Katkat
Back

Debug round-robin, DashMap, and simple cache

DoorDashSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

Last updated

This question evaluates skills in concurrent programming and stateful selection logic (round-robin load balancing), data structure invariants and debugging (custom hash map hashing vs equality, collision resolution, resizing and iterator correctness), simple in-memory caching strategies with TTL or size-based eviction, and unit test design.

You are given a service that routes requests to a list of nodes, each marked as either available or unavailable. The pickNode() function is intended to perform round-robin selection while skipping unavailable nodes, but a failing unit test shows it occasionally returns an unavailable node. Debug and fix the implementation: (

  1. maintain a global (thread-safe) index so selection does not reset per call; (
  2. ensure status checks are robust (e.g., use an enum or constant, not fragile string literals); (
  3. define behavior when all nodes are unavailable; and (
  4. update/add a unit test where one node is unavailable and one is available so the selector repeatedly returns the available node. As a separate debugging task, you are given a buggy custom hash map named DashMap. Diagnose and fix issues around key hashing vs equality, collision handling, resizing/rehashing, and iterator behavior. Finally, implement a very simple in-memory cache backed by a map with get/set and optional TTL or size-based eviction, and write basic tests for it.