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Interview Tasks — Load Balancer

Sequential user stories, one at a time, each finished with tests before the next. Budget 30 minutes for tasks 1 to 4, task 5 if time remains.

Task 1: Register (6 min)

Story: A load balancer holds up to N instance addresses. register(address) adds one and returns true; it returns false when the address is already present or the balancer is full.

Acceptance Criteria:

  • Capacity of 0 or less is rejected in the constructor
  • Duplicates return false and do not change the size
  • null is rejected

Hints:

  • Ask: "Is an address a String, or should I model it?" A String is fine; say why
  • A Set gives uniqueness for free; which Set, and why, is the follow-up

Task 2: Unregister (3 min)

Story: unregister(address) removes one and returns whether it was present.

Acceptance Criteria:

  • Removing an unknown address returns false
  • Removing frees capacity for a new registration

Task 3: Get (6 min)

Story: get() returns one registered address at random. With none registered it throws.

Acceptance Criteria:

  • Empty balancer throws a specific exception
  • Only registered addresses are ever returned
  • Over many calls every address is eventually returned

Hints:

  • Ask: "Equal probability?" and "Does the caller need to know which one?"
  • ThreadLocalRandom rather than a shared Random

Task 4: Strategy (8 min)

Story: Make the selection algorithm pluggable: random and round robin. Round robin must cycle through all addresses in registration order and wrap around.

Acceptance Criteria:

  • A SelectionStrategy interface with two implementations
  • Round robin over a, b, c yields a, b, c, a, b
  • Round robin adapts when an address is unregistered mid-cycle

Hints:

  • Registration order means an ordered set: LinkedHashSet
  • Math.floorMod keeps the counter correct after it wraps

Task 5: Many Threads (7 min)

Story: Threads register, unregister and get at the same time. Nothing may throw a ConcurrentModificationException, and round robin must stay correct.

Acceptance Criteria:

  • A test with 10 threads hammering all three operations completes without exceptions
  • Round robin over three addresses under 6 concurrent threads distributes exactly evenly

Hints:

  • The critical sections are tiny and contention is low: synchronized is the right tool, and saying when you would switch to a ReadWriteLock or CopyOnWriteArraySet is the point

Questions to Ask Afterwards

  • What is the time complexity of your duplicate check? How does HashSet.contains work? (The word they want is buckets, then equals inside the bucket. Candidates have been rejected for saying "memory addresses".)
  • What do equals and hashCode have to do with each other?
  • Why LinkedHashSet and not HashSet or ArrayList?
  • When is synchronized not enough?

Tips for Interviewers

  • This round is about code quality and communication. A candidate who writes a perfect algorithm silently scores lower than one who narrates a simple one.
  • Ask for the tests before the next story, every time.