What are the advantages of keeping linear history in Git?

Question

When I was taught using Git with central repo (project on Gitorious) I was told to always use rebase instead of merge because we want to have linear history. So, I have always been trying to work that way.

Now when I come to think about it is it really so beneficial? Rebasing branch with many commits is much more time consuming then simple merge.

There are 2 advantages that come to my mind right now:

1. git bisect
2. Possibility of submitting with history to another version control system like SVN.

Are there any other benefits?

Answer 1

Two more advantages of linear/semi-linear history that weren't mentioned in the other answers -

1. No code changes are hidden inside merge commits as conflict resolution. Each diff belongs to single commit with its message explaining the reason behind the change.
2. Conflicts are conceptually easier to resolve during rebase than during merge, as the resolution takes place in a "context" of single commit (as opposed to entire branch). This reduces potential conflict resolution mistakes.

Let me explain this by example. Assume the following non-linear history:

*   f17ba26 (HEAD -> master) Merge branch 'topic/feature'
|\
| * 1234bbb (topic/feature) adds feature foo
* | 1234aaa blah
|/
* 03f4f8d previous commit

Suppose evolution of some code line that was changed by both commits 1234aaa and 1234bbb and the resulting conflict was resolved by the merge commit:

Commit   Line contents
03f4f8d  print("Nothing is supported")
1234aaa  print("We now support blah!")
1234bbb  print("We now support foo!")
f17ba26  print("We support plenty of futures now!")

In such scenario, the logic of why this line assumes its final state is hidden under the conflict resolution in the merge commit. The merge commit might make it very difficult to follow the logic of why the final version of code was chosen. Also, amount of conflicts in the merge commit might be large, making it even more difficult to grasp and review.

If the same code evolved through rebase, and the history was semi-linear like this:

*   f17ba26 (HEAD -> master) Merge branch 'topic/feature'
|\
| * 1234bbb (topic/feature) adds feature foo
|/
* 1234aaa blah
* 03f4f8d previous commit

The conflict would have been resolved as part of the rebase, in a context of applying the 1234bbb commit. The developer resolving the conflict would have had the opportunity to revise the specific change given the already existing change from 1234aaa and document the reasoning his/her decision about the final code in the commit message.

Answer 2

If you're rebasing your work often and nobody else is working in that part of your code, it should usually be a non-event.

These are the commands more or less (from):

git checkout -b my-new-feature
git push -u origin my-new-feature

# Changes and commits

git rebase origin/master
git push origin my-new-feature --force-with-lease
git merge --no-ff my-new-feature

Also people here seem to be mistaking merges and merge commits. I'm in favour of a linear history with merge commits, like this. That way you can see the individual commits if you need to but can also jump from merge to merge.

Answer 3

As far as I am concerned, keeping a linear history is mostly for asthetics. It's a very good way for a very skilled, disciplined developer to tidy up the history and make it nice to skim through. In a perfect world, we should have a perfectly linear history that makes everything crystal clear.

However, on a large team in an enterprise environment I often do not recommend artificially keeping a linear history. Keeping history linear on a team of sharp, experienced, and disciplined developers is a nice idea, but I often see this prescribed as a 'best practice' or some kind of 'must do'. I disagree with such notions, as the world is not perfect, and there are a lot of costs to keeping a linear history that people do not do a good job of disclosing. Here's the bullet list overview:

• Rewriting history can include erasing history
• Not everybody can even rebase
• The benefits are often overstated

Now, let's dig into that. Warning: Long, anecdotal, mostly ranting

Rewriting history can include erasing history Here's the problem with rebasing all of your commits to keep everything nice and linear: Rebasing is not generally loss-less. There is information- real, actual things that were done by the developer - that may be compressed out of your history when you rebase. Sometimes, that's a good thing. If a developer catches their own mistake, it's nice for them to do an interactive rebase to tidy that up. Caught-and-fixed mistakes have already been handled: we don't need a history of them. But some people work with that one individual who always seems to screw up merge conflicts. I don't personally know any developers named Neal, so let's say it's a guy named Neal. Neal is working on some really tricky accounts receivable code on a feature branch. The code Neal wrote on his branch is 100% correct and works exactly the way we want it to. Neal gets his code all ready to get merged into master, only to find there are merge conflicts now. If Neal merges master into his feature branch, we have a history of what his code originally was, plus what his code looked like after resolving the merge conflicts. If Neal does a rebase, we only have his code after the rebase. If Neal makes a mistake when resolving merge conflicts, it will be a lot easier to troubleshoot the the former than it will be the latter. But worse, if Neal screws up his rebase in a sufficiently unfortunate way (maybe he did a git checkout --ours, but he forgot he had important changes in that file), we could altogether lose portions of his code forever.

