Concurrent Versions System

Features

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 also supports various flavours of Microsoft Windows), 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 conflicts, the server only accepts 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 an edit 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.

CVS servers can allow "anonymous read access", wherein 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 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. This works well with large text files with few changes from one version to the next. This is usually the case for source code files. On the other hand, when CVS is told to store a file as binary, it will keep each individual version on the server. Storing files as binary is important in order to avoid corruption of binary files.

In the world of open source software, the Concurrent Version System (CVS) has long been the tool of choice for version control. And rightly so. CVS itself is free software, and its non-restrictive modus operandi and support for networked operation – which allow dozens of geographically dispersed programmers to share their work – fits the collaborative nature of the open-source world very well. CVS and its semi-chaotic development model have become cornerstones of open-source.

Terminology

CVS labels a single project (set of related files) that it manages as a module. A CVS server stores the modules it manages in its repository. Programmers acquire copies of modules by checking out. The checked-out files serve as a working copy, sandbox or workspace. Changes to the working copy are reflected in the repository by committing them. To update is to acquire or merge the changes in the repository with the working copy.

History and status

Dick Grune developed CVS as a front end for the Revision Control System (RCS), an older version control system that manages individual files but not whole projects.

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.

Grune publicly released the code on June 23, 1986.

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. On November 19, 1990, CVS version 1.0 was submitted to the Free Software Foundation for development and distribution.

CVS supports distributed, multi-site and offline operations due to the unreliability of the few computer networks that existed at the time CVS evolved.

Development status

There have been no official recent announcements indicating the project status.

Development of the Microsoft Windows, Linux, Solaris, HPUX, I5os and Mac OS X port of CVS has split off into a separate project named CVSNT, which is under current, active development.

Relationship with GNU

The relationship between CVS and the GNU project has long been somewhat ambiguous: the GNU web site distributed the program, labelling it "GNU package" on one page and "other GPL-licensed project" on another. In 2008, when development of CVS was transferred from the old website (cvshome.org) to the GNU Savannah hosting platform, it was placed in the "non-GNU" section. Further, on GNU's FTP download server, CVS is distributed in the "non-gnu" directory.

Successors

Over time, developers have created new version control systems based on CVS in order to add features, alter the operational model, and improve developers' productivity. CVS replacement projects include CVSNT (first released 1998), Subversion (initially released in 2004) and EVS (first released 2008).

Criticism

Several characteristics of CVS have been frequently criticized. Defenders argue that many of these are the result of deliberate design decisions, some of which were made at a time when the software and hardware landscape were different than they are now. They also point to the existence of workarounds or approaches to the development process that can mitigate problems.