Block, Inc. migrated approximately 450 JVM repositories from a polyrepo model into a single monorepo spanning Cash App and Square engineering teams. The move addressed dependency version drift, diamond dependency issues, and cross-team coordination overhead. The monorepo now handles ~8,800 builds per week with p90 CI times of ~10 minutes. Key technical investments include a custom IntelliJ plugin that loads only relevant projects (swapping others for published JARs), dependency graph–based selective CI builds, Gradle convention plugins enforcing module boundaries, merge queues for main branch stability, and git performance tuning. The migration took longer than planned due to CI scalability challenges and projects with slow or custom builds. Block advises against monorepos when platform team investment is lacking or when projects span very different languages and frameworks.
