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14-subproject.md

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Up | Content | Intro | Concepts | Operations | Dictionary

Subproject

Very often a source code has to be reused in many projects. Code which is reused is also called a module, a library, a framework, or a package. It is not feasible to copy the code each time, as each copy must be maintained manually. It is much easier to have a central repository with the reused code and let git automate the copying and synchronization of the files. Git offers two popular solutions for this: submodule and subtree.

Submodules

  • A small footprint size
  • A submodule is a reference to a specific commit of another repository
  • A submodule must be updated manually to clone the submodule
  • A special folder .git/modules hosts the submodules objects after update
  • A submodule links a branch or a specific revision
  • A repository can have more than one submodule
  • Submodules can be nested

submodules

Why submodules?

  • Submodules are native to git
  • Submodule repositories have their own commit history
  • A new submodule version can be released without affecting the project
  • A checkout to a different revision of the submodule doesn't affect the project
  • Usually used for bigger project where checkout times can be significant

Submodules drawbacks

  • No automatic updates for the submodule
  • Use of additional set of commands to handle submodules
  • Nested submodules are omitted by default
  • Merging changes from the project into the submodules are difficult

See also: git submodule

Subtrees

  • A transparent view of the code base
  • A subtree is a full copy of another repository with files and history
  • A subtree can be handled using standard commands
  • A repository can have more than one subtree
  • Subtrees are not nested

Git Subtrees

Why subtrees?

  • A subtree can be updated using standard commands (clone or pull)
  • A subtree is easier to use for branching and merging
  • A subtree doesn't add metadata files
  • Older version of git are supported (even before v1.5.2)
  • Usually used for integrated projects with frequent commits

Subtree drawbacks

  • Subtree is a tool and not native to git
  • Subtrees require a deeper understanding of git merging strategies
  • Special attention of not mixing project and subtree code in commits
  • A checkout to a different revision will affect all the project files
  • Not recommended when the amount of dependencies is too large.

See also: git subtree

Which to use?

Git submodules is more fit to a component-based development where the project depends on a specific version (commit) of an external repository and the user doesn't change the code of the submodule or the submodule doesn't change frequently.

Git subtrees is more fit to a system-based development where the user wants to have a full copy all files and their history and the subtree will be changed frequently either by the maintainer or by the user himself.

Other tools