Zanzibar is Google's authorization system, one service that decides who can see a photo, a drive link or a document.
Hundreds of systems inside Google need authorization, and Google had five goals in mind when building it:
- A uniform data model and configuration language. If the data model changes, the older systems have to migrate and change their API connections.
- Causal ordering of actions. Gaurav adds 10 rupees to his account and Zoom deducts 10 rupees from it. The result of the second operation depends on the first one, so the two are ordered. Arpita's account is independent, and a total order would be wasteful.
- Massive scale. Trillions of access control lists, millions of requests per second and billions of end users.
- Very low latency. 10 milliseconds is the 95th percentile, so of a hundred people 95 are served within 10 milliseconds.
- Five nines of availability. The failure rate is the failures over the total, and it is subtracted from hundred percent.
Most of the paper talks about the scale.