Apache Subversion

Subversion

Developer(s)	Apache Software Foundation
Initial release	October 20, 2000 (2000-10-20)
Stable release	1.7.1  (October 23, 2011; 30 days ago (2011-10-23)) [+/−]
Development status	Active
Written in	C
Operating system	Cross-platform
Type	Revision control
License	Apache License
Website	subversion.apache.org

Apache Subversion (often abbreviated SVN, after the command name svn) is a software versioning and a revision control system distributed under a free license. 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, 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 was created by CollabNet Inc. in 2000 and is now a top-level Apache project being built and used by a global community of contributors.

History

CollabNet founded the Subversion project in 2000 as an effort to write an open-source version-control system which operated much like CVS but which fixed the bugs and supplied some features missing in CVS. By 2001, Subversion had advanced sufficiently to host its own source code.^[2] In November 2009, Subversion was accepted into Apache Incubator: this marked the beginning of the process to become a standard top-level Apache project.^[3] It became a top-level Apache project on February 17, 2010.^[4]

Features

• Commits as true atomic operations (interrupted commit operations in CVS would 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 as a cheap operation, independent of file size (though Subversion itself does not distinguish between 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 C#, PHP, Python, Perl, Ruby, 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.

Repository types

Subversion offers two types of repository storage.

Berkeley DB

Original development of Subversion used the Berkeley DB package. Subversion has some limitations with Berkeley DB usage when a program that accesses the database crashes or terminates forcibly. No data loss or corruption occurs, but the repository is offline while Berkeley DB replays the journal and cleans up any outstanding locks. When using Berkeley DB repository, the safest way to use it is by a single server process running as one user, instead of through a shared filesystem.^[5]

FSFS

In 2004, the Fast Secure File System (FSFS) was first developed. It uses the OpenSSL and Filesystem in Userspace (FUSE) packages.^[6] FSFS works faster on directories with a large number of files and takes less disk space, due to less logging.^[5] Beginning with Subversion 1.2, FSFS became the default data store for new repositories.

Repository access

Main article: Comparison of Subversion clients

Access to Subversion repositories can take place by:

1. Local filesystem or network filesystem,^[7] accessed by client directly. This mode uses the file:///path access scheme.
2. WebDAV/Delta-V (over http or https) using the mod_dav_svn module for Apache 2. This mode uses the http://host/path access scheme or https://host/path for secure connections using ssl.
3. Custom "svn" protocol (default port 3690), using plain text or over TCP/IP. This mode uses either the svn://host/path access scheme for unencrypted transport or svn+ssh://host/path scheme for tunneling over ssh.

All three means can access both FSFS and Berkeley DB repositories.

Any 1.x version of a client can work with any 1.x server. Newer clients and servers have additional features and performance capabilities, but have fallback support for older clients/servers.^[8]

Layers

Internally, a Subversion system comprises several libraries arranged as layers. Each performs a specific task and allows developers to create their own tools at the desired level of complexity and specificity.

Fs 

The lowest level; it implements the versioned filesystem which stores the user data.

Repos 

Concerned with the repository built up around the filesystem. It has many helper functions and handles the various "hooks" that a repository may have, e.g. scripts that run when an action is performed. Together, Fs and Repos constitute the "filesystem interface".

mod_dav_svn 

Provides WebDAV/Delta-V access through Apache 2.

Ra 

Handles "repository access", both local and remote. From this point on, repositories are referred to using URLs, e.g.

• file:///path/ for local access,
• http://host/path/ or https://host/path/ for WebDAV access, or
• svn://host/path/ or svn+ssh://host/path/ for the SVN protocol.

Client, Wc 

The highest level. It abstracts repository access and provides common client tasks, such as authenticating users or comparing versions. Subversion clients use the Wc library to manage the local working copy.

Filesystem

One can view the Subversion filesystem as "two-dimensional".^[9] Two coordinates are used to unambiguously address filesystem items:

• Path (regular path of Unix-like OS filesystem)
• Revision

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, though no existing clients expose this capability.

