Friday 16 December 2016

Rebase as an Integration Strategy for Feature Branches

There are generally two reasons for using git rebase, 1) To tidy up/ rearrange commits that aren't publically in use 2) As a strategy for integrating branches. This post discusses the second use case. Rebase gets a lot of bad press, I think that this is parlty due to misunderstanding, it's like a dog that people kick, it bites someone, then it gets put down, so lets try and understand it and then play fetch with it or something instead.

So most people are familiar with merging as an integration strategy, the problem with merging is that it creates a non-linear history polluted with merge commits. The murky history manifests itself as ambiguity in tools like git log and increased difficulty in using git bisect, it generally makes archeological spelunking and working with history harder. Merges do have some benefits however, they naturally work well with pull requests and also preserve branch history (that may or may not be valuable to you). For a full discussion of pros and cons check out this excellent article.

Merge and Rebase Workflows

Starting steps, create a feature branch off the tip of up-to-date/ upstream master

 $ git checkout master
 $ git pull
 $ git checkout -b feature-branch

State of Play
      C             feature-branch
A---B---D        master

A normal merge workflow
... do some work, stage it
 $ git commit -m "C"
 $ git checkout master
 $ git merge feature-branch
 $ git push origin master
       C            feature-branch
      /   \
A---B---D        master

A normal rebase workflow
... do some work, stage it
 $ git commit -m "C"
 $ git rebase master
 $ git push origin feature-branch:master
Post Rebase
        C'         feature-branch
A---B---D        master

The Rebase Workflow Analysed

The rebase command above takes master and forward ports C on top and sets this to be feature-branch, this can be considered as 'rebasing' "feature commit" on the updated master. Through this process C is rewritten, with a different SHA, it is now a different commit, C', this can be a sticking point in understanding, but an important feature of a commit is that it is immutable. Rebases rewrite commits whereas merges don't.

The git push means, push the ref before the colon to the ref after the colon on the remote origin. If your git push is being rejected as a non-fast forward, you are doing something wrong or someone has pushed in the time since you pulled, the blighters, repull master and rebase onto it again.

Consider Dry Run

If you are worried about what you are commiting, note that you can always see what you are doing before you push it using the --dry-run argument to git push, which stops short of sending the actual update, you can then run git log on the SHA range it outputs.

$ git push origin feature-branch:master --dry-run  
   d4f3294..6c53234 feature-branch -> master  
 $ git log -U3 d4f3294..6c53234  

On a personal note, I prefer generally prefer rebase over merge for integrating feature branches, I am a bit keen on a nice clean history, the importance of well-kept history is a topic for a future post.

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