Concurrent Versions System

The Concurrent Versions System (CVS) is a version control system: it keeps track of all changes to a (software) project and allows several distant developers to collaborate. It is popular in the free software world and is itself released under the GNU General Public License.

Features

CVS is a client-server system: the server keeps the current version of the project and its history, and clients connect to the server in order to check out a complete copy of the project, work on this copy and then later check in their changes. Typically, client and server connect over the internet, but it is also possible to use the client and the server on the same machine, in order to keep track of the version history of a project with only local developers. The server normally runs on Unix, while CVS clients are available for all major platforms.

Several clients may check out copies of the project concurrently. When they later check in their changes, the server attempts to merge them. If this fails, for instance because two clients attempted to change the same line in a certain file, then the server denies the second checkin operation and informs the client about the conflict, which needs to be resolved by hand. If the checkin operation succeeds, then all involved file's version numbers are automatically increased, and a user supplied description line, the date and the author's name are stored in the log files.

Clients can also compare different versions of files, request a complete history of changes, or check out a historical snapshot of the project at a given date or revision number. Many open source projects allow "anonymous read access", meaning that the clients are allowed to check out and compare versions without a password; only the checkin of changes requires a password in these scenarios.

The client can also use the "update" command in order to bring his local copy up to date with the newest version on the server. This eliminates the need for repeated downloading of the whole project.

It is possible to maintain different "branches" of a project with CVS. For instance, a released version of the software project could be maintained in a branch which is only used for bug fixes, while the currently developed version with major changes and new features is stored in a separate branch.

Terminology

A single project (set of related files) managed by CVS is called a module. A CVS server can manage several modules, and the place where it stores all the modules it manages is known as the repository. The copy of a module that has been downloaded by a client is called a working copy.

History and Status

CVS is based on an earlier versioning system called RCS which manages individual files but not whole projects and is still in use. The project based approach and conflict resolution algorithms of CVS were first conceived by Dick Grune in December 1986. The code that eventually evolved into the current version of CVS was started by Brian Berliner in April 1989, later to be helped by Jeff Polk and many other contributors. Nowadays, the CVS code is maintained by a group of volunteers.

The status of CVS as part of the GNU project is somewhat ambiguous: the GNU website distributes the program; it is labeled "GNU package" on one page and "other GPL-licensed project" on another. On the FTP site, the program is located in the /non-gnu/ directory.

See version control system for other examples.

References and external links

• Per Cederqvist et. al. : CVS Concurrent Versions System, http://www.cvshome.org/docs/manual/cvs
• Karl Fogel: Open Source Development with CVS, http://cvsbook.red-bean.com/cvsbook
• CVS web sites: http://www.loria.fr/~molli/cvs-index , http://www.cvshome.org