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Apache Subversion

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Subversion
Developer(s)CollabNet, Inc.
Stable release
1.4.5 / 27 August 2007
Repository
Operating systemCross-platform
TypeRevision control
LicenseApache License
Websitehttp://subversion.tigris.org/

In computing, Subversion (SVN) is a version control system (VCS) initiated in 2000 by CollabNet Inc. It allows users to keep track of changes made to any type of electronic data, typically source code, web pages or design documents.

Subversion is currently a popular alternative to CVS (another widely used VCS), particularly among open source projects.[1] Projects using Subversion include the Apache Software Foundation, KDE, GNOME, Free Pascal, GCC, Python, Ruby, Sakai, Samba, and Mono. SourceForge.net and Tigris.org also provide Subversion hosting for their open source projects, Google Code and BountySource systems use it exclusively. Subversion is also finding adoption in the corporate world. In a 2007 report by Forrester Research, Subversion was recognized as the sole leader in the Standalone Software Configuration Management (SCM) category and strong performer in the Software Configuration and Change Management (SCCM) category.[2]

Subversion is released under the Apache License, making it free software.

Features

  • Commits are true atomic operations. Interrupted commit operations do not cause repository inconsistency or corruption.
  • Renamed/copied/moved/removed files retain full revision history.
  • Directories, renames, and file metadata are versioned. Entire directory trees can be moved around and/or copied very quickly, and retain 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/DeltaV for protocol. There is also an independent server process that uses a custom protocol over TCP/IP.
  • Branching and tagging are cheap operations, independent of file size.
  • Natively client/server, layered library design.
  • Client/server protocol sends diffs in both directions.
  • Costs are proportional to change size, not data size.
  • Parsable output, including XML log output.
  • Open Source licensed — "CollabNet/Tigris.org Apache-style license"
  • Internationalised program messages.
  • File locking for unmergeable files ("reserved checkouts").
  • Path-based authorization for svnserve.
  • PHP, Python, Ruby, Perl, and Java language bindings.
  • Full MIME support - the MIME Type of each file can be viewed or changed, with the software knowing which MIME types can have their differences from previous versions shown.

Repository access

As of version 1.4, Subversion repositories can be accessed by the following means:

  • Local filesystem or network filesystem,[3] accessed by client directly.
  • WebDAV/DeltaV (over http or https) using the mod_dav_svn module for Apache 2.
  • Custom "svn" protocol (default port 3690), using plaintext or over SSH.

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

Layers

Subversion is composed internally of 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 are run when an action is performed. Together, Fs and Repos constitute the "filesystem interface".
mod_dav_svn
Provides WebDAV/DeltaV access through Apache 2.
Ra
Handles "repository access", both local and remote. From this point on, repositories are referred to using URLs, i.e. file:///path/ for local access, http://host/path/ (or https) for WebDAV access, or svn://host/path/ (or svn+ssh) for the SVN protocol.
Client, Wc
The highest level. It abstracts repository access and provides common client tasks, e.g authenticating the user, or comparing versions. The Wc library is used by Client to manage the local working copy.

Filesystem

The Subversion filesystem is described as a "three dimensional" filesystem. Since most representations of a directory tree (e.g tree view) are two dimensional, the added dimension is that of 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 storage space used is proportional to the number of changes made, not to the number of revisions.

The Subversion filesystem uses transactions to keep changes atomic. A transaction is begun from 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.

Properties

One important feature of the Subversion filesystem is properties, simple name=value pairs of text. Properties are used 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 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 keywords must also be referenced in the file as $keyword$. This is used to maintain certain information (e.g date of last change, revision number) in a file without human intervention.
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 EOL's match the users OS EOL style. Repositories may require this property on all files to prevent inconsistent line endings, which can be 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 to be used with the locking mechanism. The read-only permission is a reminder 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 isn't meant to be set or modified directly by users. Currently 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.

The second place in Subversion where properties are used is 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 when the revision was made.
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 interfile branching model from Perforce[4] to handle branches and tags. A new branch or tag is created with the 'svn copy' command, which should be used in place of the native operating system mechanism. Subversion does not create an entire new file in the repository with its copy. Instead, the old and new files are linked together internally and the history is preserved for both. The copied files take up only a little extra room in the repository because Subversion saves only the differences from the original files.

