Consider the following Gemfile(5):
source "https://rubygems.org" gem "rails", "3.0.0.rc" gem "nokogiri"
When you run bundle install(1) bundle-install.1.html the first time, bundler will resolve all of the dependencies, all the way down, and install what you need:
Fetching gem metadata from https://rubygems.org/......... Resolving dependencies... Installing builder 2.1.2 Installing abstract 1.0.0 Installing rack 1.2.8 Using bundler 1.7.6 Installing rake 10.4.0 Installing polyglot 0.3.5 Installing mime-types 1.25.1 Installing i18n 0.4.2 Installing mini_portile 0.6.1 Installing tzinfo 0.3.42 Installing rack-mount 0.6.14 Installing rack-test 0.5.7 Installing treetop 1.4.15 Installing thor 0.14.6 Installing activesupport 3.0.0.rc Installing erubis 2.6.6 Installing activemodel 3.0.0.rc Installing arel 0.4.0 Installing mail 2.2.20 Installing activeresource 3.0.0.rc Installing actionpack 3.0.0.rc Installing activerecord 3.0.0.rc Installing actionmailer 3.0.0.rc Installing railties 3.0.0.rc Installing rails 3.0.0.rc Installing nokogiri 1.6.5 Bundle complete! 2 Gemfile dependencies, 26 gems total. Use `bundle show [gemname]` to see where a bundled gem is installed.
As you can see, even though you have two gems in the Gemfile(5), your application needs 26 different gems in order to run. Bundler remembers the exact versions it installed in Gemfile.lock. The next time you run bundle install(1) bundle-install.1.html, bundler skips the dependency resolution and installs the same gems as it installed last time.
After checking in the Gemfile.lock into version control and cloning it on another machine, running bundle install(1) bundle-install.1.html will still install the gems that you installed last time. You don't need to worry that a new release of erubis or mail changes the gems you use.
However, from time to time, you might want to update the gems you are using to the newest versions that still match the gems in your Gemfile(5).
To do this, run bundle update --all, which will ignore the Gemfile.lock, and resolve all the dependencies again. Keep in mind that this process can result in a significantly different set of the 25 gems, based on the requirements of new gems that the gem authors released since the last time you ran bundle update --all.
For instance, in the scenario above, imagine that nokogiri releases version 1.4.4, and you want to update it without updating Rails and all of its dependencies. To do this, run bundle update nokogiri.
source "https://rubygems.org" gem "thin" gem "rack-perftools-profiler"
The thin gem depends on rack >= 1.0, while rack-perftools-profiler depends on rack ~> 1.0. If you run bundle install, you get:
Fetching source index for https://rubygems.org/ Installing daemons (1.1.0) Installing eventmachine (0.12.10) with native extensions Installing open4 (1.0.1) Installing perftools.rb (0.4.7) with native extensions Installing rack (1.2.1) Installing rack-perftools_profiler (0.0.2) Installing thin (1.2.7) with native extensions Using bundler (1.0.0.rc.3)
In this case, the two gems have their own set of dependencies, but they share rack in common. If you run bundle update thin, bundler will update daemons, eventmachine and rack, which are dependencies of thin, but not open4 or perftools.rb, which are dependencies of rack-perftools_profiler. Note that bundle update thin will update rack even though it's also a dependency of rack-perftools_profiler.
In short, by default, when you update a gem using bundle update, bundler will update all dependencies of that gem, including those that are also dependencies of another gem.
To prevent updating shared dependencies, prior to version 1.14 the only option was the CONSERVATIVE UPDATING behavior in bundle install(1) bundle-install.1.html:
In this scenario, updating the thin version manually in the Gemfile(5), and then running bundle install(1) bundle-install.1.html will only update daemons and eventmachine, but not rack. For more information, see the CONSERVATIVE UPDATING section of bundle install(1) bundle-install.1.html.
When Bundler is resolving what versions to use to satisfy declared requirements in the Gemfile or in parent gems, it looks up all available versions, filters out any versions that don't satisfy the requirement, and then, by default, sorts them from newest to oldest, considering them in that order.
Providing one of the patch level options (e.g. --patch) changes the sort order of the satisfying versions, causing Bundler to consider the latest --patch or --minor version available before other versions. Note that versions outside the stated patch level could still be resolved to if necessary to find a suitable dependency graph.
For example, if gem 'foo' is locked at 1.0.2, with no gem requirement defined in the Gemfile, and versions 1.0.3, 1.0.4, 1.1.0, 1.1.1, 2.0.0 all exist, the default order of preference by default (--major) will be "2.0.0, 1.1.1, 1.1.0, 1.0.4, 1.0.3, 1.0.2".
If the --patch option is used, the order of preference will change to "1.0.4, 1.0.3, 1.0.2, 1.1.1, 1.1.0, 2.0.0".
If the --minor option is used, the order of preference will change to "1.1.1, 1.1.0, 1.0.4, 1.0.3, 1.0.2, 2.0.0".
Combining the --strict option with any of the patch level options will remove any versions beyond the scope of the patch level option, to ensure that no gem is updated that far.
To continue the previous example, if both --patch and --strict options are used, the available versions for resolution would be "1.0.4, 1.0.3, 1.0.2". If --minor and --strict are used, it would be "1.1.1, 1.1.0, 1.0.4, 1.0.3, 1.0.2".
Gem requirements as defined in the Gemfile will still be the first determining factor for what versions are available. If the gem requirement for foo in the Gemfile is '~> 1.0', that will accomplish the same thing as providing the --minor and --strict options.
foo 1.4.3, requires: ~> bar 2.0 foo 1.4.4, requires: ~> bar 2.0 foo 1.4.5, requires: ~> bar 2.1 foo 1.5.0, requires: ~> bar 2.1 foo 1.5.1, requires: ~> bar 3.0 bar with versions 2.0.3, 2.0.4, 2.1.0, 2.1.1, 3.0.0
foo (1.4.3) bar (~> 2.0) bar (2.0.3)
# Command Line Result ------------------------------------------------------------ 1 bundle update --patch 'foo 1.4.5', 'bar 2.1.1' 2 bundle update --patch foo 'foo 1.4.5', 'bar 2.1.1' 3 bundle update --minor 'foo 1.5.1', 'bar 3.0.0' 4 bundle update --minor --strict 'foo 1.5.0', 'bar 2.1.1' 5 bundle update --patch --strict 'foo 1.4.4', 'bar 2.0.4'
In case 1, bar is upgraded to 2.1.1, a minor version increase, because the dependency from foo 1.4.5 required it.
In case 2, only foo is requested to be unlocked, but bar is also allowed to move because it's not a declared dependency in the Gemfile.
In case 3, bar goes up a whole major release, because a minor increase is preferred now for foo, and when it goes to 1.5.1, it requires 3.0.0 of bar.
In case 4, foo is preferred up to a minor version, but 1.5.1 won't work because the --strict flag removes bar 3.0.0 from consideration since it's a major increment.