Lossless merges

Let’s say you have a git history that looks like this:

    
      C - D         feature/magic
     /
A - B               main
    

A standard git merge feature/magic issued on the main branch results in this history:

    
      C - D         feature/magic
     /
A - B - C - D       main
    

This is a fast-forward merge. Since the main ref is at B and B is the parent of C when we merge feature/magic into main, main’s ref is updated to point at the commit at D.

There is no loss of fidelity from the point of development. Every development commit is kept and the relationships between commits maintained.

Lossy merges

Using --squash instead of the default merge strategy is lossy: the fidelity of git history is lost. Squash, in our example, results in a new commit being added to main’s history that is an amalgam of the commits on the feature/magic branch:

    
      C - D        feature/magic
     /
A - B - - - CD'    main
    

You can no longer see that C and D were two separate commits.

Helpful Loss

There are reasons to choose a lossy merge over a lossless merge.

There are blogs that advocate heavily for a squash workflow. Which strategy to choose is dependent on the content of the commits you are merging. The strategy chosen should maintain the principal that a commit in a mainline branch’s history should make sense on its own.

In the above example, the content of the feature branch isn’t shown. A new example might be a feature/lossless branch that contains a refactor and a new feature that depends on that refactor:

    
| * (feature/lossless) feature: method is dynamic
| * refactor: method instead of global
|/
* (main) Initial Commit
    

This is an example where the desired outcome is lossless: both of these commits are meaningful on their own and can be vectors for bugs. After the merge, in an ideal case, the main branch should look like:

    
* (main, feature/lossless) feature: method is dynamic
* refactor: method instead of global
* Initial Commit
    

Now, imagine a different branch – a bugfix, with only a single commit that is up for review.

    
| * (bugfix/lossy) bugfix: validate user input
|/
* (main) Initial Commit
    

During review, there’s a typo in a comment that needs fixing. Now the branch graph looks like:

    
| * (bugfix/lossy) fix comment typo
| * bugfix: validate user input
|/
* (main) Initial Commit
    

I would argue that the typo commit doesn’t make sense on its own. There’s no need to persist that commit into the main branch: it’s noise. It’s not a vector for meaningful error, it’s a development detail that shouldn’t leak back into the main branch. In short, a more functional history for a merge would be to use a lossy strategy:

    
* (main, bugfix/lossy) bugfix: validate user input
* Initial Commit
    

This is all an oversimplification

Of course, the example above completely ignores merge commits, repository merge strategies, and any shared agreements about the state of feature or mainline branches, code review, testing strategies, deployment pipelines, and so so (so!) much more!

Many of the blog posts on git I read make broad generalizations about the Right™ way to use some particularly controversial features of git (pull, merge, rebase, branching, commit messages …wait. 🤔 Is every feature controversial?), but the reality is that there is a lot of nuance in the world and the only right answer depends on your situation.