externals

Externals refer to resources or dependencies that originate outside a project's own repository or bundle. They enable reuse of code, data, or assets without duplication, supporting modular design and centralized maintenance.

In version control, externals bring in content from external locations. Subversion's svn:externals property maps a local path to an external URL and optional subpath. When checked out or updated, the external content is fetched. Externals reduce duplication and simplify sharing, but they can complicate builds, cause drift between repositories, and rely on network access.

Git does not provide a direct externals feature. The closest concepts are submodules and subtrees. Submodules pin to a specific commit in an external repository and require explicit initialization and updates. Subtrees merge an external history into the local repository, avoiding separate checkouts but increasing complexity and size.

In modern builds, bundlers can treat modules as externals, excluding them from the bundle and expecting them to be supplied externally. For example, Webpack's externals setting maps module names to global variables or external scripts. Other tools offer analogous mechanisms to prevent packaging of dependencies that are provided by the runtime environment.

Best practices include documenting externals, pinning to specific revisions or versions, and ensuring reliable access and reproducible builds. Externals can reduce duplication and improve modularity but require careful coordination across repositories and environments.