ACL Service for Microservices

Design a centralized ACL (Access-Control List) micro-service that every other micro-service in the company can call to answer the question: “Can subject X do action Y on resource Z?” The system must support:

• 10 000 requests per second with p99 < 20 ms
• 5 M users, 50 M resources, 100 M distinct permissions
• RBAC (users → roles → permissions) and ABAC (attributes like geo, device, time)
• Hierarchical resources (folder/file, org/team/project, db/schema/table)
• Instant revocation: once an ACL is deleted anywhere in the hierarchy, no subsequent call may return ALLOW older than 1 s after the delete
• High availability: 99.99 % uptime, no single AZ failure may deny service
• Eventually-consistent reads for dashboards, strongly-consistent reads for sensitive operations (e.g., DROP TABLE)

You do not need to implement the policy language parser; assume policies are stored as tuples (subject, action, resource, allow/deny). Outline the APIs, storage layer, caching strategy, cache-invalidation mechanism, and how you would guarantee freshness and horizontal scale. Be prepared to discuss trade-offs (latency vs. consistency, storage cost vs. throughput, cache hit ratio vs. revocation delay).