Start by clarifying requirements and scale, then design the core real-time messaging architecture with WebSockets and a persistent message store. Next, address group chat and message history retrieval, and finally design the email notification pipeline with a scheduled job that checks for inactive users with unread messages.
Pro tip: Emphasize trade-offs between consistency and availability for message delivery, and discuss how to avoid duplicate email notifications using idempotent processing and a last_notified_at timestamp.
Ask about expected user count, concurrent connections, message volume, and latency requirements. Confirm whether group chat is truly optional and how history should be stored/retrieved.
Propose a WebSocket-based service for 1:1 and group messaging, with a message queue (e.g., Kafka) for reliable delivery and a database (e.g., Cassandra) for message persistence.
Extend the design to support group chats by using a fan-out approach or a shared group message stream. For history, discuss pagination, indexing by conversation and timestamp, and caching recent messages.
Create a scheduled service that periodically queries for users inactive for 10+ days with unread messages, then sends email notifications via a queue to avoid duplicates and ensure scalability.
Discuss trade-offs like consistency vs. latency, handling offline users, message ordering, and failure recovery. Mention monitoring and alerting for the notification pipeline.
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