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What is Revision Control?

Revision control is an aspect of documentation control wherein changes to documents are identified by incrementing an associated number or letter code, termed the "revision level", or simply "revision". It has been a standard practice in the maintenance of engineering drawings for as long as the generation of such drawings has been formalized.

A simple form of revision control, for example, has the initial issue of a drawing assigned the revision level "A". When the first change is made, the revision level is changed to "B" and so on.

Revision Control Overview

In computer software engineering, revision control is any practice which tracks and provides controls over changes to a project's source code.

Software developers sometimes use revision control software to maintain documentation and configuration files as well as source code.

In theory, revision control can be applied to any type of information record. In practice, however, the more sophisticated techniques and tools for revision control have rarely been used outside software development circles (though they could actually be of benefit in many other areas). However, they are beginning to be used for the electronic tracking of changes to CAD files, supplanting the "manual" electronic implementation of traditional revision control.

As software is developed and deployed, it is extremely common for multiple versions of the same software to be deployed in different sites, and for the software's developers to be working privately on updates.

Bugs and other issues with software are often only present in certain versions (because of the fixing of some problems and the introduction of others as the program evolves). Therefore, for the purposes of locating and fixing bugs, it is vitally important for the debugger to be able to retrieve and run different versions of the software to determine in which version(s) the problem occurs. It may also be necessary to develop two versions of the software concurrently (for instance, where one version has bugs fixed, but no new features, where the other is where new features are worked on).

Another problem that occurs in large software development projects is that of multiple developers seeking to work on the program at the same time. If two developers try to change the same file at the same time, without some method of managing access the developers may well end up overwriting each other's work.

Some systems attempt to manage who is allowed to make changes to different aspects of the program, for instance, allowing changes to a file to be checked by a designated reviewer before being added.

At the simplest level, users can simply retain multiple copies of the different versions of the program, and number them appropriately. This simple approach has been used on many large software projects. Whilst this method can work, it is inefficient (as many near-identical copies of the program will be kept around), requires a lot of self-discipline on the part of developers, and often leads to mistakes. Consequently, systems to automate some or all of the revision control process have been developed.

Most revision control systems use a system called delta compression, in which only the differences between successive versions of files are retained, thus allowing the efficient storage of many, many different versions of files.

Some systems also provide methods for preventing "concurrent access" problems, by simply locking files so that only one developer has write access to the central "repository" at once; others, such as CVS, provide facilities to automatically or semi-automatically merge changes. In the latter type, the concept of a reserved edit means to explicitly lock a file for exclusive write access, even though a merging capability exists.

The merits and risks for file locking are hotly debated; while it can provide some protection against difficult-to-resolve merge conflicts when a user is making radical changes to many sections of a large file (or group of files) that is constantly being maintained, if the files are left exclusively locked for too long, other developers can be tempted to simply bypass the revision control software and change the files locally anyway; this can lead to more serious problems.

Some of the more advanced version control tools offer many other facilities, allowing deeper integration with other tools and software engineering processes. Plug-ins are often available for IDEs such as Visual Studio.

In particular the Wikipedia:Page history features of Wikipedia are identical in concept and practice to the revision control software discussed above, which was developed for source code control, in the decades before the inception of Wikipedia.

Popular Version Control Software

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Technical Writing.

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