Design a configuration subscription service

Q: Design a configuration subscription service

This is a System Design interview question from Amazon for Software Engineer roles. View the full question and solution on PracHub.

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Question

System Design: Configuration Service with Subscriptions

Context

Design a multi-tenant configuration service that stores key–value configuration and lets applications subscribe to changes for specific keys or namespaces (prefixes). When a value changes, subscribers must be notified promptly and reliably.

Requirements

API surface
- set
- get
- subscribe
- unsubscribe
Notification semantics
- Delivery guarantees (at-least-once vs at-most-once)
- Ordering guarantees
- Debouncing/coalescing
Versioning and change management
- Versioning of keys
- Rollbacks
- Handling partial failures
Correctness
- Thread safety (client library and server)
- Concurrency control for writes
Durability and recovery
- Persistence and backups
Security
- Access control and auditing
Scale and performance
- Scale to thousands of clients
- Fan-out strategy
- Caching strategy
- Protocol choice: long polling vs WebSocket (and/or SSE)
- Time/space complexity of core operations

Assume namespaces are path-like prefixes (for example, /env/prod/serviceA/). Specify assumptions as needed.

Design a configuration subscription service

System Design: Configuration Service with Subscriptions

Context

Requirements

Solution (Locked)

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