“New” subversion structure using svn:externals

October 29, 2008 by
Filed under: Configuration Management, Development, Projects 

Me and the boss deviced a new structure for the project during the last few weeks, and it’s been slowly refining in our heads until yesterday when we finally implemented it. I think we made a rather refined and complex structure, but once we got it into place physically and once we get the general idea into the developers heads (including me), I think it will prove very powerful.

That being said, I don’t think this is a new structure, I just think people are very quiet about how they use subversion, and it’s a problem. Newcomers do the same old errors over and over again. So, let’s get on to try and explain it all.

Most projects uses a single BTT root, where BTT stands for Branches, Tags and Trunk. Ie, they start a project, and then straight in the root put the BTT, and then inside that, they create the project structure. For example:

  • project-root/
    • Branches
    • Tags
    • Trunk
      • admin
      • src
      • out
      • test

This is a good basic structure for very small projects, containing perhaps 10’ish files, or where the actual implementation is perfectly homogenous and has no need for separated versioning. Every time we want to make a release, we cheap copy the content to Tags as a new tag (called perhaps /Tags/Milestone1-RC1). We now have a release that we can provide to people.

The problem comes if it isn’t so homogenous. For example, let’s say you are developing a calculator. It has two objects, a numpad and a display. What if you want to make a new version just of the display? You need to make a completely new version, including for the numpad.
Or how about wanting to branch just a small part of the project? Ie, I want to use a branch for the numpad, and then use the trunk for the display. You’d then have to make a cheap copy for the entire tree. Admittedly, it isn’t costing too much.

Our “new” structure deals with this on a different level. Basically, the idea is to have multiple BTT roots, and then use svn:externals to connect the correct tags to create
1) a complete releasable project and
2) a complete workarea project.

For the calculator example, you get the following structure:

  • calculator/
    • Calculator_Modules/
      • Display/
        • Branches/
        • Tags/
        • Trunk/
      • Numpad/
        • Branches/
        • Tags/
        • Trunk/
    • Calculator/
      • Branches/
      • Tags/
      • Trunk/

As you can see, it looks much more complex, and it is, but the possibilities are infinitely much better.

The Calculator/Trunk/ directory contains a svn:externals property linking in the Calculator_Modules/Display/Trunk as Display and Calculator_Modules/Numpad/Trunk as Numpad. This works by linking external resources into the current directory structure, so basically I would get the trunks into my Calculator trunk, but properly renamed, without them actually being there in the repository. This also works on “real externals” by the way, such as linking in a specific version of a library from some repository on the Internet.

To create a Calculator/Tags/MS1 we could either just set a -rXX to the correct subversion revision, or we would create svn:externals to the correct Display and Numpad Tags, not their trunk. This way, we can say that “Calculator 1.0 contains Display 2.0 and Numpad 2.1”, not “Calculator 1.0 contains Display revision 439 and Numpad revision 587”, or even worse “Calculator 1.0 is revision 587” which completely lacks granularity.

I’m not completely sure it’s perfect, and others have probably already tested it, but I think it will be pretty sweet :-).

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