Start by clarifying requirements and constraints (e.g., single machine, expected throughput, priority levels, delivery guarantees). Then design the core components: topic management, subscription handling, and a priority-aware message queue, discussing data structures and trade-offs. Finally, outline the implementation details and potential optimizations.
Pro tip: Emphasize the trade-offs between different priority queue implementations (e.g., heap vs. multiple queues) and how they affect latency, throughput, and fairness. Also, mention how you would handle slow subscribers to prevent them from blocking the system.
Ask questions to understand the scope: expected message volume, number of topics/subscribers, priority levels, delivery semantics (at-least-once, at-most-once), and persistence needs.
Outline the main components: a topic registry, subscriber registry, and a message broker that routes messages to subscribers based on topic and priority. Consider using a priority queue per subscriber or per topic.
Choose appropriate data structures: e.g., a min-heap or multiple FIFO queues for priorities, hash maps for topic/subscriber lookup. Discuss time complexity for publish and subscribe operations.
Describe how to implement the core operations: publish (enqueue with priority), subscribe (register and receive messages), and unsubscribe. Address concurrency (locks, thread-safe queues) and message delivery (push vs. pull).
Discuss trade-offs: e.g., strict priority vs. fairness, memory usage, and potential bottlenecks. Mention how the design could scale to multiple machines if needed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.