Subversion - where should the svn:externals come from?

Question

I am about to establish a rule at work here that all svn:externals references should come from the one of the other project's tag, never from its trunk or from any of its branches. Is this a reasonable rule or do you see problems / issues with this approach? I am trying to achieve a stable development environment and I wonder if this rule would make development slower or more difficult.

Answer 1

Your concern is that a project with "svn:externals" can change without any commits to that project. That's a problem because it's hard to discover the breaking change or to roll back to a good version.

So yes, demanding that svn:external references are stable is a good rule. Only allowing references to tags is one way to achieve that. Another way is to use the -r syntax to pin the external to a fixed revision. Example from the subversion book:

third-party/skins -r148 http://svn.example.com/skinproj

This will also protect you against changes in tags, a bad practice which is more common than I like.

That being said, there still is a trade-off between stability and continuous integration. Often you need the changes and bug-fixes in the external dependencies anyway. In that case you want to be notified by your CI server as quickly as possible that some change in the dependencies broke your project, so that the problem can be fixed ASAP. Most continuous integration servers have support for checking externals.

For this reason I think it's OK to keep the externals in trunk tracking the trunk HEAD of your dependencies (if you have a CI server). Just pin your externals to fixed revisions when tagging and when creating stable maintenance branches.