Thread-safe Organization Hierarchy: API, Concurrency, and Consistency

Context

You are building an in-memory organization directory that manages groups (hierarchical) and users. Multiple threads will concurrently read and mutate the hierarchy. The workload is read-heavy with periodic updates.

Assume:

• Groups form a tree (or forest) identified by stable IDs; groups can have child groups.
• Users have unique IDs and can be members of multiple groups.
• The service runs in a single process but must be crash-safe via durable storage.

Task

Design thread-safe add/remove operations for users and groups.

1. Propose a minimal API for:
   • Creating/deleting users and groups
   • Adding/removing a user to/from a group
   • Moving a group under a new parent
   • Querying users, groups, and memberships
2. Propose a concurrency strategy (e.g., reader–writer locks, fine-grained locking with lock ordering, immutable snapshots with copy-on-write, or MVCC/RCU). Specify:
   • How writes occur atomically
   • How readers are isolated from in-progress writes
   • Deadlock avoidance strategy (if using locks)
3. Consistency guarantees:
   • How to guarantee linearizable writes (or at least read-consistent snapshots) while updates occur
   • How to serve long-running reads without blocking writers
4. Discuss:
   • Contention hot spots and performance trade-offs of your approach
   • Failure recovery (durability model, recovery steps)
   • How to test concurrency correctness (stress tests, race detection, linearizability/snapshot checks)