| Developer(s) | The CVS Team |
|---|---|
| Initial release | November 19, 1990 |
| Stable release | 1.11.23 / May 8, 2008 |
| Preview release | 1.12.13 / July 26, 2006 |
| Written in | C |
| Operating system | Unix-like, Windows |
| Type | Revision control |
| License | GNU General Public License |
| Website | savannah.nongnu.org/projects/cvs |
The Concurrent Versions System (CVS), also known as the Concurrent Versioning System, is a client-server free software revision control system in the field of software development. A version control system keeps track of all work and all changes in a set of files, and allows several developers (potentially widely separated in space and time) to collaborate. Dick Grune developed CVS as a series of shell scripts in July 1986.
In addition to commercial software developers, CVS became popular with the open source software world and was released under the GNU General Public License. While there was regular development to add features and fix bugs in the past, including regular builds and test results, there have been no new releases since 2008. The product is mature: new releases are not produced until there are requests for new features or bug reports. For more on the status of CVS development, see the section below.
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 the possibility of people stepping on each other's toes, 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.
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 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.
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.
- Collins-Sussman, Version Control with Subversion For Subversion 1.1, 2005
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.
CVS developed from an earlier versioning-system called Revision Control System (RCS) (still[update] in use) that manages individual files but not whole projects. Dick Grune provides some brief historical notes 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.
- Dick Grune, Dick Grune's website
Grune publicly released the code to mod.sources on June 23, 1986: Google Groups continues to archive and serve the original usenet post.
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 introduced the implementation of branching into version control systems: the branching techniques in other systems all derive from the CVS implementation as documented in 1990. Whilst RCS did incorporate the concept of branches - they were for individual files only.
CVS has always solidly supported distributed, multi-site and offline operations due to the unreliability of the few computer networks that existed at the time CVS evolved.
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.
Historically, the relationship between CVS and the GNU project could appear somewhat ambiguous: the GNU Web site distributed the program, labelling it "GNU package" on one page and "other GPL-licensed project" on another. Clarification came recently[update] 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 resided in the /non-gnu/ directory and still does.
The GNU project specifically states that it does not maintain ownership of CVS.
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. This has occurred frequently enough to lead to the phrase YACC: "Yet Another CVS Clone" (a play on the Unix command named yacc, which stands for "yet another compiler compiler"). CVS replacement projects include CVSNT (first released 1998), Subversion (initially released in 2004), EVS (first released 2008), OpenCVS (not released as of 3 May 2012), and numerous systems that support distributed revision control.
Several characteristics of CVS have been frequently criticized[by whom?]. Defenders[who?] 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.
| Criticism | Response |
|---|---|
| Revisions created by a commit are per file, rather than spanning the collection of files that make up the project or spanning the entire repository. | When a release is made, a tag can be created to associate the set of revisions with a meaningful release name. |
| CVS does not version the moving or renaming of files and directories. | In 1984, when CVS originated, refactoring was less common in development processes; the first known paper to discuss refactoring was written in 1992. Hence, there was less of a requirement for moving and renaming files and directories. However, moving and renaming can still be accomplished with the help of a knowledgeable administrator by directly moving the RCS file in the repository. |
| No versioning of symbolic links. | Symbolic links stored in a version control system can pose a security risk. For instance, a symbolic link to a sensitive file can be stored in the repository, making the sensitive file accessible even when it is not checked in. In place of symbolic links, scripts that require certain privileges and conscious intervention to execute may be checked into CVS. |
| Support for Unicode and non-ASCII filenames is limited. | Use of native non-ASCII character sets was traditionally avoided due to the problems caused when multiple encodings were used. Now many Unix systems run in UTF-8, and CVS on such systems handles UTF-8 filenames natively. |
| Commits are not atomic. | Some networks and servers have insufficient resilience to complete a commit without crashing; this was even more common in the past. The lack of atomicity is mitigated by the fact that in many code management processes, development work is performed on branches and then merged into the trunk after code review. That final merge is atomic, and performed in the data center by QA. The term atomic is sometimes used in the transactional database sense, where a commit automatically rolls back if it fails for any reason, and sometimes in the sense that each commit can be uniquely identified. Tracking each commit can be accomplished by modifying the correct trigger. |
| Branch operations are expensive. | CVS assumes that the majority of work takes place on the trunk, and that branches should generally be short-lived or historical. When used as designed, branches are easily managed and branch operations are efficient and fast. |
| CVS treats files as text by default. | Text files are expected to be the primary file type stored in the CVS repository. However, binary files are also supported, and files with a particular file extension can automatically be recognized as being binary. |
| No support for distributed revision control or unpublished changes. | CVS developers favored an approach in which programmers frequently committed changes to the centrally checked-in copies of files in order to aid merging and foster rapid distribution of changes to all users. |
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