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Design a Read-Once Chat System

GrammarlySoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

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This question evaluates a candidate's competence in large-scale system design and distributed messaging, including API and data model design, read-once delivery semantics, consistency trade-offs, and operational concerns like scaling, partitioning, and failure handling.

Design a chat messaging system where one sender can select multiple recipients at send time, but the message is not a group chat message. Instead, the system creates separate recipient-specific deliveries.

Core requirements:

  • A sender can send the same message to many recipients at once.
  • Each recipient should only be able to view the message one time.
  • After a recipient reads the message, that recipient's copy must be deleted or made permanently unavailable.
  • One recipient reading a message must not affect another recipient's copy.
  • The system should support online delivery, offline recipients, retries, and large-scale traffic.

Please describe:

  • APIs
  • data model
  • high-level architecture
  • message fan-out strategy
  • read-once enforcement
  • deletion and retention behavior
  • scaling, load balancing, and partitioning choices
  • failure handling and consistency tradeoffs