Microsoft Office's Epic Codebase Migration: From Source Depot to Git

This article recounts the epic journey of the Microsoft Office team migrating their massive codebase from the outdated Source Depot to Git. The migration was fraught with challenges, including building a 'parallel universe' to synchronize codebases, proving functional equivalence between the two systems, and coordinating communication with over 4,000 engineers. The author details each stage, from overcoming the differences between Source Depot and Git's branching models, building a Virtual File System to optimize performance, and training engineers in Git. The successful migration significantly improved developer efficiency and code quality, offering valuable lessons for large-scale technical migrations.