← Robinhood Interview Insights

Robinhood·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Coding round at Robinhood for a software engineer role. One question, straightforward enough if you've worked with inventory tracking before.

Questions Asked (1)

Q1

Design a system to handle buying and selling fractional shares, including tracking and updating inventory state.

Algorithms & Data StructuresSystem Design
Author's notes

A hashmap gets you there.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design a high-level architecture that separates order handling from inventory management. Focus on data consistency and concurrency, using appropriate data models and transactional guarantees to handle fractional shares accurately.

Pro tip: Emphasize idempotency and exactly-once processing for order execution to prevent duplicate trades, and discuss how you would handle partial fills and reconciliation with external systems.

1. Clarify Requirements and Scale

Ask about expected throughput, latency, consistency requirements, and regulatory constraints. Determine if the system needs to support real-time trading, batch processing, or both.

2. Design High-Level Architecture

Outline components: API gateway, order service, matching engine, inventory service, and database. Decide on synchronous vs asynchronous communication and how to handle failures.

3. Data Model and Storage

Design schemas for orders, trades, and inventory. Use decimal types for fractional shares to avoid floating-point errors. Choose a database that supports ACID transactions for inventory updates.

4. Concurrency and Consistency

Implement locking or optimistic concurrency control to prevent race conditions when updating inventory. Consider using a distributed transaction or saga pattern if services are separate.

5. Scalability and Fault Tolerance

Discuss partitioning, replication, and caching strategies. Ensure idempotent operations and implement retries with exponential backoff. Plan for monitoring and auditing.

Key Points to Mention

  • Use of decimal or fixed-point arithmetic for fractional shares to avoid precision issues.
  • Idempotency keys for order submission and processing to prevent duplicate trades.
  • Transactional integrity for inventory updates, possibly using database transactions or distributed sagas.
  • Concurrency control mechanisms like optimistic locking or serializable isolation levels.
  • Event sourcing or change data capture for auditability and real-time inventory tracking.
  • Handling partial fills and order cancellation with proper inventory adjustments.

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