How to achieve code review small & early and reduce merge hell.

1. Code Reviews
vs.
Merge Requests
- Continuous Code Review

2. Why Phabricator
• Tool is not our focus today, process is
• Gitflow (gitlab, github) is not TBD friendly
• Use PR/MR for (post-commit) pre-push code
review is hard

3. Why Code Review
It’s not just for catching bugs …

4. Efficiency

5. But
How to do it more efficiently …

6. Large Diffs Hurts (I)
• Large diffs won’t get reviewed
✴ It’s hard to spot bugs in large diffs

8. src: http://blogs.atlassian.com/2011/07/creating_optimal_reviews/

9. Large Diffs Hurts (II)
• Large diffs won’t get reviewed
✴ It’s hard to spot bugs in large diffs
• Reverting large diffs is hard
src: https://medium.com/@kurtisnusbaum/large-diffs-are-hurting-your-ability-to-ship-
e0b2b41e8acf#.3l2trqfsr

10. git commit -m "fixed issue with fan" [Pull Request code review]

11. Large Diffs Hurts (III)
• Large diffs won’t get reviewed
• It’s hard to spot bugs in large diffs
• Reverting large diffs is hard
• Rebasing/merging large diffs is error-prone

13. TBD

14. The Right Direction
Avoid overloading
• concise – don’t let cruft get in the way
• contained – if there are two parts, create two
reviews
• guided – tell people how to review it

15. The Right Direction
Avoid overcrowding
• selected with care – don’t blindly choose reviewers
• focused – if its a specialist change, choose a
specialist dev
• guided – tell people individually what they should
look out for

16. Arc Workflow
git co -b <branchname>
<edit edit edit>
git commit
arc diff --rr reviewer1,reviewer2 [--cc person3] --verbatim
<edit based on review feedback>
git commit
arc diff
<maybe run git pull if you want some updates>
<edit + commit + arc diff some more>
arc land

17. Arc Workflow
<edit edit edit> ＃ No new branch
git commit
arc diff --rr reviewer1,reviewer2 [--cc person3] --verbatim
<edit based on review feedback>
git commit
arc diff
<maybe run git pull if you want some updates>
<edit + commit + arc diff some more>
arc amend
git push

18. Code Review vs. Push
• Differential: Pre-push code review
• Diffusion/Audit: Post-push audit (non-blocking)
• User Guide: Review vs Audit

19. Post-push Audit
<edit edit edit>
git commit
# put 'Auditors: foo,bar,baz' in the commit message
git push
Fix the foobaz test now that we no longer frob our foos.
I can't believe I forgot to run this test when I changed the
foo-frobbing code!
Auditors: chris, james
Test plan:
tools/runtests.py foobaz_test.py

20. Rebase vs Merge
• The major benefit of rebasing is that you get a much cleaner
project history. First, it eliminates the unnecessary merge commits
required by git merge. Second, as you can see in the above
diagram, rebasing also results in a perfectly linear project history
—you can follow the tip of feature all the way to the beginning of
the project without any forks. This makes it easier to navigate your
project with commands like git log, git bisect, and gitk.
• But, there are two trade-offs for this pristine commit history: safety
and traceability. If you don’t follow the Golden Rule of Rebasing,
re-writing project history can be potentially catastrophic for your
collaboration workflow. And, less importantly, rebasing loses the
context provided by a merge commit—you can’t see when
upstream changes were incorporated into the feature.

21. Merge PR
• Branches
• Merge conflicts
• Guitar Hero

22. Rebase
• linearized, rebased, fast-forward commits is the
default behavior of Phabricator.

23. Benefits
code evolution history
vs
commit logs

24. Import Gitlab Repo

25. Community Resources
Arcanist-support: additional testing and linting engines
• SBTTestEngine: Runs unit tests for Scala using SBT
• Scalastyle Lint Engine: Runs Scalastyle linter
Phabricator Jenkins Plugin
• Integrates with Phabricator's Differential and
Harbormaster apps

26. Jenkins Integration

27. Current Workflow

28. Diff Build

29. Diff by Commit vs master

30. Other Usage Examples
Test plan of a diff:
• https://secure.phabricator.com/D16974
Phab TBD vs wikimedia branches model:
• https://secure.phabricator.com/source/libphutil/repository/master/
• https://phabricator.wikimedia.org/diffusion/ECOE/repository/master/
Diff with feature-toggle example:
• https://secure.phabricator.com/D17200
Use Phab wiki for release note example:
• https://secure.phabricator.com/w/changelog/2016.49/

31. Phabricator vs. GitHub vs.
Atlassian
• https://rekinyz.wordpress.com/2015/01/18/
phabricator/

32. Goal
• Test driven
• Code review small and early
• Test before delivery
• Continuous Improvement

33. Reference
• Writing Reviewable Code (Facebook)!!
• Pull Requests vs Code Reviews
• Branch recommendation in Phabricator
• Using phabricator guide [khanacademy]
• Phabricator Project Mgmt Example