Object-Oriented Design: Publish/Subscribe Notification Service

Design an object-oriented publish/subscribe notification service for user–topic subscriptions.

Provide the following APIs:

• addSubscription(userId, topicId)
• unsubscribe(userId, topicId)
• publishNews(topicId, newsId, payload)
• onNewsReceived(userId, newsId) // client acknowledgement

Requirements:

1. Delivery semantics
   • Each published item is delivered at most once to users who are subscribed at the time of delivery.
   • Unsubscribes prevent any future deliveries for that user–topic pair, including items published earlier but not yet delivered.
   • Per-topic per-user ordering: for a given user and topic, deliver items in the order they were published to that topic.
2. Design deliverables
   • Class/interface design for the service and its components.
   • In-memory and persistence data models.
   • Concurrency control (ordering, idempotency, race handling).
   • Failure handling (restarts, partial failures, retries policy).
   • Scalability plan to millions of users.
   • Time and space complexity analysis of core operations.

Assume:

• newsId is unique per topic (idempotency key).
• Per-topic ordering is defined by a monotonically increasing sequence (offset) assigned at publish time.
• If a user unsubscribes after a publish but before delivery, do not deliver that item.
• New subscriptions start from the current end of the topic (no historical replay).