How is dvcs (git/mercurial) branching and merging support better than svn's?

Question

Lots of articles on dvcs systems claim superior branching and merging support as one reason to move from svn to dvcs systems. How exactly do these systems do branching and merging differently that makes it better?

Answer 1

Historically, the difference between merge-tracking in git and svn was this: git has merge-tracking, and until version 1.5, svn didn't. At all. If you wanted to make a merge you had to always specify exactly what changes were to be merged, and if you merged one branch into another more than once, you would have to manually keep track of which revisions had and hadn't been merged, and manually select only the changes that hadn't been merged yet, to avoid conflicts. Good luck with that if you ever cherry-picked any changes.

Beginning with version 1.5 (released in 2008), if your client, server, and repository are all up-to-date, then svn is capable of acting a lot more intelligently; it uses properties to keep track of where a branch came from and what changes have already been merged into it. The upshot is that in many cases you can just svn merge BRANCHNAME and have the right thing happen. But due to its "bolted on" nature it's still not very fast and not entirely robust. Git, on the other hand, has to handle merge scenarios well because of its DVCS nature, and it was designed from the beginning with data structures (like the particular kind of DAG it uses) and algorithms (such as recursive-merge and octopus-merge) that are suited to the task.

Answer 2

The difference is not, contrary to popular perception, due to the distributed nature of DVCS's, vs Subversion's centralised model. There is nothing inherent in a centralised model that entails that branching and merging will be substandard.

My take is that Subversion made a massive design gaffe by deciding to model code-base directory structure, branching and tagging (and all manner of other code management patterns) in a single, unified directory tree, which made the problem of reliably detecting branching activity one hundred times more difficult than it would have been if branching were explicit in the model.