Netflix Develops Quasar to Measure Impact of Library Changes Across Repositories

The Challenge of Library Evolution at Netflix

Netflix operates with a large ecosystem of independently evolving Java repositories, where changes in one shared library can affect many others. Library producers can see API differences but often lack visibility into which downstream consumers depend on the symbols being changed. This challenge is amplified as automated coding agents become more prevalent, as they lack the cross-repository context to assess downstream risks or plan consumer migrations effectively.

Addressing the Impact of Deprecated APIs

A common scenario involves removing deprecated APIs. While removing old entry points can improve library evolution, producers face difficulties in determining the actual usage of these deprecated methods. Key questions include whether consumers still call the method, the distribution of consumer versions, and if other libraries re-implement or wrap the API, potentially multiplying its impact. Without this information, producers risk either releasing breaking changes or indefinitely preserving old APIs due to unknown risks.

Introducing Quasar for Impact Assessment

To address these challenges, Netflix built Quasar, a tool designed to index symbol definitions and references across all Netflix repositories. Quasar provides library producers with two critical pieces of information: what has changed in the library's public API and which repositories use the exact symbols that have changed. This combined insight transforms generic compatibility warnings into actionable evidence.

Enabling Informed Decisions

By providing a clear view of the "blast radius" of a change, Quasar allows producers to make informed decisions before merging or releasing updates. This capability helps prevent consumer failures and supports intentional library evolution, moving beyond the limitations of manual checks or small-scale repository searches.