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Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Stripe coding round for a software engineer role. The problem was a load balancer simulation with some state management twists. Not the hardest thing I've done but there were a few edge cases that tripped me up mid-way.

Questions Asked (1)

Q1

You're given a connection load balancer that assigns connections to servers via round-robin. Extend it to support a DISCONNECT operation that removes a connection, decrements the target server's connection count, and handles edge cases like disconnecting an unknown ID or reconnecting after a disconnect.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

The base round-robin part was fine, I had that sketched out quickly.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and data structures, then walk through the DISCONNECT operation step-by-step, covering edge cases and trade-offs. Emphasize correctness, efficiency, and how you would test the solution.

Pro tip: Discuss how you would handle concurrency and idempotency, especially in a distributed system like Stripe's, to show you think beyond the basic algorithm.

1. Clarify Requirements and Assumptions

Ask questions to confirm the data structures (e.g., map of connection ID to server, server connection counts) and expected behavior for edge cases like unknown IDs or duplicate disconnects.

2. Design the Data Structures

Propose using a hash map to track connection ID to server mapping and a counter per server, ensuring O(1) lookup and updates.

3. Implement DISCONNECT Logic

Outline the steps: check if connection ID exists, if not handle gracefully; otherwise, retrieve server, decrement its count, remove mapping, and update round-robin state if needed.

4. Address Edge Cases and Reconnection

Explain how to handle disconnecting an unknown ID (return error or no-op), and how reconnecting after disconnect works (new connection ID or reuse).

5. Discuss Trade-offs and Testing

Mention time/space complexity, potential concurrency issues, and how you would test the implementation with unit tests covering normal and edge cases.

Key Points to Mention

  • Use a hash map for O(1) connection lookup and a counter per server.
  • Handle unknown connection ID gracefully (e.g., return false or throw exception).
  • Ensure round-robin state remains consistent after disconnects.
  • Consider idempotency: disconnecting the same ID twice should not corrupt state.
  • Discuss concurrency: use locks or atomic operations if multiple threads.
  • Reconnection: assign a new connection ID or reuse if allowed, and update server count accordingly.

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