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Design a thread-safe event dispatcher

PurestorageSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSoftware Engineering Fundamentals

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This question evaluates understanding of concurrent programming and thread-safe design, including synchronization primitives, race-condition avoidance, listener lifecycle semantics, and event-delivery guarantees.

Problem: "Event fire" / event dispatcher with multithreading

Design an in-memory event system that supports registering listeners and firing events.

Core requirements

Implement an EventDispatcher (or EventBus) supporting:

  • subscribe(eventType, listener)
  • unsubscribe(eventType, listener)
  • fire(eventType, eventPayload) — invokes all listeners currently subscribed to eventType

Where listener is a callback/function.

Follow-ups (multithreading)

Assume subscribe, unsubscribe, and fire can be called concurrently from multiple threads.

Answer/discuss:

  1. What thread-safety guarantees will you provide? (e.g., “listeners registered before fire starts must be called exactly once”, handling concurrent unsubscribe, etc.)
  2. How will you avoid race conditions and deadlocks?
  3. How will you handle performance trade-offs under high read (fire) vs high write (subscribe/unsubscribe) workloads?
  4. Will fire be synchronous (caller thread) or asynchronous (thread pool / queue)? How does that change your design?

You may state reasonable assumptions (e.g., per-event-type ordering is or isn’t required; at-least-once vs exactly-once delivery within a single process).