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Fri, 04 Mar 2005

This is an archived blog post. I've switched to using WordPress as my blogging software and have not migrated all my old posts. I believe strongly in not letting an old link die, so this page continues to work. Please do visit mikemason.ca/blog to read newer posts.

Subversion FAQ

Pragmatic Version Control using Subversion launched Tuesday night in Calgary. I went down to a local bookstore and spent a few minutes talking about version control, Subversion, and what the book covered. The audience had a bunch of questions about Subversion and I took this as a really good sign — people are doing their own research and wanted to find out more.

Here’s a few of the questions people are asking about Subversion:

How does Subversion compare to other tools? Is there a feature matrix I can look at to decide what tool to use?

Subversion stacks up really well against CVS, fixing the bugs and fragility of CVS whilst keeping the proven development model. Subversion also adds features like change sets, atomic commit, decent networking performance, and a reliable back end. I’m wary of comparisons that read like a school book report, checking boxes if a tool has a particular feature. Those kinds of comparisons always tend to be biased by the person writing them — if you really want to know whether Subversion is right for you try it out on a small project. If things don’t work out you can try something else, if things do work out you’ll know more about the tool and will be better able to roll it out to larger projects.

I’ve heard that Subversion’s database can become corrupted. That doesn’t sound good!

Subversion 1.0 uses the Berkeley DB for storing your files, and this has been a source of some problems. BDB is very reliable when used properly, but unfortunately it’s quite finicky about permissions on its database files. If you set up a Subversion repository, usually on Unix, and have two different users access the repository, if their umask isn’t quite right they can grab control of those database files. This usually happens when you have more than one access mechanism, say svn+ssh as well as Apache. If BDB can’t write to its files it gets stuck, or “wedged.” People often confuse this with database corruption, which has only happened in a few cases and was traced to hardware problems.

Subversion 1.1 introduced the new “fsfs” back end which doesn’t use Berkeley DB and instead uses plain files on disk. This works much better for people using NFS, for example, and helps avoid some of the permissions problems. Most people can stick with BDB as long as they don’t try to mix network servers for Subversion.

I’ve heard Subversion supports “meta-data.” What’s that?

Using Subversion, you can attach named data to files and directories. Each name defines a property and properties can have textual or binary content. The nifty thing is that properties are version controlled in exactly the same way as files — Subversion tracks how their contents change over time, and can perform merges, deletes, and updates just like file contents. Subversion uses special properties to do stuff like ignoring certain files in a directory or setting the “execute bit” for a file.

Since properties are editable just like file contents, you could write a tool that used them in some special way. An example often given is a system that stores big graphic files — you could store a thumbnail inside a Subversion property for each file, then use that in your system.

Why would someone spend thousands on a commercial tool when they can get Subversion for free?

This is a good question, and one that I think a lot of people are beginning to ask. In the case of a version control tool, it may be that a company is happier using a product for which they can pay for support – if something goes wrong they can call someone and get it fixed. But open-source software is challenging the notion that you must pay for support. Subversion has an extremely active user community and you can often get a response in minutes, for free.

I like Perforce, and I actually think it’s better than Subversion in certain circumstances (usually when your branching has got out of control and you’re in a bit of a mess). But is Perforce several hundred dollars per head better than Subversion? I think probably not.

So what’s in Subversion that’s not in your new book?

This was actually the toughest question I faced during the book launch and I had to think for a long time before answering. I think the book covers 95% of Subversion’s features, and easily covers everything you’ll need when using Subversion on a typical project. I couldn’t cover all the advanced usages of Subversion, but I think having read the book you’ll be able to adapt what’s in there to cover any new situation you face.

The book sticks to the Subversion command line, and only covers GUI tools briefly, so you’ll need to experiment a little to figure out how Tortoise works, for example. I think it’s useful to understand what a GUI is doing “under the hood,” so I don’t see this as a serious omission. The book also doesn’t cover IDE integration because those tools are still evolving rapidly.

It looks like we’ll be doing another print run of the book, so be sure to get a copy of the first printing before we correct the typos!

Posted 01:30, 04 Mar 2005.

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