I started with requirements and that part went fine.
Start by clarifying requirements and scale, then design a high-level architecture that separates real-time messaging (WebSockets) from persistent storage and push notifications. Focus on the message flow for 1:1 and group chats, and discuss trade-offs for delivery guarantees, presence, and read receipts.
Pro tip: Emphasize idempotency and ordering: use client-generated message IDs and per-conversation sequence numbers to handle retries and ensure consistent ordering, which is critical for a reliable chat system.
Ask about expected user base, message volume, latency requirements, and features like message editing or typing indicators. Define scope for 1:1 vs group chats and offline scenarios.
Propose a microservices-based architecture with separate services for connection management (WebSocket gateways), message handling, presence, and notifications. Use a message queue (e.g., Kafka) for asynchronous processing.
Design schemas for users, conversations, messages, and receipts. Choose databases: Cassandra for messages (high write throughput), Redis for presence and recent messages, and a relational DB for user metadata.
Explain how messages are routed: sender -> WebSocket gateway -> message service -> persistence -> fan-out to recipients via their gateways. Discuss delivery guarantees (at-least-once, idempotency) and ordering.
Detail presence tracking using heartbeats and Redis pub/sub. For read receipts, update per-conversation read markers. For offline users, trigger push notifications via APNs/FCM and store undelivered messages.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.