I'm looking into svn externals for my company, and it seems like it would be a good feature for us to use. We have several products that often reference shared components, but have a bad habit of falling behind into older versions and even differently branched codebases sometimes.
I've read a decent bit about how they work now, and I think I understand the concept OK. What I'm not 100% sure on is how different revisions of multiple repositories link together.
Let's say I have a Product and a Library. The Product is built against the Library, so its repo has a svn:externals property that links to the Library source. In the absence of a specific version in the svn:externals definition, when I check out HEAD of Product I also get HEAD of Library.
I build several versions of Product over the years, each time referencing the latest version of Library. One day though I have to go back and check out Product version 1, by manually selecting the correct revision. When I do so, which version of Library do I get, HEAD or the revision that I used when I built it the first time?
Hopefully I've been a good developer and remembered to tag every version of Product that I release. When I apply my tag 'Product-1-0-0' to the repository, does the correct revision of the Library repository get tagged too? If I later check out Product based on the tag 'Product-1-0-0', does the correct revision of Library get checked out with it?
What you have to watch out for with svn:externals is that you need to explicitly specify the revision if you want something other than trunk. Google "pinning svn:externals" for the details. If you are using a fairly modern version, 1.5 or newer IIRC, then relative externals are at least supported. Older versions, like the one that I am currently using, requires us to explicitly pin the revision using the -rNNNNN option on the svn:externals property for every damned folder.
We ended up using a modification of a perl script named svncopy.pl from tigris.org to do all of our branching and tagging. It's not that bad but I wish that we had known how much work it was before we decided to use them so heavily.