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This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Optiver.

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This question is commonly asked for Software Engineer candidates at Optiver during technical interviews.

Design a subscription push service | Optiver Interview Question

Quick Overview

This question evaluates object-oriented system design skills and knowledge of publish/subscribe messaging, delivery semantics (at-most-once delivery, ordering, idempotency), concurrency control, persistence models, failure handling, and scalability in the System Design domain.

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:

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.
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).

Quick Overview

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:

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.

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).

Design a subscription push service

Quick Overview

Object-Oriented Design: Publish/Subscribe Notification Service

Solution

Comments (0)

Design a subscription push service

Quick Overview

Object-Oriented Design: Publish/Subscribe Notification Service

Solution

Comments (0)