Design Car Rental System
A car rental system is a service or software platform that allows customers to book, use, and return vehicles for a temporary period in exchange for payment.
It helps people rent cars for a few hours, days, or weeks without owning them.
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In this chapter, we will explore the low-level design of a car rental system in detail.
Let's start by clarifying the requirements:
1. Clarifying Requirements
Before starting any design, it's important to ask thoughtful questions to uncover hidden assumptions, clarify ambiguities, and define the system's scope.
Here is an example of how a discussion between the candidate and the interviewer might unfold:
Candidate: "Is this a single-location rental agency or a multi-location company? Can customers return cars to a different location than where they picked up?"
Interviewer: "Multi-location. Customers should be able to pick up from one location and return to another."
Candidate: "Should we support different vehicle categories with different pricing? How granular should the categories be?"
Interviewer: "Yes. Support categories like Economy, Compact, SUV, Luxury, and Van. Each category has its own daily rate."
Candidate: "When a customer makes a reservation, do they reserve a specific car or just a vehicle type? When is the actual car assigned?"
Interviewer: "They reserve a vehicle type. The specific car gets assigned at pickup time, based on what's available at the location."
Candidate: "Should we support additional equipment rentals like GPS, child seats, or insurance?"
Interviewer: "Yes, those should be add-ons with their own daily rates that get included in the final bill."
Candidate: "How should pricing work? Is it always a flat daily rate, or should we support different pricing models like weekend rates or seasonal pricing?"
Interviewer: "Let's support configurable pricing strategies. Standard daily rate is the default, but we should be able to swap in weekend pricing or other models."
Candidate: "Do we need to handle concurrent reservations? Two customers trying to book the last SUV at the same location at the same time?"
Interviewer: "Yes, the system should be thread-safe. Only one reservation should succeed when inventory is limited."
Candidate: "Should we handle late returns? What about vehicle maintenance after returns?"
Interviewer: "Yes to both. Late returns should incur additional fees, and we should track vehicle status through maintenance before it becomes available again."
After gathering the details, we can summarize the key system requirements.
- Support multiple rental locations where vehicles are stored and managed
- Support different vehicle types (Economy, Compact, SUV, Luxury, Van) with distinct daily rates
- Allow customers to make reservations by vehicle type, pickup location, and date range
- Support different pickup and return locations (one-way rentals)
- Assign a specific vehicle at pickup time based on availability at the location
- Track vehicle status through its lifecycle: Available, Reserved, Rented, Under Maintenance
- Support add-on equipment (GPS, Child Seat, Insurance) with daily rates
- Calculate rental cost based on duration, vehicle type, equipment, and pricing strategy
- Support configurable pricing strategies (standard, weekend)
- Handle late returns with additional fees
- Notify observers when reservations are created, vehicles picked up, and vehicles returned
- The design should follow object-oriented principles with clear separation of concerns
- The system should handle concurrent reservation requests without double-booking
- The system should be modular and extensible to support future enhancements
- The code should be thread-safe for concurrent access
- The components should be testable in isolation