CVS

In the field of software development, the Concurrent Versions System (CVS), also known as the Concurrent Versioning System, provides a version control system based on open-source code. Version control system software keeps track of all work and all changes in a set of files, and allows several developers (potentially widely separated in space and/or time) to collaborate. Dick Grune developed CVS in the 1980s. CVS has become popular in the open source software world and is released under the GNU General Public License.

CVS uses a client-server architecture: a server stores the current version(s) of a 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, the client and server connect over a LAN or over the Internet, but client and server may both run on the same machine if CVS has the task of keeping track of the version history of a project with only local developers. The server software normally runs on Unix (although at least the CVSNT server supports various flavors of Microsoft Windows and Linux), while CVS clients may run on any major operating-system platform.

Several developers may work on the same project concurrently, each one editing files within their own "working copy" of the project, and sending (or checking in) their modifications to the server. To avoid the possibility of people stepping on each other's toes, the server will only accept changes made to the most recent version of a file. Developers are therefore expected to keep their working copy up-to-date by incorporating other people's changes on a regular basis. This task is mostly handled automatically by the CVS client, requiring manual intervention only when a conflict arises between a checked-in modification and the yet-unchecked local version of a file.

If the check-in operation succeeds, then the version numbers of all files involved automatically increment, and the CVS-server writes a user-supplied description line, the date and the author's name to its log files. CVS can also run external, user-specified log processing scripts following each commit. These scripts are installed by an entry in CVS's loginfo file, which can trigger email notification or convert the log data into a Web-based format.

Clients can also compare versions, request a complete history of changes, or check out a historical snapshot of the project as of a given date or as of a revision number. Many open-source projects allow "anonymous read access", a feature pioneered by OpenBSD. This means that clients may check out and compare versions with either a blank or simple published password (e.g., "anoncvs"); only the check-in of changes requires a personal account and password in these scenarios.

Clients can also use the "update" command in order to bring their local copies up-to-date with the newest version on the server. This eliminates the need for repeated downloading of the whole project.

CVS can also maintain different "branches" of a project. For instance, a released version of the software project may form one branch, used for bug fixes, while a version under current development, with major changes and new features, can form a separate branch.

CVS uses delta compression for efficient storage of different versions of the same file. The implementation favors files with many lines (usually text files) - in extreme cases the system may store individual copies of each version rather than deltas.