Start by clarifying requirements and scale, then sketch a high-level architecture with TCP connection handling, a message broker, and persistent storage. Walk through the data model and protocol design, and discuss trade-offs for reliability, reconnection, and persistence.
Pro tip: Emphasize idempotency and message ordering—use client-generated message IDs and server-side sequence numbers to handle reconnection and deduplication gracefully. Also, mention that you'd start with a simple design and iterate based on metrics, showing pragmatic engineering.
Ask about expected number of concurrent users, message throughput, latency requirements, and persistence guarantees. Define functional requirements like slash commands, chat rooms, history, and reconnection.
Outline components: TCP gateway for connection handling, a message broker (e.g., Redis Pub/Sub or Kafka) for fan-out, a database for persistence, and a service for slash command processing. Consider load balancers and horizontal scaling.
Design a simple length-prefixed or newline-delimited JSON protocol over TCP. Define data schemas for users, rooms, messages, and commands. Discuss indexing for efficient history retrieval.
Explain how clients reconnect and resume: use session tokens, last-received message ID, and server-side message buffers. Ensure at-least-once delivery with deduplication.
Choose storage (e.g., PostgreSQL for messages, Redis for presence) and discuss trade-offs: consistency vs. availability, latency vs. durability, and cost. Mention sharding and replication for scale.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.