What is the difference between a "fast-forward" and a "recursive" merge in Git? — Git Interview Question #21

In Git, a merge is used to combine changes from two or more branches into a single branch. There are two types of merges in Git: fast-forward and recursive.

A fast-forward merge occurs when the branch being merged has all of its changes in a straight line from the current branch. In other words, there are no changes on the current branch that are not already on the branch being merged. When a fast-forward merge occurs, Git simply moves the current branch pointer to the commit of the branch being merged.

Here’s an example of a fast-forward merge:

    # Create a new branch called "feature"
    git branch feature

    # Switch to the "feature" branch
    git checkout feature

    # Make changes and commit them
    echo "Some changes" > file.txt
    git add file.txt
    git commit -m "Add changes"
    
    # Switch back to the main branch
    git checkout main
    
    # Merge the changes from the "feature" branch
    git merge feature

In this example, the git merge command will perform a fast-forward merge, since the changes made in the "feature" branch are already in a straight line from the current branch.

A recursive merge, on the other hand, occurs when the branch being merged has changes that are not already in a straight line from the current branch. When a recursive merge occurs, Git will create a new commit that combines the changes from both branches.

Here’s an example of a recursive merge:

    # Create a new branch called "feature"
    git branch feature
    
    # Switch to the "feature" branch
    git checkout feature
    
    # Make changes and commit them
    echo "Some changes" > file.txt
    git add file.txt
    git commit -m "Add changes"
    
    # Switch back to the main branch
    git checkout main
    
    # Make changes and commit them
    echo "More changes" > file.txt
    git add file.txt
    git commit -m "Add more changes"
    
    # Merge the changes from the "feature" branch
    git merge feature

In this example, the git merge command will perform a recursive merge, since there are changes on both the "main" and "feature" branches that are not in a straight line from each other.

In summary, a fast-forward merge occurs when the branch being merged has all of its changes in a straight line from the current branch, while a recursive merge occurs when the branch being merged has changes that are not in a straight line from the current branch. Understanding the difference between these two types of merges can help you choose the right merge strategy for your project.