Concurrent Versions System
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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.
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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 store of different versions of the same file.
Terminology
CVS terminology dubs a single project (set of related files) managed by CVS as a module. A CVS server can manage several modules; it stores all the modules it manages in its repository. The copy of a module that has been downloaded by a client serves as a working copy.
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 (http://www.cs.vu.nl/~dick/CVS.html#History) about CVS on his site. To quote:
I created CVS to be able to cooperate with my students Erik Baalbergen and Maarten Waage on the ACK (Amsterdam Compiler Kit) C compiler. The three of us had vastly different schedules (one student was a steady 9-5 worker, the other was irregular, and I could work on the project only in the evenings). Their project ran from July 1984 to August 1985. CVS was initially called cmt, for the obvious reason that it allowed us to commit versions independently.
The code was publicly released to mod.sources on June 23, 1986. You can still see the original usenet post (http://groups.google.com/groups?:mod.sources.*&hl=en&lr=lang_en&ie=UTF-8&c2coff=1&safe=off&selm=122%40mirror.UUCP&rnum=2) on 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 (http://citeseer.ist.psu.edu/berliner90cvs.html) 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.
The relationship between CVS and the GNU project can appear somewhat ambiguous: the GNU website distributes the program, labelling it "GNU package" on one page and "other GPL-licensed project" on another. On the FTP site, the program resides in the /non-gnu/ directory.
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.
See also
- Tools for CVS - many tools related to CVS
- Version control system, general overview
External links
- the CVS text-based program web site: cvshome (http://www.cvshome.org) (the main web site for CVS)
- Documentation at CVSHome.org (http://www.cvshome.org/docs/)
- Introduction to CVS (http://www.linuxdevcenter.com/pub/a/linux/2002/01/03/cvs_intro.html)
- Getting Started with CVS (http://www.linux.ie/articles/tutorials/cvs.php)
- CVS Administration introduction (http://www.linuxdevcenter.com/pub/a/linux/2002/01/17/cvsadmin.html)
- CVS Best Practices (http://tldp.org/REF/CVS-BestPractices/) (2nd link (http://www.magic-cauldron.com/cm/cvs-bestpractices/index-cvs-bestpractices.html))
- Top 10 CVS Tips (http://opensource.oreilly.com/news/cvs_0900.html)
- CVSNT Home Page (http://www.cvsnt.org/) - this is a different project, derived from the original CVS as a windows server/client but now runs on many platforms including linux.
- CVS information on Sourceforge (http://sourceforge.net/cvs/?group_id=34373)
Literature
- David Thomas, Andrew Hunt: Pragmatic Version Control Using CVS (The Pragmatic Programmers, 2003), ISBN 0974514004
- Jennifer Vesperman: Essential CVS (O'Reilly, 2003), ISBN 0596004591
- Official reference manual by Per Cederqvist et al: Version Management with CVS (http://www.network-theory.co.uk/cvs/manual/), ISBN 0954161718
- Karl Fogel, Moshe Bar: Open Source Development with CVS (http://cvsbook.red-bean.com/), ISBN 1932111816
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