Stripe·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026Remote

Summary

Stripe SWE interview with a coding round focused on systems thinking as much as correctness. The problem looked like a simple round-robin scheduler but they kept pushing on extensibility and state design, which I was not fully prepared for.

Questions Asked (1)

Implement a connection load balancer that assigns incoming connections to servers in round-robin order. For each connection, output the connection ID and the assigned server index. The data structures should support future extensions like idempotent reconnects, disconnects, capacity limits, and server shutdowns.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the basic round-robin working pretty quickly, a pointer advancing mod the number of servers and a map from connection ID to server index.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a modular data structure that separates connection assignment from server state management. Implement round-robin using a circular index, and ensure the design supports future extensions like idempotent reconnects and capacity limits through clean interfaces.

Pro tip: Demonstrate foresight by discussing how to handle server failures and rebalancing connections without disrupting existing ones, showing you think beyond the basic algorithm.

1. Clarify Requirements and Constraints

Ask questions to understand expected scale, concurrency needs, and specific extension requirements like idempotent reconnects or capacity limits. This ensures your solution aligns with the interviewer's expectations.

2. Design Core Data Structures

Propose a Server class to track server state (e.g., active connections, capacity) and a LoadBalancer class that maintains a list of servers and a current index for round-robin. Use appropriate data structures like a circular array or modulo arithmetic for efficiency.

3. Implement Round-Robin Assignment

Describe the algorithm: for each incoming connection, assign the next server in sequence, skipping any unavailable servers (e.g., at capacity or shut down). Update the index accordingly, ensuring O(1) time per assignment.

4. Extend for Future Requirements

Explain how to modify the design to support idempotent reconnects (e.g., using a map from connection ID to server), disconnects (decrementing connection counts), capacity limits (checking before assignment), and server shutdowns (marking server inactive and rebalancing).

5. Analyze Trade-offs and Complexity

Discuss time and space complexity, potential bottlenecks (e.g., concurrency), and alternative approaches like weighted round-robin or consistent hashing. Highlight why your design is suitable for the given context.

Key Points to Mention

Round-robin algorithm with modulo arithmetic for O(1) assignment
Server state management including active connections, capacity, and health status
Idempotent reconnects via a connection-to-server mapping
Handling server shutdowns and rebalancing connections gracefully
Thread safety and concurrency considerations for high-throughput scenarios
Trade-offs between simplicity and extensibility, and potential future enhancements

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