Concurrent Versions System

From Free net encyclopedia

The Concurrent Versions System (CVS), also known as the Concurrent Versioning System, implements a version control system: it keeps track of all work and all changes in a set of files, typically the implementation of a software project, and allows several (potentially widely separated) developers to collaborate. CVS has become popular in the open-source world. CVS is released under the GNU General Public License.

1 Features
2 CVS Terminology
3 History and status
- 3.1 Relationship with GNU
4 Limitations
5 See also
6 References
7 External links

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Features

CVS utilizes a client-server architecture: a server stores the current version(s) 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 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 a Windows NT CVS server also exists), while CVS clients may run on any major operating-system platform.

Several clients may edit 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 check-in operation and informs the client about the conflict, which the user will need to resolve by hand. 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.

Clients can also compare different versions of files, 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 that was 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, forms a separate branch.

CVS uses delta compression for efficient storage of different versions of the same file. The implementation favours files with many lines (usually text files) - in extreme cases individual copies of each version are stored rather than a delta.

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CVS Terminology

A single project (set of related files) managed by CVS is called a module. A CVS server stores the modules it manages in its repository. Acquiring a copy of a module is called checking out. The checked out files serve as a working copy. One's changes to the working copy will be reflected in the repository by commit[ting] them. To update is to acquire the latest changes from the repository in the working copy.

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History and status

CVS developed from an earlier versioning system called Revision Control System (RCS), still in use, which manages individual files but not whole projects. Dick Grune has provided some brief historical notes about CVS on his site. To quote: Template:Quotation The code was publicly released to mod.sources on June 23, 1986: the original usenet post is still visible via Google Groups.

The code that eventually evolved into the current version of CVS started with Brian Berliner in April 1989, with later input from Jeff Polk and many other contributors. Brian Berliner wrote a paper introducing his improvements to the CVS program which describes how the tool was extended and used internally by Prisma, a third party developer working on the SunOS kernel, and was released for the benefit of the community under the GPL.

Nowadays, a group of volunteers maintains the CVS code. Notably, the development of the Microsoft Windows version of CVS has split off into a separate project named CVSNT and has been more active in extending the feature set of the system, even porting the changes back to the UNIX platform under the name CVSNT.

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Relationship with GNU

Historically, the relationship between CVS and the GNU project could appear somewhat ambiguous: the GNU website distributed the program, labelling it "GNU package" on one page and "other GPL-licensed project" on another. This was recently clarified when CVS development moved from cvshome.org to savannah.nongnu.org, with CVS officially assigned to the non-gnu category. On the FTP site, the program has traditionally and still does reside in the /non-gnu/ directory.

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Limitations

Files in a CVS repository cannot be renamed; they must be removed and re-added.
Limited support for Unicode files as non ASCII filenames

A number of key developers who have worked on CVS are now responsible for Subversion (SVN), released in early 2004, which aims to replace CVS by addressing some of its limitations. Also, tools like WANdisco for CVS have addressed many of the limitations of CVS like lack of atomic commit, role based access control, multisite features.

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