Beyond API Compatibility: Building a Tool to Measure the Blast Radius of Library Changes

By Roberto Perez Alcolea , Java Platform Team Quasar connects a proposed API change with observed references in consumer repositories, giving the producer evidence for a release decision At Netflix, shared Java libraries support applications developed across a large, independently evolving repository ecosystem. A library change begins in one repository, but its impact can reach many others. Producers can see the API diff in front of them. What they often cannot see is who still depends on the symbols that changed. The need for that visibility grows as coding agents take on more of the software development lifecycle. An agent can accelerate a local edit, but without cross-repository context it cannot judge downstream risk or help plan consumer migrations. The evidence a producer uses to assess a change can also guide the agent working on it. Consider a common case: removing a deprecated API. Deprecated entry points expand the surface producers must test, document, and support, and they can constrain how a library evolves. Suppose a replacement has been available for several releases and the old method cannot express options required by new behavior. The library’s tests pass, and a compatibility checker flags the removal. The producer still cannot answer the questions that determine what to do next: Does any consumer still call this method? Are consumers concentrated on one version or spread across several generations? Does another library implement or wrap the API, multiplying its impact? Is direct coordination sufficient, or does this require a deprecation and migration campaign? At small scale, producers can ask nearby teams or search a handful of repositories. Across many repositories, it is harder to find every caller of a method and the library version each one uses. Those answers are scattered across consumer codebases and builds. Without that evidence, a producer may release a breaking change and discover the impact through consumer failures, or preserve an old API indefinitely because the risk is unknown. Libraries need to evolve, and their producers need evidence that helps them do so intentionally. We built Quasar to index symbol definitions and references across Netflix repositories. For library producers, that shared index helps answer two questions: What changed in the library’s public API? Which repositories use the exact symbols that changed? The first identifies a potentially incompatible change. The second estimates its observed blast …