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Bundler is a dependency-management tool for Ruby applications. Bundler reads a list of required Ruby libraries, called gems, from a configuration file named Gemfile. Bundler can be used to install the required gems, resolving their interdependencies and version constraints to create a set of gem versions that will work together; it then saves the solution into a file, Gemfile.lock.
During the execution of a bundler-enabled application, Bundler takes over the gem location process, ensuring that the application will find gem versions that are part of the known-good solution.