Grokking Microservices for System Design Interviews

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Case Study: A Real-Time Chat and Presence System

1. The Question

"Design a real-time chat system with presence. One-to-one and group conversations, delivery and read receipts, and a reliable indicator of who is online. Say 10 million concurrent connections."

Every service so far could be restarted without a user noticing. Here the connection layer is the whole problem, and five dimensions are new.

  • Stateful nodes. The connection layer holds state no database can rebuild. Restarting one node drops a hundred thousand people.
  • **Routing to a machine, not to a service

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