SnowFlake 统一配额服务系统设计题——高频系统设计面试（类似 Google/Apple/Microsoft 云存储）

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.

这是一道典型的 统一存储配额（Quota）系统设计题 ，类似 Google Drive、iCloud、OneDrive 的后端模式。多个服务（例如相册、文档、邮箱等）共享同一个配额，系统需要做到

• ✅ 跨服务的总配额一致性
• ✅ 并发写入不重复计数、不卡死
• ✅ 对每个服务的独立计数
• ✅ 面向客户端的查询与写入 API
• ✅ 高并发、高扩展

这道题考察的是你对分布式计数器、事务、锁机制、配额检查流程以及系统扩展性的理解，是高频的系统设计面试题。

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