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Design a real-time messaging system at 500M DAU — persistent WebSocket connections, guaranteed message delivery, offline queuing, message ordering, read receipts, and inbox at scale.

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• Requirements & Base Architecture

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  • Functional Requirements
  • Estimation
  • Non-Functional Requirements
  • API Design
  • Real-Time Comms
  • Base Architecture

• Deep Dives

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  • Database
  • Message Ordering
  • Offline Delivery
  • Message Status
  • Inbox
  • Caching
  • Peak Traffic
  • Fault Isolation

• Final Design & Observability

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  • Final Architecture
  • SLIs & SLOs
  • Measuring Latency
  • Measuring Availability
  • Alerting
  • Error Budget