Instacart·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say

May 2026

Summary

System design round at Instacart focused entirely on inventory management for e-commerce. The problem had a lot of moving parts and I felt like I was playing catch-up the whole time trying to cover schema, concurrency, and APIs without losing the thread.

Questions Asked (1)

Design an inventory management system for an e-commerce platform that handles stock updates, checkout reservations to prevent overselling, and a curbside pickup or warehouse picking workflow.

System DesignData ModelingAPI & Integrations

Author's notes

This was one question but it sprawled into like five different conversations.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design a high-level architecture that separates inventory data into available and reserved buckets. Focus on the checkout reservation flow to prevent overselling, using atomic operations and idempotency, and finally outline the picking workflow with real-time updates and consistency guarantees.

Pro tip: Emphasize the importance of idempotency and handling race conditions in reservation APIs, as overselling is a critical business risk. Also, discuss how to handle partial failures and timeouts in distributed transactions to maintain data consistency.

1. Clarify Requirements and Scale

Ask about expected traffic, inventory size, consistency needs, and whether overselling is absolutely prohibited. Understand the difference between online orders and curbside pickup workflows.

2. High-Level Architecture

Propose a microservices-based system with separate services for inventory, reservations, and picking. Use a database that supports atomic operations and consider caching for read-heavy inventory checks.

3. Data Modeling and Reservation Logic

Design tables for inventory (with available and reserved quantities) and reservations. Explain how to atomically decrement available and increment reserved using transactions or conditional updates.

4. Checkout and Reservation Flow

Detail the API endpoints for reserving items during checkout, including idempotency keys and expiration of reservations. Discuss how to handle concurrent requests and prevent overselling.

5. Picking and Fulfillment Workflow

Outline the process for warehouse picking or curbside pickup, including how pickers are assigned, how inventory is updated upon picking, and how to handle substitutions or out-of-stock items.

Key Points to Mention

Use of atomic operations (e.g., database transactions or Redis Lua scripts) to prevent race conditions during reservation.
Idempotency keys for reservation APIs to handle retries safely.
Reservation expiration and release mechanism to avoid dead stock.
Separation of available and reserved inventory to accurately reflect stock.
Event-driven architecture for real-time updates to inventory and picking status.
Handling of partial failures and compensating transactions (e.g., saga pattern) for distributed consistency.

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