Design a scalable parking lot system

Q: Design a scalable parking lot system

This question evaluates a candidate's system design competencies, including scalable architecture, concurrency-safe resource allocation, data consistency versus availability trade-offs, fault tolerance, real-time availability tracking, payment integration, and domain/API modeling.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Amazon.

Q: What role is this question designed for?

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

Question

System Design: Multi-Level Parking Lot Service

Context

Design a production-grade parking lot system for a large, multi-level facility. The system should support multiple vehicle and spot types, multiple entry/exit gates, real-time availability, ticketing, payments, and pricing rules. Assume the system may operate across many lots and must remain available under concurrent use.

Requirements

Functional
- Support multiple vehicle types (e.g., motorcycle, car, truck/van, EV) and spot types (compact, large, handicapped, EV-charging).
- Support multiple levels and multiple entry/exit gates per lot.
- Entry flow: issue ticket (or identify by license plate), assign or admit to a spot/group, start a parking session.
- Exit flow: compute charges based on pricing rules (hourly, daily), accept payment, end session, open gate.
- Real-time availability tracking by lot/level/spot type.
- Handle failure scenarios: lost tickets, payment failures, network issues, and full capacity.
Non-functional
- High availability and concurrency-safe spot allocation.
- Data consistency for session state and payments; reasonable eventual consistency for availability counters.
- Observability and auditability of financial transactions.
Deliverables
- Define core classes/entities and their relationships.
- Define key APIs for parking, un-parking, payments, and querying availability.
- Address concurrency control, data consistency, failure handling, and full-capacity behavior.
Assumptions (make minimal, explicit choices if needed)
- License plate recognition (LPR) is available but may be imperfect; tickets (QR/barcode) serve as fallback.
- Option A: “Free parking” model where the driver selects a spot; Option B: “Assigned spot” model at entry. Design should support both; default to Option A for throughput, with optional assignment.
- Pricing supports hourly and daily with configurable grace and rounding.

Design a scalable parking lot system

Quick Overview

System Design: Multi-Level Parking Lot Service

Context

Requirements

Solution

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