Companies like Google, Apple, Microsoft, etc. offer various services such as drive, photos, mail, and documents.
Typically, they provide users with a single shared quota. For example, if you purchase a 100GB storage plan,
all these services will consume capacity from the same quota bucket.
Under the hood, such systems usually look like this:
+--------------------------+
| QUOTA SERVICE |
+--------------------------+
/ | \
/ | \
S1 Storage S2 Storage ...Applications and services consume from the shared quota through this centralized service.
Your task:
Design a quota service that supports multiple independent services (S1, S2, …),
all sharing the same quota limit, ensuring consistency, concurrency safety, and accuracy.
This question asks you to design a unified quota management service similar to those used by cloud providers like Google Drive, iCloud, and OneDrive. All different applications (photos, documents, mail, etc.) share a single storage quota, and the system must ensure:
- ✅ Correct usage tracking across multiple services
- ✅ Strong concurrent consistency (no double counting)
- ✅ API for checking and consuming quota
- ✅ Isolation between services while sharing the same quota
- ✅ Scalability to millions of users and operations
This is a system design challenge focused on distributed counters, concurrency control, storage metadata management, and API design.
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