Design a scalable inventory system

Last updated: Mar 29, 2026

Quick Overview

Design a scalable inventory system evaluates requirements, scale assumptions, API/data design, architecture, trade-offs, failure modes, and rollout in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

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Design a scalable inventory system

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Jul 31, 2025, 12:00 AM

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Design a scalable inventory system

System Design: Multi-Warehouse, Multi-Channel Inventory Service

You are designing an inventory service for an e-commerce platform that sells across multiple sales channels (e.g., web, marketplace, retail partners) and fulfills from multiple warehouses. The service must prevent overselling under concurrent load and support cart reservations and order commits.

Requirements

Scope and Responsibilities
- Track inventory at the granularity of SKU × warehouse.
- Support per-channel inventory policies (e.g., safety stock, backorder rules).
- Maintain real-time availability for browsing and checkout.
Core Operations and APIs
- Adjust: apply deltas from physical counts, transfers, damages, or WMS/ERP updates.
- Reserve: hold stock for a cart or pre-authorized order with TTL.
- Commit: finalize reservation on payment/ship confirmation.
- Release: free a reservation on cancel, expiration, or failure.
- Define request/response shapes, idempotency, and retry behaviors.
Data Model
- Entities for SKU, Warehouse, Inventory (with on_hand, reserved, inbound, safety_stock, damaged), and Reservation.
- Support fields for channel-specific constraints and policies.
Consistency and Concurrency
- Define invariants and the consistency model needed to prevent oversell under concurrent updates.
- Describe how you handle multi-warehouse reservations and cross-row transactions.
Idempotency and Retries
- Strategy to make all mutation APIs idempotent and safe under at-least-once delivery.
Eventing
- Publish stock change events for downstream systems (e.g., search, merchandising, marketplaces).
- Ensure ordering and delivery guarantees; describe the outbox/CDC approach.
Reconciliation with ERP/WMS
- How the service integrates with external systems, which system is the source of truth for physical stock, and how periodic reconciliation works.
Scaling and Partitioning
- Partitioning strategy, hot-key mitigation, read/write scaling, and latency targets.
- Caching and materialized views for browse availability.
Business Scenarios and Failure Handling
- Backorders and pre-sell against inbound.
- Returns and restocking flows.
- Degraded modes during outages (DB, message bus, ERP/WMS).

Deliver:

A clear data model.
API designs for adjust, reserve, commit, and release with idempotency.
Consistency model and concurrency control strategy.
Eventing model and reconciliation plan.
Scaling/partitioning approach.
Handling of backorders, returns, and outages.

Constraints & Assumptions

Preserve the scope, facts, inputs, and requested outputs from the prompt above.
If the prompt leaves a detail unspecified, state a reasonable assumption before relying on it.
Keep the answer interview-ready: concise enough to present, but concrete enough to implement or evaluate.

Clarifying Questions to Ask

Clarify users, core use cases, read/write patterns, scale, latency, availability, and data retention.
State explicit assumptions before making sizing or architecture decisions.
Prioritize the functional path first, then address reliability, security, observability, and rollout.

What a Strong Answer Covers

A scoped requirements summary with concrete non-goals and success metrics.
API, data model, architecture, consistency, capacity, and operations.
Reasoned trade-offs among simple and scalable designs, including bottlenecks and failure modes.
A validation, monitoring, migration, and launch plan appropriate for the risk level.

Follow-up Questions

What breaks first at 10x traffic or data volume?
How would you degrade gracefully during dependency failures?
What metrics and alerts would prove the design is healthy after launch?

Submit Your Answer to Earn 20XP

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