Subversion

Engineering revision control developed from formalized processes based on tracking revisions of early blueprints or bluelines. Implicit in this control was the option to be able to return to any earlier state of the design, for cases in which an engineering dead-end was reached in iterating any particular engineering design.
Likewise, in computer software engineering, revision control is any practice which tracks and provides controls over changes to 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, while the other version is where new features are worked on).
At the simplest level, developers 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.