core practice · system design prompt
In-process pub/sub bus
Build an in-memory event bus: publishers fire events, subscribers register handlers by topic. Decide sync vs async delivery, error isolation, and unsubscribe semantics.
The mechanism
The observer pattern decouples who emits an event from who listens. Publishers fire events, subscribers register handlers, and the event bus is the registry that connects them. The tradeoff: flows are no longer visible in a single call stack, so debugging gets harder.
Expected evidence
Bus interface (publish, subscribe, unsubscribe) + the subscriber registry data structure.
Try this drill in the practice workspace →
Hints when you need them
- What happens if one subscriber throws?
- Keep the bus simple — no ordering guarantee unless asked.