Merge Strategies in Git: Fast-Forward vs. Recursive

When we talk about merging branches in Git, there are different strategies that Git can use to combine changes. The two most common strategies are Fast-Forward and Recursive. Understanding which one to use and when can help you maintain a clean and comprehensible project history.

Fast-Forward Merge

The Fast-Forward merge occurs when the target branch has not advanced since the point of divergence with the source branch. In other words, there are no additional commits in the target branch since the source branch was created or updated.

Characteristics:

• Does not create a merge commit.
• Simply moves the target branch pointer forward.

Advantages:

• Linear and clean history.
• Easier to follow and understand.

Example of Fast-Forward Merge:

bash

If main has not changed since feature-xyz was created, Git will move the main pointer forward, integrating all commits from feature-xyz into main.

Forcing a Fast-Forward Merge

To force a Fast-Forward merge when possible:

bash

If a Fast-Forward is not possible, Git will abort the merge.

Recursive Merge

The Recursive merge is used when the branches have evolved independently, and both the source and target branches have additional commits since their common point of divergence.

Characteristics:

• Creates a merge commit.
• Combines changes from both branches using a "three-way" strategy.

Advantages:

• Maintains a detailed history of when and how branches were merged.
• Useful for integrating branches with complex developments clearly.

Example of Recursive Merge:

bash

If main has changed since feature-xyz was created, Git will perform a three-way merge and create a merge commit.

Handling Recursive Merges

By default, Git uses the Recursive strategy to merge when a Fast-Forward is not possible:

bash

Forcing a Merge Commit (No Fast-Forward)

If you want to create a merge commit even when a Fast-Forward is possible, you can use:

bash

This creates a merge commit, which is useful for keeping an explicit record of the merge.

Example:

bash

This command will merge feature-xyz into main and create a merge commit, even if a Fast-Forward is possible.

Considerations for Choosing a Strategy

Use Fast-Forward:

• When you want to maintain a simple, linear history.
• For minor or quick changes where an explicit merge commit is not necessary.

Use Recursive (or --no-ff):

• For complex projects with multiple developers.
• When you want to maintain a clear and detailed record of all merges.
• To integrate large features or branches that have undergone many changes.

Summary

1. Fast-Forward Merge: Moves the branch pointer forward, without creating a merge commit.
2. Recursive Merge: Combines changes with a merge commit, using the three-way strategy.
3. Force Fast-Forward: git merge --ff-only <branch-name>
4. Always create a merge commit: git merge --no-ff <branch-name>

Knowing when and how to use each merge strategy will help you maintain an organized and easy-to-understand project history. In the next chapter, we will cover rebase in Git, another method for integrating changes.