gem 'rack' requirement in your Gemfile.

In short, by always evaluating all of the dependencies in your Gemfile, regardless of the dependencies you intend to use in a particular environment, you avoid nasty surprises when switching to a different set of groups in a different environment. And because we just download (but do not install) the gems, you won't have to worry about the possibility of a difficult **installation** process for a gem that you only use in production (or in development).

I Have a C Extension That Requires Special Flags to Install

Q: I have a C extension gem, such as mysql, which requires special flags in order to compile and install. How can I pass these flags into the installation process for those gems?

A: First of all, this problem does not exist for the mysql2 gem, which is a drop-in replacement for the mysql gem. In general, modern C extensions properly discover the needed headers.

If you really need to pass flags to a C extension, you can use the bundle config command:

$ bundle config build.mysql --with-mysql-config=/usr/local/mysql/bin/mysql_config

Bundler will store this configuration in ~/.bundle/config, and bundler will use the configuration for any bundle install performed by the same user. As a result, once you specify the necessary build flags for a gem, you can successfully install that gem as many times as necessary.

I Do Not Have an Internet Connection and Bundler Keeps Trying to Connect to the Gem Server

Q: I do not have an internet connection but I have installed the gem before. How do I get bundler to use my local gem cache and not connect to the gem server?

A: Use the --local flag with bundle install. The --local flag tells bundler to use the local gem cache instead of reaching out to the remote gem server.

$ bundle install --local

Bundling From RubyGems is Really Slow

Q: When I bundle from rubygems it is really slow. Is there anything I can do to make it faster?

A: Add the --full-index flag when bundling from the rubygems server. This downloads the index all at once instead of making numerous small requests to the api.

$ bundle install --full-index