Back in February, Microsoft made the surprising announcement that the Windows development team was going to move to using the open source Git version control system for Windows development. A little over three months after that first revelation, and about 90 percent of the Windows engineering team has made the switch.
The switch to Git has been driven by a couple of things. In 2013, the company embarked on its OneCore project, unifying its different strands of Windows development and making the operating system a more cleanly modularized, layered platform. At the time, Microsoft was using SourceDepot, a customized version of the commercial Perforce version control system, for all its major projects.
SourceDepot couldn’t handle a project the size of Windows, so rather than having the whole operating system in a single repository, the Windows code was actually divided among 65 different repositories, with a kind of virtualization layer on top to produce a unified view of all the code. Some of these 65 repos contained nicely isolated, standalone components; others took vertical or horizontal slices through the operating system; others were just grab bags of different code. As such, the repo structure didn’t correspond with OneCore’s module boundaries.