Properties

One important feature of the Subversion filesystem is properties: simple name=value pairs of text. Properties occur in two different places in the Subversion filesystem. The first is on filesystem entries (i.e., files and directories). These are versioned just like other changes to the filesystem. Users can add any property they wish, and the Subversion client uses a set of properties, which it prefixes with 'svn:'.

svn:executable 

Makes files on Unix-hosted working copies executable.

svn:mime-type 

Stores the Internet media type ("MIME type") of a file. Affects the handling of diffs and merging.

svn:ignore 

A list of filename patterns to ignore in a directory. Similar to CVS's .cvsignore file.

svn:keywords 

A list of keywords to substitute into a file when changes are made. The file itself must also reference the keywords as $keyword$ or $keyword:...$. This is used to maintain certain information (e.g., author, date of last change, revision number) in a file without human intervention.
The keyword substitution mechanism originates from rcs^[10] and from cvs.

svn:eol-style 

Makes the client convert end-of-line characters in text files. Used when the working copy is needed with a specific EOL style. "native" is commonly used, so that EOLs match the user's OS EOL style. Repositories may require this property on all files to prevent inconsistent line endings, which can cause a problem in itself.

svn:externals 

Allows parts of other repositories to be automatically checked-out into a sub-directory.

svn:needs-lock 

Specifies that a file is to be checked out with file permissions set to read-only. This is designed for use with the locking mechanism. The read-only permission reminds one to obtain a lock before modifying the file: obtaining a lock makes the file writable, and releasing the lock makes it read-only again. Locks are only enforced during a commit operation. Locks can be used without setting this property. However, that is not recommended, because it introduces the risk of someone modifying a locked file; they will only discover it has been locked when their commit fails.

svn:special 

This property is not meant to be set or modified directly by users. As of 2010^[update] only used for having symbolic links in the repository. When a symbolic link is added to the repository, a file containing the link target is created with this property set. When a Unix-like system checks out this file, the client converts it to a symbolic link.

svn:mergeinfo 

Used to track merge data (revision numbers) in Subversion 1.5 (or later). This property is automatically maintained by the merge command, and it is not recommended to change its value manually.^[11]

Subversion also uses properties on revisions themselves. Like the above properties on filesystem entries the names are completely arbitrary, with the Subversion client using certain properties prefixed with 'svn:'. However, these properties are not versioned and can be changed later.

svn:date 

the date and time stamp of a revision

svn:author 

the name of the user that submitted the change(s)

svn:log 

the user-supplied description of the change(s);

Branching and tagging

Subversion uses the inter-file branching model from Perforce^[12] to handle branches and does not support tagging. A branch is a separate line of development.^[13] Tagging refers to labeling the repository at a certain point in time so that it can be easily found in the future.

The system sets up a new branch by using the 'svn copy' command, which should be used in place of the native operating system mechanism. Subversion does not create an entire new file version in the repository with its copy. Instead, the old and new versions are linked together internally and the history is preserved for both. The copied versions take up only a little extra room in the repository because Subversion saves only the differences from the original versions.

All the versions in each branch maintain the history of the file up to the point of the copy, plus any changes made since. One can "merge" changes back into the trunk or between branches. Due to the differencing algorithm, creating a copy takes very little additional space in the repository.

Visualization of a simple Subversion project

Limitations and problems

A known problem in Subversion affects the implementation of the file and directory rename operation. As of 2010^[update], Subversion implements the renaming of files and directories as a "copy" to the new name followed by a "delete" of the old name. Only the names change, all data relating to the edit history remains the same, and Subversion will still use the old name in older revisions of the "tree". However, Subversion may become confused^{[citation needed]} when files are modified and moved in the same commit. This can also cause problems when a move conflicts with edits made elsewhere,^[14] for example during merging branches.^[15] The Subversion 1.5 release addressed some of these scenarios while others remain problematic.^[16]

As of 2010^[update], Subversion lacks some repository-administration and management features. For instance, someone may wish to edit the repository to permanently remove all historical records of certain data. Subversion does not have built-in support to achieve this simply.^[17]

Subversion stores additional copies of data on the local machine, which can become an issue with very large projects or files, or if developers work on multiple branches simultaneously. These .svn directories on the client side can become corrupted by ill-advised user activity.^[18]

Subversion does not store the modification times of files. As such, a file checked out of a subversion repository will have the 'current' date (instead of the modification time in the repository), and a file checked into the repository will have the date of the check-in (instead of the modification time of the file being checked in). This might not always be what is wanted.^[19] To mitigate this third party solutions exist that allow for preserving modification time and other filesystem meta-data.^{[20] [21]} However, giving checked out files a current date is important as well — this is how tools like make(1) will take notice of a changed file for rebuilding it.

