Colin's Journal

Subversion should, if all goes to plan, have its version 1.0 release on Monday. This news should be of great interest to anyone who uses CVS, either professionally or personally. Subversion started development back in 2000 (I’ve been following its progress on and off since October 2001) and so reaching the 1.0 milestone is a very significant achievement.

Subversion has been developed to replace CVS as the free version control system of choice. Although the working philosophy behind Subversion is very similar to CVS (such as using copy-modify-merge instead of locking) it has a significantly improved feature set, and a very different architecture to support those features. The top three most important features that Subversion has, and that CVS lacks, are:

• Version control of directories as well as files. This means that you can now copy, move, and delete directories in exactly the same way as you can files.
• Atomic commits. When you commit changes to the repository it happens as a single transaction that either fully works or fully fails.
• Cheap copies to implement branches and tags. This means that you can easily browse through the branches and tags of a repository, instead of having to remember what tags and branches you have created.

Subversion can either work on a local repository (and it must be local – network shares will not work), or over a network. Currently there are two different server implementations, one that sits inside Apache and uses the WebDAV/DeltaV protocol, and a standalone server that uses a custom protocol. The standalone server is probably more interesting because it doesn’t depend on Apache, and can act as a traditional daemon (either running in the background or through inetd), or be used through SSH in a similar fashion to CVS.

Subversion includes many utilities, including a not-yet 1.0 quality CVS to Subversion converter. By running this on my small (20MB) home repository I successfully created a new Subversion repository in about 15 minutes. The resulting repository is very similar in size to the CVS one, despite the completely different storage mechanisms (Berkeley DB versus RCS files).

It will take some considerable time for Subversion to take over from CVS as the major free version control tool. For new projects it is easy to choose Subversion over CVS, but for existing projects it will take a 1.0 quality converter before many people will be willing to switch. For large repositories there are questions regarding the scalability of Subversion, which will only be answered through performance testing. As these questions are answered, and the converter is improved in quality, there will be little to prevent most people from migrating from CVS to Subversion.

To learn more about Subversion take a look at Subversion: The Definitive Guide, an excellent free book on the subject.

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