Design an online reservation system that supports creating, modifying, and canceling reservations while preventing double-booking. Specify APIs, data model, consistency guarantees, idempotency, concurrency control, and handling of capacity limits, waitlists, and expiration/TTL. Discuss trade-offs between alternative approaches (e.g., optimistic vs. pessimistic locking; relational vs. NoSQL; centralized vs. sharded inventory). Explain how you would scale the design as traffic grows (partitioning, caching, queues, rate limiting, backpressure, monitoring, failure recovery). Identify key security and privacy concerns and mitigations. Which deployment strategy (blue/green, canary, rolling) would you choose and why? When designing features or services in general, what principles and risks do you prioritize first?

This question evaluates a candidate's ability to design scalable, concurrent backend systems, covering data modeling, consistency and concurrency control, API design, capacity and waitlist management, as well as reliability and security considerations.

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This is a hard difficulty System Design question, commonly asked during Onsite rounds at Nordstrom.

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This question is commonly asked for Software Engineer candidates at Nordstrom during technical interviews.

Design and evolve a reservation system | Nordstrom Interview Question

System Design: Online Reservation System

You are designing a backend platform to book time-based slots for services or events (e.g., appointments, classes, tables). Each resource offers time slots with a fixed capacity. The system must prevent double-booking under high concurrency and support lifecycle operations.

Assume a single region to start, with the need to scale reads (availability lookups) more than writes (bookings). Payments are out of scope; focus on reservation integrity.

Requirements

Core capabilities
- Create, modify, and cancel reservations.
- Prevent double-booking and overselling capacity.
APIs
- Specify external-facing endpoints and request/response behaviors.
- Define idempotency strategy for write APIs.
Data model
- Entities, relationships, and key indexes/constraints.
Consistency and concurrency
- Consistency guarantees for reads/writes.
- Concurrency control approach to avoid double-booking.
Capacity, waitlists, and expiration
- Capacity checks, holds, waitlist mechanics, and TTL/expiration handling.
Trade-offs
- Optimistic vs. pessimistic locking.
- Relational vs. NoSQL stores.
- Centralized vs. sharded inventory.
Scaling and reliability
- Partitioning, caching, queues, rate limiting, backpressure.
- Monitoring, alerting, and failure recovery patterns.
Security and privacy
- Key risks and mitigations.
Deployment
- Choose between blue/green, canary, and rolling; justify your choice.
Product engineering principles

What principles and risks do you prioritize when designing features/services?

System Design: Online Reservation System

Assume a single region to start, with the need to scale reads (availability lookups) more than writes (bookings). Payments are out of scope; focus on reservation integrity.

Requirements

Core capabilities

Create, modify, and cancel reservations.
Prevent double-booking and overselling capacity.

APIs

Specify external-facing endpoints and request/response behaviors.
Define idempotency strategy for write APIs.

Data model

Entities, relationships, and key indexes/constraints.

Consistency and concurrency

Consistency guarantees for reads/writes.
Concurrency control approach to avoid double-booking.

Capacity, waitlists, and expiration

Capacity checks, holds, waitlist mechanics, and TTL/expiration handling.

Trade-offs

Optimistic vs. pessimistic locking.
Relational vs. NoSQL stores.
Centralized vs. sharded inventory.

Scaling and reliability

Partitioning, caching, queues, rate limiting, backpressure.
Monitoring, alerting, and failure recovery patterns.

Security and privacy

Key risks and mitigations.

Deployment

Choose between blue/green, canary, and rolling; justify your choice.

Product engineering principles

What principles and risks do you prioritize when designing features/services?

Design and evolve a reservation system

Quick Overview

System Design: Online Reservation System

Requirements

Solution

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Design and evolve a reservation system

Quick Overview

System Design: Online Reservation System

Requirements

Solution

Submit Your Answer