Monorepo vs Polyrepo: Lessons from Meta, AWS, and a 15-Repo Migration

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A software engineer who worked at both Meta and AWS shares firsthand observations on how each company manages code organization at scale. Meta's single Mercurial monorepo enabled cross-cutting changes in one commit and made code discoverable, but demanded distributed build tooling and careful dependency management. AWS's polyrepo model offered clear team ownership and independent deployments, yet led to duplicated infrastructure patterns and poor discoverability across thousands of repositories. Drawing on both experiences, the engineer migrated SID Technologies from 15 separate repositories into a single monorepo over a weekend. The key takeaway is that the monorepo versus polyrepo choice is fundamentally an organizational decision rather than a purely technical one.

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