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Commits and DAG

Git history is better understood as a directed acyclic graph, or DAG, than as a simple list of commits.

What A Commit Stores

A commit typically stores:

  • the root tree object ID,
  • one or more parent commit IDs,
  • author and committer metadata,
  • timestamps,
  • a message.

Importantly, the branch name is not stored inside the commit. Branches are refs that point to commits.

Why History Is A Graph

If every commit had only one parent and no merges ever happened, history would look like a chain.

But Git allows:

  • branching from earlier history,
  • parallel development,
  • merge commits with multiple parents.

As soon as multi-parent commits appear, history stops being a simple line and becomes a graph.

What DAG Means

DAG stands for Directed Acyclic Graph.

  • Directed means commit relationships have a direction: newer commits point to parents.
  • Acyclic means the graph does not loop back into itself.

Those properties make it possible to reason about:

  • ancestry,
  • lowest common ancestors,
  • fast-forward possibilities,
  • merge bases,
  • history traversal.

Why Branches Are Lightweight

Once you see history as a commit graph, branch behavior becomes easier to explain.

A branch is just a movable ref pointing at a commit. Creating a branch does not duplicate the repository snapshot; it creates another name in the graph.

That is one reason Git branching is so lightweight compared with many centralized version-control systems.

Why DAG Knowledge Matters

Commands such as log, merge, branch, checkout, and reset all make more sense when you think in terms of commit graph movement rather than in terms of a flat sequence of saved folders.