Submodules, subtrees or vendoring: three ways to include another repository

What it is

Three mechanisms put code from another repository into yours:

• Submodule. The superproject's tree contains a gitlink entry holding the commit ID the submodule should be at, and .gitmodules records path and URL. The submodule keeps its own history and Git directory. The documentation states that after git submodule update the recorded commit is checked out on a detached HEAD.
• Subtree. git subtree add --prefix=<dir> <repository> <ref> imports the other project's files into a subdirectory as ordinary tracked files; with --squash only a single commit is imported instead of the whole history. git subtree pull and git subtree push move changes in both directions. The manual stresses that subtrees need no special constructions such as .gitmodules or gitlinks and do not force end users to do anything.
• Vendoring. Copy the files, commit them, and write the upstream version, source URL and any local patches into a file next to them (VENDOR.md or a lock file).

Why it matters

The choice decides what a fresh clone looks like, how an upgrade is performed and whether local modifications are possible. Submodules are exact but demand git clone --recurse-submodules or git submodule update --init --recursive from everyone, including CI; forgetting this yields empty directories. Subtrees and vendoring produce a self-contained clone at the cost of a fatter history and less obvious provenance.

How to apply

• Prefer a package manager when one exists for the language; these three are for cases where none fits (shared configuration, assets, a private fork).
• Use submodules when the dependency is large, changes independently and must be pinned to an exact commit that you also develop in.
• Use subtrees when consumers should not need to know, and upgrades are occasional (git subtree pull --squash keeps the history short).
• Vendor when the dependency is small and stable; note the version and licence so the copy can be audited and refreshed.
• Whatever you pick, put the upgrade command in the README and run the build from a fresh clone in CI.

Pitfalls

Submodule commits pushed to the superproject but not to the submodule's remote break every other clone. Commits made inside a submodule on its detached HEAD sit on no branch and are easy to lose unless a branch is created first. The subtree manual asks for consistency with --squash: if all merges are squashed, split --rejoin must be squashed too, or the log shows a copy of every commit. Vendored copies drift silently when someone patches them without updating the record.