The minimal layout recommended for a project repository looks similar to this with branches and tags added:

  • project/
    • trunk/
    • branches/
    • tags/

On larger projects, the above structure is repeated throughout the repository at different levels in the project tree. The paths listed under tags are snapshots of the trunk on the same repository level, and they are commonly named after the release dates or version related to snapshot. The paths under tags are generally for read-only access once created.

  • tags/
    • 20050516/
    • 20060320/
    • 20070801/

The paths listed under branches are commonly named after subsequent minor versions (or subversions) of the current repository level, given names of certain features, or have cross reference information. Branches can form more repository levels and also have a trunk node. For example, the layout as shown above is repeated as if the branches form new sub-projects of the project.

  • branches/
    • 1.2/
      • trunk/
      • tags/
        • 1.2.3/
      • branches/
    • 2.0/
      • trunk/
      • tags/
      • branches/
        • 2.0.1/
          • trunk/
          • tags/
          • branches/

The tags directory contains snapshots of the trunk at particular milestones, usually major releases or perhaps prior to a major departure in development.

The trunk directory holds the head version for each branch level.

The branches directory holds versions (or subversions) of the project that a developer or group of developers may be working on. Each trunk path is in alignment to merge changes with the trunk in the level above each other. The changes can be merged into the trunk above or into other branches. This not only allows changes to be checked into the repository and shared separately; it also allows implementations models to be distributed across the repository and provides access to previous versions for maintenance.

It is not intended for someone to check-out the entire project tree; instead, they need only to check-out one of the trunk paths or one of the tagged snapshots.

The tags were made by svn mechanisms, like the usual command line to copy a trunk to a new path:

svn copy trunk tags/my-new-snapshot

Likewise, branch creation only requires a few extra steps to set up a new layout:

svn mkdir branches/my-new-branch
svn mkdir branches/my-new-branch/tags
svn mkdir branches/my-new-branch/branches
svn copy trunk branches/my-new-branch/trunk

All the files in each branch maintain the history of the file up to the point of the copy, plus any changes made since. Changes can be 'merged' back into the trunk or between branches. To Subversion, the only difference between tags and branches is that changes should not be checked into the tagged versions. Due to the differencing algorithm, creating a tag or a branch takes very little additional space in the repository.

Visualization of very simple Subversion project.

Current problems

A known problem in Subversion is the implementation of the file and directory rename operation. Subversion currently 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 are changed, 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 be confused when files are modified and moved - the normal workaround is to make a separate commit just for the move. This can also cause problems when a move conflicts with edits made elsewhere. This is documented in the Subversion issue database.[5]

Subversion currently lacks some repository administration and management features. For instance, it is sometimes desired to make permanent edits to the repository to permanently remove all historical records of certain data being in the repository. Subversion does not have built-in support to allow this to be done simply. The standard user-level interface allows files and directories to be moved or deleted, but earlier versions of the repository can always be retrieved. The current solution to this sort of problem involves 'dumping' the repository, editing the resulting (possibly large) text file, and then recreating the repository. For simple renaming or removal of files this is fairly straight-forward, but other alterations can be more complex and hence error-prone.[6]

Subversion requires the use of extra .svn directories which can become mangled. The SVK project aims to improve this situation.

Releases

Main article: Software which uses Subversion

CollabNet is still involved with Subversion but the project is run as an independent open source community. The home of Subversion is on Tigris.org, an open source community dedicated to software engineering tools.

The Subversion open source community does not provide binaries but these can be downloaded from volunteers and from CollabNet, the initiator of the Subversion open source project. While the Subversion project does not include an official graphical user interface (GUI) for use with Subversion, a number of different GUIs have been developed, along with a wide variety of additional ancillary software.

See also

References

  1. ^ http://cia.navi.cx/stats/vcs
  2. ^ "The Forrester Wave: Software Change and Configuration Management, Q2 2007". Forrester Research.
  3. ^ Berkeley DB relies on file locking and thus should not be used on (network) filesystems which do not implement them
  4. ^ http://www.perforce.com/perforce/branch.html
  5. ^ http://subversion.tigris.org/issues/show_bug.cgi?id=898
  6. ^ http://subversion.tigris.org/issues/show_bug.cgi?id=516
  • 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 at http://svnbook.red-bean.com/)
  • 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)

External links

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