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World 10: Deep Git

Refs, the HEAD file and reftable

You will learn: what a branch really is inside .git, and where Git 3.0 will keep it.

The problem

You make a branch in a huge project. It appears at once. No copying, no waiting. So what is a branch, inside .git?

Watch

A ref: A name that points to a commit, like a branch or a tag. is a name for a commit. Branch main is a tiny file in .git/refs/heads. Inside is one commit id.

Try it

.git/HEAD says where you are. Read it. Then detach HEAD, and read it again.

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Predict

You make a new commit on main. What changes in .git/refs/heads/main?

Try it

Thousands of tiny ref files get slow. git pack-refs --all puts them in one file: packed-refs: One file in .git that holds many refs together, instead of one small file each.. Pack them, then read it.

Check

reftable: A newer, faster way to store refs in a few compact tables. Git 3.0 plans it for new repos. is a newer way. Refs live in a few compact tables. Which format fits each fact?

Drag a card into a box. Or tap the card, then tap the box.

      Check

      Your repo uses reftable. Which command still shows the commit id of main?

      You did it!

      You can now read refs and the HEAD file, and know reftable.

      Go deeper (optional)

      git show-ref lists every ref with its id. Want reftable today? git init --ref-format=reftable makes a new repo with it. The setting init.defaultRefFormat does it for every new repo. In scripts, ask Git (rev-parse, show-ref). Don’t read .git files.