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Design a Real-Time Chat Service for One-to-One and Group Messaging

MetaSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

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A system design question on a real-time chat service with one-to-one and group conversations, instant delivery to online users, offline catch-up, and history across devices. It tests connection routing, per-conversation ordering, idempotent sends, message storage, group fan-out, and read receipts.

Design a real-time messaging service for a social network's chat product. Users exchange text messages in one-to-one conversations and in group conversations. Recipients who are online receive messages immediately, recipients who are offline receive them when they reconnect, and a user's conversation history is available on every device they use.

Clarifying Questions

  • How many daily active users are there, how many messages are sent per day, and what is the largest group size?
  • Which features are in scope: delivery and read receipts, typing indicators, presence, media attachments, end-to-end encryption?
  • Must every one of a user's devices receive every message and share the full history?
  • How long is message history retained?
  • Should offline users receive push notifications?

Hints

Hint 1

Find the recipient's connection: Think about how the server locates the live connection of a recipient who may be attached to any one of many gateway machines, possibly from several devices.

Hint 2

Decide who sets the order: Decide what "in order" must mean for a conversation and which single component is allowed to assign that order.

Follow-up questions

  • How do you guarantee that no message is lost or duplicated when a client's connection drops in the middle of a send?
  • How would end-to-end encryption change the server design, especially for multiple devices and groups?
  • A group grows very large. What changes in fan-out, receipts, and notifications?
  • How would you implement "seen by" for group messages efficiently?