svnexternals

svn externals is a Subversion feature that lets a working copy include files or directories from other locations, such as another repository or a different path within the same repository. By defining a set of externals, projects can share common libraries or components without duplicating their history in every checkout.

Externals are defined by the svn:externals property on a directory. Each line of the property specifies an external item: a local directory path followed by a URL, with an optional revision specifier. When a user checks out or updates the parent directory, Subversion fetches the external item into the specified local path. Externals can point to subtrees in the same repository or to external repositories altogether, and you can pin a specific revision to ensure reproducible builds.

Common syntax examples include:

- libs http://example.com/svn/libs/trunk

- third_party http://example.org/externals/third_party/trunk -r 1234

Notes on usage: you typically edit the svn:externals property via svn propset or svn propedit and commit the change. After that, running svn update or svn checkout will populate the external directories. Peg revisions (URL@PEGREV) and operative revisions (-r REV) allow control over which state of the external you fetch.

Limitations and caveats: externals introduce dependencies on external sources, which can affect build reliability, network reliability, and repository history. They can complicate merges and make the build less portable if external repositories change or become unavailable. Careful management, including pinning revisions and validating external sources, is advised.

Alternatives exist in other version control systems, such as git submodules or vendor directories, but svn externals remain a core mechanism in Subversion for reusing shared components.