Subversion does not use a distributed revision control model. Ben Collins-Sussman, one of the designers of Subversion, believes a centralised model would help prevent "insecure programmers" from hiding their work from other team members.^[22] Some users of version control systems see the centralised model as detrimental; famously, Linus Torvalds attacked^[23] Subversion's model and its developers.

While Subversion stores filenames as Unicode, it does not specify if precomposition or decomposition is used for certain accented characters (such as é). Thus, files added in SVN clients running on some operating systems (such as OS X) use decomposition encoding, while clients running on other operating systems (such as Linux) use precomposition encoding, with the consequence that those accented characters do not display correctly if the local SVN client is not using the same encoding as the client used to add the files.^[24][25]

By design, the svn log command is always recursive: trying to access the history of a directory systematically pulls out the history of its entire hierarchy. A workaround is not to use the command line but use a sophisticated SVN client with filtering capabilities.

Subversion tags

This subsection focuses on tags but parts of it also apply to branches.

Revision numbers are difficult to remember in any version-control system. For this reason, most systems offer symbolic tags as user-friendly references to them. Subversion does not have such a feature and what its documentation recommends to use instead is very different in nature. Instead of implementing tags as references to points in history, Subversion recommends making snapshot copies into a well-known subdirectory ("tags/") in the space of the repository tree. Only a few predefined references are available: HEAD, BASE, PREV and COMMITTED.

This history-to-space projection has multiple issues:

1. When a snapshot is taken, the system does not assign any special meaning to the name of the tag/snapshot. This is the difference between a copy and a reference. The revision is recorded and the snapshot can be accessed by URL. This makes some operations less convenient and others impossible. For instance, a naive svn diff -r tag1:tag2 myfile does not work; it is slightly more complicated than that to achieve, requiring the user to know and input URL/paths to the snapshots instead of just the names: svn diff <URL-TO-TAG1>/myfile <URL-TO-TAG2>/myfile. Other operations like for instance svn log -r tag1:tag2 myfile are just impossible.

2. When two (ideally independent) object types live in the repository tree, a "fight to the top" can ensue. In other words, it is often difficult to decide at which level to create the "tags/" subdirectory:

trunk/componentfoo/ /componentbar/ tags/1.1/componentfoo/ /componentbar/

or

componentfoo/trunk/ /tags/1.1/ componentbar/trunk/ /tags/1.1/

3. Tags, by their conventional definition are both read-only and light-weight, on the repository and client. Subversion copies are not read-only, and while they are light-weight on the repository, they are incredibly heavy-weight on the client.

To address such issues, posters on the Subversion mailing lists have suggested a new feature called "labels" or "aliases".^[26] SVN labels would more closely resemble the "tags" of other systems such as CVS or git. As of 2010^[update], this suggestion has not progressed and is not in the list of the most wanted features.^[27]

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.^[28] Since March 2010, the project is formally known as Apache Subversion, being a part of the Apache Top-Level Projects.^[29]

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.^[30]

The Subversion open-source community does not provide binaries, but potential users can download binaries from volunteers.^[31] 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.

Work anounced in 2009 included SubversionJ (a Java API) and implementation of the Obliterate command, similar to that provided by Perforce. Both of these enhancements were sponsored by WANdisco.^[32]

The Subversion committers normally have at least one or two new features under active development at any one time. The 1.7 release of Subversion in October 2011 included a streamlined HTTP transport to improve performance and a rewritten working-copy library.^[33]

Source code hosting

See also: Comparison of open source software hosting facilities

The following websites provide free source code hosting for SVN repositories:

• Alioth
• Assembla
• BerliOS
• Betavine
• Freepository
• Google Code
• SourceForge

See also

Free software portal

• List of revision control software
• Comparison of revision control software
• Comparison of Subversion clients
• TortoiseSVN
• DotSVN
• UberSVN

