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Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Stripe coding round for a software engineer role, one question but it had enough layers to keep me busy for the whole session. The base problem was manageable but the capacity extension is where things got interesting.

Questions Asked (1)

Q1

You're given a connection load balancer with round-robin assignment and basic connect/disconnect support. Extend it so each target server has a max connection capacity, and the round-robin skips full targets cyclically. If all targets are full, silently drop the connection.

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

I got the basic round-robin part going pretty quickly, the connId map and next_target pointer are fairly standard stuff.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then outline a data structure that tracks connection counts per target and supports efficient round-robin skipping. Walk through the algorithm for assigning connections, handling full targets, and dropping when all are full, and discuss trade-offs like time complexity and concurrency.

Pro tip: Mention that the round-robin pointer should advance only when a connection is successfully assigned, and that a full cycle without success indicates all targets are full—this avoids unnecessary scanning and shows attention to efficiency.

1. Clarify requirements and constraints

Ask about expected scale, concurrency, and whether targets can be added/removed dynamically. Confirm that 'silently drop' means no error is returned to the client.

2. Design the data structures

Propose a circular list or array of targets, each with a current connection count and max capacity. Maintain a pointer to the next target to consider.

3. Define the assignment algorithm

Starting from the pointer, check each target cyclically: if it has capacity, assign the connection, increment its count, and advance the pointer to the next target. If a full cycle completes without finding capacity, drop the connection.

4. Analyze complexity and trade-offs

Discuss time complexity: O(n) worst-case per assignment when many targets are full, but typically O(1) amortized. Mention alternatives like maintaining a separate list of available targets for O(1) assignment, at the cost of extra bookkeeping.

5. Address concurrency and edge cases

Explain how to handle concurrent connections (e.g., locks, atomic counters) and edge cases like zero targets, all targets full, and dynamic capacity changes.

Key Points to Mention

  • Use a circular pointer to implement round-robin skipping of full targets.
  • Track per-target connection counts and max capacity; increment on connect, decrement on disconnect.
  • Detect 'all full' by completing a full cycle without finding a target with capacity.
  • Time complexity: O(n) worst-case, O(1) amortized; can optimize with a separate available-targets list.
  • Concurrency considerations: use locks or atomic operations to avoid race conditions.
  • Trade-offs: simplicity vs. performance, and how to handle dynamic target addition/removal.

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