Saturday, January 10, 2009

Difference in POV between Centralized VCS and Distributed VCS

'However, the total cost of branching is paid by reduced code velocity to main, merge conflicts and additional testing can be expensive. Throughout this guidance we ask the user to confirm that a branch is really needed and always ask the question "how does this branch support my development project?"'
-- from the TFS branching guide.


I came across this interesting passage yesterday, and I was struck by the profound difference in point of view between centralized version control and distributed version control.

In distributed version control systems (dvcs), branching is fundamental (in most scenarios): every "checkout" is really a branch of the repository, with full history. Therefore the focus in developing dvcs has been on making branching and merging as simple, quick and painless as possible.

Now, arguably, branching is also fundamental to all version control; after all, when you check out a copy of the code and modify it, you make a kind of lightweight, local branch for your changes that is quickly merged back into the trunk. But, with centralized version control, the focus is on control of the codebase, which is concentrated in the trunk. So, not only is branching not optimized (because of the lack of focus), but it is indirectly made more costly by the controls put on the trunk. And, because local checkouts aren't recognized as 'branches', you lack most of the usual tools for managing them separately from the trunk/server, like full history and an easy way to exchange patches with another developer.

Now, this isn't to say that the above advice isn't germane (PSP: take a shot!). If you are using a centralized version control tool, then you should absolutely consider the costs associated with branching, because it is quite expensive. And, arguably, you could go crazy with dvcs as well, but it takes a lot more work, I think. But the elaborate systems that the aforementioned branching guide mentions for scenarios just seem baroque and unnecessary when considered from the context of a dvcs.
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