Merging Branches: Fast-Forward vs Three-Way Merges (git merge)

Once work on an isolated feature branch is completed and tested, it must be integrated back into the main project trunk. The process of combining disparate lines of development is called merging (git merge).

1. How Merging Works

To merge branch B into branch A:

1. 1
   You must first switch into the receiving branch (A).
2. 2
   Run git merge B.

Depending on the commit history structure, Git automatically chooses between two merging strategies: Fast-Forward Merge or Three-Way Merge.

2. Strategy 1: Fast-Forward Merge

A Fast-Forward merge occurs when there is a direct linear path from the current branch tip to the incoming feature branch tip. In other words, no new commits were made on main while the feature branch was being developed!

Key Properties of Fast-Forward:

• No merge commit is created.
• Git simply moves the [main] pointer forward along the commit chain to C4.
• Fast, clean, and creates zero overhead.

Forcing a Merge Commit (--no-ff):

Some teams prefer to preserve explicit historical evidence that a branch existed:

3. Strategy 2: Three-Way Merge (Recursive / Ort)

A Three-Way Merge occurs when both branches have diverged: commits were made on main AND commits were made on feature-login at the same time:

Why is it called "Three-Way"?

Git uses three distinct snapshots to calculate the merged result:

1. 1
   The Common Ancestor Commit (C2): The point where the two branches split.
2. 2
   The Current Branch Tip (C6 - main).
3. 3
   The Incoming Branch Tip (C4 - feature-login).

By comparing both branch tips against their common base, Git can automatically deduce what each developer intended to change:

• If a line was changed only in feature-login and untouched on main, Git applies the feature change.
• If a line was changed only on main and untouched in feature-login, Git preserves the main change.

Git generates a brand new Merge Commit (C7) that uniquely has two parents (C4 and C6).

4. Summary: Merge Types Comparison