Notes

1. ^ "The Forrester Wave: Software Change and Configuration Management, Q2 2007". Forrester Research. http://www.collab.net/forrester_wave_report/index.html. 
2. ^ "Subversion's History", section of Version Control with Subversion, version 1.4
3. ^ Subversion joins forces with Apache by David Rubinstein in SD Times November 4, 2009.
4. ^ [1] via subversion community
5. ^ ^{a b} Ben Collins-Sussman, Brian W. Fitzpatrick, C. Michael Pilato (2011). "Chapter 5: Strategies for Repository Deployment". Version Control with Subversion: For Subversion 1.7. O'Reilly. http://svnbook.red-bean.com/en/1.7/svn.reposadmin.planning.html#svn.reposadmin.basics.backends. 
6. ^ "FSFS - the Fast Secure File System". http://fsfs.sourceforge.net/. Retrieved October 29, 2011. 
7. ^ Berkeley DB relies on file locking and thus should not be used on (network) filesystems which do not implement them
8. ^ SVN 1.5 release notes
9. ^ Basic Merging
10. ^ http://www.openbsd.org/cgi-bin/man.cgi?query=rcs&sektion=1#KEYWORD+SUBSTITUTION Keyword substitution keywords in cvs(1)
11. ^ Subversion Properties
12. ^ Inter-File Branching: A Practical Method for Representing Variants
13. ^ Branching / Tagging — TortoiseSVN
14. ^ Implement true renames
15. ^ Advanced Merging
16. ^ Copy/move-related improvements in Subversion 1.5
17. ^ svn obliterate
18. ^ Fierens, Sergio. "Downsides of Subversion 1.4 for configuration management in large-scale software development"
19. ^ Issue 1256 at Tigris.org
20. ^ FreezeAttrib (saves/restores file attributes using properties)
21. ^ FSVS (Fast System VerSioning)
22. ^ Programmer Insecurity @ iBanjo
23. ^ Google Tech Talk video and its [2]
24. ^ subversion: Issue 2464
25. ^ SmartSVN - Subversion/SVN Client: Problems with umlauts in file names on Mac OS X
26. ^ Subversion mailing lists
27. ^ Subversion Roadmap
28. ^ http://www.open.collab.net/news/press/2009/svn-asf.html Collabnet Press Release
29. ^ http://www.open.collab.net/news/press/2010/apache.html Collabnet Press Release regarding Apache subversion
30. ^ WANdisco (January 7, 2010). "WANdisco Names Hyrum Wright to Lead Subversion Open Source Efforts". News release (Open Source magazine). http://opensource.sys-con.com/node/1239202. Retrieved October 29, 2011. 
31. ^ "Apache Subversion Binary Packages". Official project website. http://subversion.apache.org/packages.html. Retrieved October 29, 2011. 
32. ^ WANdisco (October 28, 2009). WANdisco "WANdisco Presents New Initiatives for the Subversion Open Source Project". News release (CM Crossroads). http://www.cmcrossroads.com/index.php?Itemid=100152&catid=101:news-and-announcements&id=13065:wandisco-presents-new-initiatives-for-the-subversion-open-source-project-&option=com_content&view=article WANdisco. Retrieved October 29, 2011. 
33. ^ "Apache Subversion Roadmap". Official project website. http://subversion.apache.org/roadmap.html. Retrieved October 29, 2011. 

References

• C. Michael Pilato, Ben Collins-Sussman, Brian W. Fitzpatrick; Version Control with Subversion; O'Reilly; ISBN 0-596-00448-6 (1st edition, paperback, 2004, full book online, mirror)
• Garrett Rooney; Practical Subversion; Apress; ISBN 1-59059-290-5 (1st edition, paperback, 2005)
• Mike Mason; Pragmatic Version Control Using Subversion; Pragmatic Bookshelf; ISBN 0-9745140-6-3 (1st edition, paperback, 2005)
• William Nagel; Subversion Version Control: Using the Subversion Version Control System in Development Projects; Prentice Hall; ISBN 0-13-185518-2 (1st edition, paperback, 2005)

Further reading

• Dispelling Subversion FUD by Ben Collins-Sussman (Subversion developer), as of 2004-12-21

External links

• Official website
  • Previous official site Not all content has yet been migrated to the new official site.
• Version Control with Subversion, an O'Reilly book available for free online
• Subversion at the Open Directory Project