Apache Subversion
Apache Subversion (often abbreviated SVN, after its command name "svn") is a software versioning and revision control system distributed as open source under the Apache License. Software 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, SourceForge, and from 2006 to 2019, GCC. CodePlex was previously a common host for Subversion  repositories. 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, and in February 2004, version 1.0 was released. In November 2009, Subversion was accepted into Apache Incubator: this marked the beginning of the process to  become a standard top-level Apache project. It became a top-level Apache project on February 17, 2010. Release dates are extracted from Apache Subversion's file, which records all release history. Features. Repository types. Subversion offers two types of repository storage. Berkeley DB (deprecated). The 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 remains offline while Berkeley DB replays the journal and cleans up any outstanding locks. The safest way  to use Subversion with a Berkeley DB repository involves a single server-process running as one user (instead of through a shared filesystem). The Berkeley DB backend was deprecated in version 1.8. FSFS. In 2004, a new storage subsystem was developed and named FSFS. It works faster than the Berkeley DB backend on directories with a large number of files and takes less disk space, due to less logging. Beginning with Subversion 1.2, FSFS became the default data store for new repositories. The etymology of "FSFS" is based on Subversion's use of the term "filesystem" for its repository storage  system. FSFS stores its contents directly within the operating system's filesystem, rather than a structured system like Berkeley DB. Thus, it is a "[Subversion] FileSystem atop the FileSystem". FSX. A new filesystem, called FSX, is under development to remove some limitations of FSFS. As of Version 1.9, it was not considered production-ready. Repository access. Access to Subversion repositories can take place by: 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. 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. Filesystem. One can view the Subversion filesystem as "two-dimensional". Two coordinates are used to unambiguously address filesystem items: 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. Potentially, 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. Most properties occur on filesystem entries (i.e., files and directories). These are versioned just like other changes to the filesystem. The Subversion client reserves the 'svn:' prefix for built-in properties, but other names can be used to define custom properties. 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 they can be changed later if allowed by a pre-revprop-change hook. Branching and tagging. Subversion uses the inter-file branching model from Perforce to implement branches and tagging. A branch is a separate line of development. Tagging refers to labeling the repository at a certain point in time so that it can be easily found in the future. In Subversion, the only difference between branches and tags is how they are used. A new branch or tag is set up by using the "" command, which should be  used in place of the native operating system mechanism. The copied directory is linked to the original in the repository to preserve its history, and the copy takes very little extra space in the repository. 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. Limitations and problems. A known problem in Subversion affects the implementation of the file and directory rename operation. , 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 when a move conflicts with edits made elsewhere, both for regular commits and when merging branches. The Subversion 1.5 release addressed some of these scenarios while others remained problematic. The Subversion 1.8 release addressed some of these problems by making moves a first-class operation on the client, but it is still treated as copy+delete in  the repository. , 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. 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. In versions prior to 1.7 these codice_2 directories on the client side could become corrupted by ill-advised user activity like global search/replace operations. Starting with version 1.7 Subversion uses a  single centralized folder per working area. 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. To mitigate this, third-party tools exist that allow for preserving modification time and other filesystem meta-data. 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 uses a centralized 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. Some users of version control systems see the centralised model as detrimental; famously, Linus Torvalds attacked Subversion's model and its developers. Subversion often does not deal well with the filename normalization performed by the HFS+ filesystem. This can cause problems when files with accented characters in their names  are added to the repository on a non-HFS+ filesystem and the repository is then used with HFS+. Subversion tags and 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 ("") in the space of the repository tree. Only a few predefined "references" are  available: , , and . This history-to-space projection has multiple issues: To address such issues, posters on the Subversion mailing lists have suggested a new feature called "labels" or "aliases". SVN labels would more closely resemble the "tags" of other systems such as CVS or Git. The fact that Subversion has global revision numbers opens the way to a very simple label → revision implementation. Yet as of 2013, no progress has been made and symbolic tags are not in the list of the most wanted features. Development and implementation. 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. Since March 2010, the project is formally known as Apache Subversion, being a part of the Apache Top-Level Projects. 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. The Subversion open-source community does not provide binaries, but potential users can download binaries from volunteers. 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 announced 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. 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. In 2002, a design contest was held to select the logo for Subversion. The original entries can be found here as well as the votes for each logo. The current logo received the most votes in the contest.