Matt Mitchell takes a deep dive into .NET’s transition from distributed builds to the Unified Build strategy, revealing behind-the-scenes engineering and DevOps lessons.

Reinventing .NET Build and Release: Unified Build Approach

After years of balancing distributed repository development and rapid product composition, the .NET team faced mounting challenges: high build complexity, unpredictability in release timelines, and increasing infrastructure overhead. Matt Mitchell outlines the journey from distributed builds (multiple interdependent repositories, complex dependency flow, and significant overhead) to the new Unified Build system—pivoting .NET into a virtual monolithic repository with vertical build orchestration.

Key Themes

  • Historical Build Structure: .NET began with distributed repositories (Runtime, SDK, ASP.NET Core, WindowsDesktop, etc.), enabling autonomy and incremental development, but suffering from slow product coherency and shipping delays.
  • Complexity and Overhead: Building and shipping required traversing lengthy dependency graphs, causing delays, increased coordination needs, and maintenance headaches (illustrated via build graphs and real-world stats).
  • Source Build and Linux Distro Integration: Linux distributions needed reproducible, offline, single-platform builds, leading to the Source Build project—contrasting Microsoft’s multi-phase distributed builds.
  • Unified Build Vision: The team established clear goals for product coherency—single git commit for source, single build environment per platform, two-way code flow, downstream verification, and upstream patching flexibility for distro maintainers.

Unified Build Mechanics

  • VMR (Virtual Monolithic Repository): Centralizes all source and dependency logic, orchestrating builds and enabling contributors to work both in individual repositories and in a monolithic layout.
  • Vertical Builds: Segment product construction by platform/runtime, drastically reducing overhead and complexity.
  • Source Build Compliance: Enforces build-from-source rules, with solutions for reference-only packages, legacy dependencies, cross-platform joins, and automation of compliance coverage.
  • Two-way Code Flow: Changes propagate between the VMR and component repositories, increasing agility and enabling rapid iteration across the product.

Infrastructure and Results

  • Performance: Build times fell from 24+ hours to less than 7 (signed), enabling multiple daily validation cycles.
  • Flexibility and Predictability: Developers gain the ability to ship late-breaking fixes, respond to security incidents quickly, and predict when builds will be ready.
  • Scalability for Community and Linux Distros: Distro partners and contributors can build, validate, and ship their own distributions independently and securely.
  • Future Directions: Emphasis on AI agents for code flow monitoring, further removal of cross-platform build joins, and continuous improvements to developer workflow.

Technical Highlights

  • Dependency Flow Tools: Maestro and orchestrator scripts automate build graph traversal and artifact packaging.
  • Compliance & Licensing: Automated checks insure licensing standards for open-source distribution are maintained.
  • Scenario Testing: End-to-end tests validate full product builds, catching regressions before release.

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Conclusion

The Unified Build project marks a transformative milestone for .NET, with massive reductions in build time, complexity, and overhead—yielding a more robust, flexible, and scalable DevOps and release engineering process. These changes strengthen .NET’s position for contributors, maintainers, and Microsoft’s developer ecosystem.

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