Are subversion externals an antipattern?

Question

Subversion lets you embed working copies of other repositories using externals, allowing easy version control of third-party library software in your project.

While

Answer 1

An old thread, but I want to address the concern that a changing external could break your code. As pointed out previously, this is most often due to an incorrect usage of the external property. External references should, in almost all instances, point to a specific revision number in the external repository URI. This ensures that the external will never change unless you change it to point to a different revision number.

For some of our internal libraries, which we use as externals in our end-user projects, I've found it useful to create a tag of the library at Major.Minor version, where we enforce no breaking changes. With a four-point versioning scheme (Major.Minor.BugFix.Build), we allow the tag to be kept current with BugFix.Build changes (again, enforcing no breaking changes). This allows us to use an external reference to the tag without a revision number. In the case of major or other breaking changes, a new tag is created.

Externals themselves aren't bad, but that doesn't stop people from creating bad implementations of them. It doesn't take much research, just a little bit of reading through some documentation, to learn how to use them safely and effectively.