I get it, I get it. His unit tests should have caught the mistake. The code reviewer should have caught the mistake. QA should have caught the mistake. He shouldn't have messed up resolving the merge conflicts in the first place. Blah, blah, don't care. Neal is retired, the CFO is pissed because our ledger is all screwed up, and telling the CFO 'according to our development philosophy this shouldn't have happened' is going to get me punched in the face.

Not everybody can even rebase, bro. Yes, I've heard: You work at some space age startup, and your IoT coffee table uses only the coolest and most modern, reactive, block-chain based recurrent neural network, and the tech stack is sick! Your lead developer was literally there when Go was invented, and everybody who works there has been a Linux kernel contributor since they were 11. I'd love to hear more, but I just don't have time with how often I'm being asked 'How do I exit git diff???'. Every time someone tries to rebase to resolve their conflicts with master, I get asked 'why does it say my file is their file', or 'WHY DO I ONLY SEE PART OF MY CHANGE', and yet most developers can handle merging master into their branch without incident. Maybe that shouldn't be the case, but it is. When you have junior devs and interns, busy people, and people who didn't find out what source control is until they had already been a programmer for 35 years on your team, it takes a lot of work to keep the history pristine.

The benefits are often overstated. We've all been on that one project where you do git log --graph --pretty and suddenly your terminal has been taken over by rainbow spaghetti. But history is not hard to read because it's non-linear...It's hard to read because it's sloppy. A sloppy linear history where every commit message is "." is not going to be easier to read than a relatively clean non-linear history with thoughtful commit messages. Having a non-linear history does not mean you have to have branches being merged back and forth with each other several times before reaching master. It does not mean that your branches have to live for 6 months. An occasional branch on your history graph is not the end of the world.

I also don't think doing a git bisect is that much more difficult with non-linear history. A skilled developer should be able to think of plenty of ways to get the job done. Here's one article I like with a decent example of one way to do it. https://blog.smart.ly/2015/02/03/git-bisect-debugging-with-feature-branches/

tldr; I'm not saying rebases and linear history aren't great. I'm just saying you need to understand what you're signing up for and make an informed decision about whether or not it's right for your team. A perfectly linear history is not a necessity, and it certainly isn't free. It can definitely make life great in the right circumstances, but it will not make sense for everyone.

Answer 4

Here is a pro/con list of rebase or squashing commits when merging.

Pro:

• Easy to read chronological history
• No extraneous commits in history
• Splitting branches/PR's into smaller chunks for easier review, and testing is easier without merge commits
• Being able to rebase feature branches easily (Fx. to change merge order)
• It encourages meaningful commit messages (but does not enforce it)
• When doing a release, one can more easily look at the history and use it to write a change-log, for use by testers, consumers etc.
• The commit by commit rebase encourages PR's with less/smaller commits

Con:

• It's more complicated, less people have experience with it
• Can lead to problems if used on remote branches without care. Examples, lost commits, inconsistent history across local environments
• It's an extra thing to maintain (Does x project really need this?)
• More time consuming if you rebase a branch with many conflicting commits

Personal note

I mentioned squashing before merge as well because it has a similar effect in terms of history.

Also, most github and other services have options to rebase-merge and squash-merge, right there in the GUI.

Making it practically a free action for PR's in some cases.

Answer 5

With linear history, you can easily track history for a single file across renames with git log --follow. Quoting the documentation on the log.follow config option:

If true, git log will act as if the --follow option was used when a single <path> is given. This has the same limitations as --follow, i.e. it cannot be used to follow multiple files and does not work well on non-linear history.

Answer 6

A linear Git history (preferably consisting of logical steps), has many advantages. Apart from the two things already mentioned, there is also value in:

3. Documentation for the posterity. A linear history is typically easier to follow. This is similar to how you want your code to be well structured and documented: whenever someone needs to deal with it later (code or history) it is very valuable to be able to quickly understand what is going on.
4. Improving code review efficiency and effectiveness. If a topic branch is divided into linear, logical steps, it is much easier to review it compared to reviewing a convoluted history or a squashed change-monolith (which can be overwhelming).
5. When you need to modify the history at a later time. For instance when reverting or cherry-picking a feature in whole or in part.
6. Scalability. Unless you strive to keep your history linear when your team grows larger (e.g. hundreds of contributors), your history can become very bloated with cross branch merges, and it can be hard for all the contributors to keep track of what is going on.

In general, I think that the less linear your history is, the less valuable it is.