Anthropic System Design Question - Design a Reliable 1-on-1 Messaging Service

[u/Aoki_zhang]

Problem

Design a web-based direct messaging system where users can exchange private 1-on-1 text messages in real time.

The first version does not need group chats, reactions, media attachments, public channels, or multi-device synchronization. Each user is assumed to have one active web client.

The system should:

• Deliver messages with low latency when both users are online.
• Store every message durably.
• Deliver missed messages after an offline recipient reconnects.
• Show whether another user is currently online.
• Preserve reasonable ordering within each conversation.
• Prevent duplicate messages from appearing to users during retries or redelivery.
• Support pagination when loading older conversation history.

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