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

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

How Netflix Accurately Predicts the Blast Radius of Library Changes Across Hundreds of Repositories

Inside Quasar: Measuring Cross-Repository API Impact Using SCIP and Amazon Athena

Removing deprecated APIs in a massive multi-repository ecosystem often leads to unexpected downstream failures. Netflix solved this by creating Quasar, a tool that indexes code symbols across repositories to map out actual consumer dependencies. This article explores how Quasar transforms generic API diffs into actionable evidence during code reviews and provides context for AI coding agents.

  • 01Language-agnostic symbol indexing across repositories built on SCIP format
  • 02Fast cross-repository reference querying with Amazon S3 (Parquet) and Athena
  • 03Automated pull-request reports showing affected repo counts and version distributions
  • 04Model Context Protocol (MCP) integration providing semantic context to AI coding agents
  • 05Multi-language support tracing Protobuf definitions through generated Java code

RECOMMENDATION

Essential reading for platform engineers managing shared libraries in large multi-repo architectures and DevOps engineers embedding AI agents into CI/CD workflows.

The Problem

When library producers update or deprecate public APIs, traditional compatibility tools cannot identify which downstream consumer repositories still rely on the changed symbols, masking the true blast radius.

The Solution

Netflix built Quasar, an internal system leveraging SCIP to index symbol definitions and references across repositories, using Amazon S3 (Parquet) and Athena to query exact consumer usage during pull requests.

The Result

Library maintainers can inspect affected repository counts, library version distributions, and exact call locations during code review, preventing breaking changes and enabling safe migration plans prior to release.

Trade-off

Dynamic reflection, incomplete indexing, or stale commits can hide actual usages, making index freshness, coverage tracking, and explicit confidence reporting essential.

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Key Concepts

Concept · 01

SCIP (Sourcegraph Code Intelligence Protocol)

An open interchange format designed to record code definitions, references, and relationships across programming languages using stable symbol identifiers.

  • Used by Quasar to index code symbol definitions and usages from enrolled CI builds into a shared graph
  • Enables cross-repository code navigation and dependency linking across polyglot ecosystems
Concept · 02

Blast Radius

The extent of downstream impact or potential breakage across consumer applications resulting from a library API edit or removal.

  • Quantified in Quasar as the number of distinct caller repositories and their active library version tiers
  • Evaluated during pull requests to empower maintainers with actionable data before merging breaking edits
Concept · 03

MCP (Model Context Protocol)

An open standard that enables AI models and autonomous agents to seamlessly interface with external databases, APIs, and dev tools.

  • Powers Quasar's MCP server so AI coding agents can check API blast radius during active development
  • Allows agents to proactively plan and automate downstream consumer migrations with precise call-site evidence