| Revision Control |
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| CATEGORIES ABOUT REVISION CONTROL | |
| software engineering | |
| version control systems | |
| technical communication | |
| software development process | |
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Standalone version control systems mostly come from the software engineering industry, but revision control is also embedded in various types of software like Word Processor s (e.g. MSWord, OOwriter, Koffice), spreadsheets (e.g. OOcalc), in various Content Management System s. Integrated revision control is a key feature of Wiki Software packages such as MediaWiki, TWiki, etc. In wikis, revision control allows for the ability to revert a page to a previous revision, which is critical for defending a public wiki against vandalism and spam, to allow legitimate users to correct their mistakes, and to allow groups of editors to track each other's edits. Software Tools For Revision Control are increasingly recognized as being necessary for the organization of multi-developer projects. OVERVIEW Engineering revision control developed from formalized processes based on tracking revisions of early blueprints or Bluelines . Implicit in this control was the ability to return to any earlier state of the design, for cases in which an engineering dead-end was reached in the development of the design. Likewise, in computer Software Engineering , revision control is any practice that tracks and provides control over changes to source code. Software Developer s sometimes use revision control software to maintain documentation and Configuration File s as well as source code. Also, version control is widespread in business and law. Indeed, "contract redline" and "legal blackline" are some of the earliest forms of revision control, and are still employed with varying degrees of sophistication. An entire industry has emerged to service the document revision control needs of business and other users, and some of the revision control technology employed in these circles is subtle, powerful, and innovative. The most sophisticated techniques are beginning to be used for the electronic tracking of changes to CAD files (see Product Data Management ), supplanting the "manual" electronic implementation of traditional revision control. As software is designed, 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 simultaneously 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 develops). Therefore, for the purposes of locating and fixing bugs, it is vitally important 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 could 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. While this method can work, it is inefficient as many near-identical copies of the program have to be maintained. This 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. Moreover, in software development and other environments, including in legal and business practice, it is increasingly common for a single document or snippet of code to be edited by a team, the members of which may be geographically diverse and/or may pursue different and even contrary interests. Sophisticated revision control that tracks and accounts for ownership of changes to documents and code may be extremely helpful or even necessary in such situations. Another use for revision control is to track changes to configuration files, such as those typically stored in /etc or /usr/local/etc on Unix systems. This gives system administrators another way to easily track changes to configuration files and a way to roll back to earlier versions should the need arise. Compression Most revision control software can use Delta Compression , which retains only the differences between successive versions of files. This allows more efficient storage of many different versions of files. SOURCE MANAGEMENT MODELS Traditional revision control systems use a centralized model, where all the revision control functions are performed on a shared Server . If two developers try to change the same file at the same time, without some method of managing access the developers may end up overwriting each other's work. Centralized revision control systems solve this problem in one of 2 different "source management models": file locking and version merging. File locking The simplest method of preventing " Concurrent Access " problems is to lock files so that only one developer at a time has write access to the central "repository" copies of those files. Once one developer "checks out" a file, others can read that file, but no one else is allowed to change that file until that developer "checks in" the updated version (or cancels the checkout). File locking has merits and drawbacks. It can provide some protection against difficult merge conflicts when a user is making radical changes to many sections of a large file (or group of files). But 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. That can lead to more serious problems. Version merging Most version control systems, such as CVS , allow multiple developers to be editing the same file at the same time. The first developer to "check in" changes to the central repository always succeeds. The system provides facilities to merge changes into the central repository, so the improvements from the first developer are preserved when the other programmers check in. The concept of a ''reserved edit'' can provide an optional means to explicitly lock a file for exclusive write access, even though a merging capability exists. DISTRIBUTED REVISION CONTROL Distributed revision control takes a peer-to-peer approach, as opposed to the client-server approach of centralized systems. Rather than a single, central repository on which clients synchronize, |
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