Design a pub-sub replay system

Q: Design a pub-sub replay system

This question evaluates a candidate's skills in distributed systems and streaming architectures, including pub-sub replay semantics, message ordering, storage and indexing strategies, scalability, and operational reliability.

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Question

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Assume you have a distributed publish-subscribe messaging system with topics, publishers that write messages, and subscribers that consume them. Design a replay feature that lets a subscriber re-read historical messages after a failure, a deployment, or when a new subscriber joins.

Your design should cover:

replay from a timestamp, message ID, or offset
replay for an individual subscriber or a subscriber group
message ordering guarantees within a partition
storage and retention of historical messages
high throughput for normal delivery while also supporting replay traffic
failure recovery, duplicate handling, and operational trade-offs

Discuss the APIs, data model, storage architecture, indexing strategy, scaling approach, and how replay works end to end.

Design a pub-sub replay system

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