Design car rental schema and APIs

Q: Design car rental schema and APIs

This question evaluates data modeling, transactional integrity, concurrency control, REST API and workflow design, and system scalability within the car rental domain in the System Design category.

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 Retool.

Q: What role is this question designed for?

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

Question

System Design: Car Rental Platform

Objective

Design a car rental platform with:

Core entities and relationships (customer, vehicle, vehicle_type, branch/location, inventory/unit, reservation, rental/contract, payment, pricing rule, promotion, insurance, availability/hold, staff, damage report).
A relational database schema with primary/foreign keys, unique constraints, and essential indexes (e.g., availability lookups by location and date/time windows).
End-to-end workflows and consistency requirements for:
1. Searching availability by location and date/time window.
2. Creating, modifying, and canceling reservations with holds to prevent double-booking.
3. Pick-up (vehicle assignment, contracts, inspection) and return (mileage, fuel, damage, late fees).
4. Pricing (base rate, mileage, insurance add-ons, taxes, promotions).
5. Payments (authorization, capture, refunds).
3–5 representative REST endpoints with request/response fields (e.g., POST /search, POST /reservations, PATCH /reservations/{id}, POST /rentals/{id}/return).
Cross-cutting concerns: idempotency, pagination, authentication/authorization, concurrency control (row locks or optimistic versioning), and scaling (read replicas, partitioning by branch/region, caching hot availability queries).

Constraints and Assumptions

Use a relational database (e.g., PostgreSQL). Time stored in UTC; branch has local timezone metadata.
Availability is primarily per vehicle type at a branch; specific unit assignment occurs at pickup.
Overlaps and double-booking are prevented via transactional holds and constraints.

Design car rental schema and APIs

Quick Overview

System Design: Car Rental Platform

Objective

Constraints and Assumptions

Solution

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