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Version Control with Git by Jon Loeliger

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Precedents

The complete history of version control systems is beyond the scope of this book. However, there are several landmark, innovative systems that set the stage for or directly led to the development of Git. (This section is selective, hoping to record when new features were introduced or became popular within the free software community.)

The Source Code Control System (SCCS) was one of the original systems on Unix and was developed by M. J. Rochkind in the very early 1970s.[1] This is arguably the first VCS available on any Unix system.

The central store that SCCS provided was called a repository, and that fundamental concept remains pertinent to this day. SCCS also provided a simple locking model to serialize development. If a developer needed files to run and test a program, she would check them out unlocked. However, in order to edit a file, she had to check it out with a lock (a convention enforced through the Unix filesystem). When finished, she would check the file back into the repository and unlock it.

In the early 1980s, Walter Tichy introduced the Revision Control System (RCS)[2] RCS introduced both forward and reverse delta concepts for efficient storage of different file revisions.

The Concurrent Version System (CVS), designed and originally implemented by Dick Grune in 1986 and then crafted anew some four years later by Berliner et al., extended and modified the RCS model with great success. CVS became very popular and was the de facto standard within the open source community for many years. CVS provided several advances over RCS, including distributed development and repository-wide change sets for entire modules.

Furthermore, CVS introduced a new paradigm for the lock. Whereas earlier systems required a developer to lock each file before changing it and thus forced one developer to wait for another in serial fashion, CVS gave each developer write permission in his private working copy. Thus, changes by different developers could be merged automatically by CVS unless two developers tried to change the same line. In that case, the conflict was flagged and the developers were left to work out the solution. The new rules for the lock allowed different developers to write code concurrently.

As often occurs, perceived shortcomings and faults in CVS eventually led to a new version control system. Subversion (SVN), introduced around 2001, quickly became popular within the free software community. Unlike CVS, SVN committed changes atomically and had significantly better support for branches.

BitKeeper and Mercurial were radical departures from all the aforementioned solutions. Each eliminated the central repository; instead, the store was distributed, providing each developer with his own shareable copy. Git is derived from this peer-to-peer model.

Finally, Mercurial and Monotone contrived a hash fingerprint to uniquely identify a file’s content. The name assigned to the file is a moniker and convenient handle for the user and nothing more. Git features this notion as well. Internally, the Git identifier is based on the file’s contents, a concept known as a content-addressable file store. The concept is not new. (For example, see The Venti Filesystem, (Plan 9), Bell Labs, http://www.usenix.org/events/fast02/quinlan/quinlan_html/index.html.) Git borrowed the idea immediately from Monotone, according to Linus.[3] Mercurial was implementing the concept simultaneously with Git.



[1] “The Source Code Control System,” IEEE Transactions on Software Engineering 1(4) (1975): 364–370.

[2] “RCS—A System for Version Control,” Software Practice and Experience 15 (7) (July 1985): 637–654.

[3] Private email.

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