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Subversion Review…..

January 23, 2010

In software development, Subversion (SVN) is a version-control system initiated in 1999 by CollabNet Inc. Developers use Subversion to maintain current and historical versions of files such as source code, web pages, and documentation. Its goal is to be a mostly-compatible successor to the widely used Concurrent Versions System (CVS).

The open source community has used Subversion widely: for example in projects such as Apache Software Foundation, Free Pascal, FreeBSD, GCC, Django, Ruby, Mono, SourceForge, ExtJS, Tigris.org, PHP and MediaWiki. Google Code also provides Subversion hosting for their open source projects. BountySource systems use it exclusively. CodePlex offers access to Subversion as well as to other types of clients.

The corporate world has also started to adopt Subversion. A 2007 report by Forrester Research recognized Subversion as the sole leader in the Standalone Software Configuration Management (SCM) category and as a strong performer in the Software Configuration and Change Management (SCCM) category.[1]

Subversion uses the Apache License, making it open source.

Features

  • Commits as true atomic operations (interrupted commit operations would otherwise cause repository inconsistency or corruption).
  • Renamed/copied/moved/removed files retain full revision history.
  • The system maintains versioning for directories, renames, and file metadata (but not for timestamps). Users can move and/or copy entire directory-trees very quickly, while retaining full revision history.
  • Versioning of symbolic links.
  • Native support for binary files, with space-efficient binary-diff storage.
  • Apache HTTP Server as network server, WebDAV/Delta-V for protocol. There is also an independent server process called svnserve that uses a custom protocol over TCP/IP.
  • Branching and tagging as cheap operations, independent of file size (though Subversion itself does not distinguish between a tag, a branch, and a directory)
  • Natively client/server, layered library design.
  • Client/server protocol sends diffs in both directions.
  • Costs proportional to change size, not to data size.
  • Parsable output, including XML log output.
  • Open source licensed — Apache License in the projected 1.7 release; prior versions use a derivative of the Apache Software License, v1.1
  • Internationalized program messages.
  • File locking for unmergeable files (“reserved checkouts”).
  • Path-based authorization.
  • Language bindings for PHP, Python, Perl, and Java.
  • Full MIME support – users can view or change the MIME type of each file, with the software knowing which MIME types can have their differences from previous versions shown.

Development and implementation

Main article: Software that uses Subversion

CollabNet has continued its involvement with Subversion, but the project runs as an independent open source community. In November 2009 the project was accepted into the Apache Incubator, aiming to become part of the Apache Software Foundation‘s efforts [20].

In October 2009 WANdisco announced the hiring of core Subversion committers as the company moved to become a major corporate sponsor of the project. This included Hyrum Wright, president of the Subversion Corporation and release manager for the Subversion project since early 2008, who joined the company to lead its open source team[21].

The Subversion open-source community does not provide binaries, but potential users can download such from volunteers, from CollabNet, the initiator of the Subversion project or WANdisco. While the Subversion project does not include an official graphical user interface (GUI) for use with Subversion, third parties have developed a number of different GUIs, along with a wide variety of additional ancillary software.

Filesystem

Svn 3D-tree.svg

One can view the Subversion filesystem as “three dimensional”. In addition to the two dimensions of a standard directory tree (e.g., tree view), the Subversion filesystem’s third dimension is revisions. Each revision in a Subversion filesystem has its own root, which is used to access contents at that revision. Files are stored as links to the most recent change; thus a Subversion repository is quite compact. The system consumes storage space proportional to the number of changes made, not to the number of revisions.

The Subversion filesystem uses transactions to keep changes atomic. A transaction operates on a specified revision of the filesystem, not necessarily the latest. The transaction has its own root, on which changes are made. It is then either committed and becomes the latest revision, or is aborted. The transaction is actually a long-lived filesystem object; a client does not need to commit or abort a transaction itself, rather it can also begin a transaction, exit, and then can re-open the transaction and continue using it. Multiple clients can access the same transaction and work together on an atomic change.

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