The base round-robin part was fine, I had that working pretty quick.
First, clarify the current round-robin implementation and the meaning of 'already-connected ID'—likely a client or session ID that maps to a backend server. Then propose adding a map from ID to assigned server, checking it before advancing the round-robin pointer, and returning the stored assignment if present. Finally, discuss trade-offs like memory overhead, thread safety, and whether to expire mappings.
Pro tip: Emphasize that idempotency here is about preserving the original assignment and not disturbing the round-robin sequence, which is crucial for predictable load distribution and debugging. Mention that you'd consider making the map bounded or TTL-based to avoid unbounded memory growth in long-running services.
Ask whether the ID is client-provided or generated, what 'already-connected' means (e.g., active session), and whether the mapping should persist across restarts. Confirm that idempotency means returning the same backend without side effects.
Propose a hash map (dictionary) from connection ID to backend server index or reference. Ensure it supports O(1) lookup and insertion, and consider thread-safety if the load balancer is concurrent.
On CONNECT, first check if the ID exists in the map. If yes, return the stored assignment immediately. If no, perform the normal round-robin selection, store the mapping, and then return the assignment.
Discuss handling of disconnections (removing the mapping), memory growth (using TTL or LRU), and concurrency (locking or lock-free structures). Also consider what happens if a backend goes down.
Compare the map approach with alternatives like consistent hashing or storing state on the client. Discuss impacts on load balancing fairness, latency, and complexity.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.