Google's Zanzibar authorization system powers permission checks across services like Drive, YouTube, and Photos, handling over 10 million requests per second while maintaining correctness at massive scale. Built before 2019, Zanzibar stores more than two trillion permission records across dozens of data centers worldwide, ensuring users only access content they're authorized to see.

Traditional access control lists become unwieldy at Google's scale, where a single user action might trigger hundreds of permission checks. Zanzibar solves this through a tuple-based data model that represents permissions as simple relationships between objects and users. The system uses namespaces to organize different types of resources and configuration languages to define complex permission rules through userset rewrites, enabling inheritance without data duplication.

A critical innovation addresses the 'new enemy' problem, where stale permission data could allow revoked users to access newly created content. Zanzibar's zookie protocol ensures freshness by encoding timestamps from Google Spanner's TrueTime technology, guaranteeing that permission checks reflect all prior changes. The architecture distributes over 10,000 servers across 30 geographic locations, with each cluster containing hundreds of servers that cooperate to answer authorization requests efficiently.