Implement round-robin load balancer
Company: DoorDash
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: Medium
Interview Round: Onsite
Implement a load balancer that distributes incoming requests across N backend servers in strict round-robin order. Requirements:
(
1) Support addServer(id), removeServer(id), and nextServer() operations in O(
1) average time;
(
2) Skip servers marked unhealthy via markHealthy(id, isHealthy) without breaking round-robin fairness;
(
3) Ensure thread-safe behavior under concurrent calls to nextServer and server updates;
(
4) Discuss failure scenarios (e.g., a server crashes mid-selection) and how you would design health checks;
(
5) Provide unit tests and analyze time/space complexity.
Quick Answer: This question evaluates a candidate's ability to implement a strict round-robin load balancer with O(1) average operations while handling concurrent nextServer and update calls, health-check-driven skipping, and failure scenarios, testing competencies in data structures, concurrent programming, and fault-tolerant system behavior.