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

1. 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.
2. 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.
3. 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.
4. 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.