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Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Stripe coding round for a software engineer position, focused on extending a load balancer simulation. The problem started simple enough but the SHUTDOWN requirement added real complexity around state cleanup and I had to think carefully about keeping everything consistent.

Questions Asked (1)

Q1

You're given a connection load balancer with round-robin assignment, per-target capacity limits, and basic connect/disconnect operations. Extend it to support a SHUTDOWN command that takes a target offline, evicts all its current connections in insertion order, and permanently removes it from future round-robin assignments. How do you handle the state cleanup?

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

The base system wasn't bad to reason through but SHUTDOWN is where it gets messy.

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AI HintsAI Generated

Suggested Approach

First clarify the data structures and invariants (e.g., round-robin pointer, per-target connection lists, capacity tracking), then walk through the SHUTDOWN operation step-by-step: mark the target as removed, evict connections in insertion order, update capacity and round-robin state, and ensure thread safety. Discuss trade-offs like lazy vs eager cleanup and how to handle concurrent operations.

Pro tip: Mention that you would use a doubly-linked list for each target's connections to achieve O(1) eviction in insertion order, and a concurrent hash map for targets to avoid global locks. Also, consider idempotency and error handling for repeated SHUTDOWN calls.

1. Clarify requirements and assumptions

Ask about concurrency, expected load, and whether SHUTDOWN is graceful or immediate. Confirm that eviction order is strictly insertion order and that the target is permanently removed.

2. Identify core data structures

Describe structures: a list of targets for round-robin (e.g., circular array or linked list), per-target connection queues (e.g., doubly-linked list for O(1) removal), and a map from target to its state (active/removed).

3. Design the SHUTDOWN algorithm

Outline steps: atomically mark target as removed to prevent new connections, then iterate its connection list in insertion order, closing each and updating global connection count. Finally, remove target from round-robin list and adjust the pointer if needed.

4. Handle concurrency and consistency

Discuss locking strategy: per-target locks for connection list and target state, and a global lock for round-robin list modifications. Ensure that in-flight connections are handled (e.g., wait for them to finish or force close).

5. Address edge cases and trade-offs

Cover idempotency (repeated SHUTDOWN), target not found, empty connection list, and round-robin pointer adjustment. Compare eager vs lazy eviction and memory reclamation.

Key Points to Mention

  • Use of a doubly-linked list per target to maintain insertion order and allow O(1) eviction.
  • Atomic marking of target as removed to prevent new connections during shutdown.
  • Updating the round-robin pointer correctly when removing a target (e.g., if pointer points to removed target, advance to next).
  • Thread safety: per-target locks vs global lock, and avoiding deadlocks.
  • Idempotency: handling multiple SHUTDOWN calls for the same target gracefully.
  • Trade-offs: eager eviction (immediate resource release) vs lazy eviction (deferred cleanup), and impact on latency.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.