Requirements

Imagine you're building an e-commerce platform, and you need a robust system to manage coupons and promo codes. This system shouldn't be tied to specific products or users. It should be flexible enough to support various discount types (percentage, fixed amount, free shipping) and different application scenarios (apply to the entire order, specific items, or categories). Further, the marketing team wants to run A/B tests on different coupon strategies, so the system needs to support that with minimal code changes. The scalability of this system is critical, as the number of coupons and users can grow exponentially. Therefore, designing with performance and maintainability in mind is key to our success. Let's dive into building a generic and extensible coupon/promo code system that can meet these challenges. # Requirements ## Functional Requirements 1. **Coupon Creation:** * Support different coupon types: Percentage discount, Fixed amount discount, Free shipping. * Allow specifying the discount value (e.g., 10% off, $5 off). * Allow specifying a coupon code (e.g., "SUMMER10") or generating a unique code automatically. * Set a usage limit for each coupon (total uses and/or uses per user). * Set a start and end date for the coupon validity. * Apply coupons to: * The entire order. * Specific products (identified by product ID). * Specific product categories (identified by category ID). * Set minimum order value for coupon eligibility. * Restrict coupon usage to specific user groups (e.g., new customers, premium members). * Implement coupon stacking rules: Determine if multiple coupons can be applied to the same order, and define priority if needed. * Support for 'buy X get Y free' type promotions. * Coupons can be flagged as "exclusive," preventing combination with other non-exclusive coupons. * Ability to define complex conditions (e.g., spend X amount on category A and get Y% off on category B). * Support for hierarchical categories, for instance, if a coupon applies to "Electronics," it should also apply to "Smartphones," which is a subcategory. 2. **Coupon Application:** * Apply a coupon to an order and calculate the discount. * Validate coupon eligibility based on the above-mentioned criteria (date, usage limits, product restrictions, user group restrictions, minimum order value, complex conditions). * Handle cases where a coupon is invalid or cannot be applied (e.g., expired, usage limit reached, conditions not met). * Provide informative messages to the user about why a coupon was/was not applied. * Log all coupon applications and their outcomes for auditing and reporting. 3. **Coupon Management:** * Ability to create, update, and delete coupons. * Search and filter coupons based on various criteria (code, type, status, validity dates). * Retrieve coupon details by code or ID. * View coupon usage statistics (total uses, uses per user). * Disable/enable coupons. 4. **A/B Testing:** * Ability to create A/B tests for coupons (e.g., test different discount values or restrictions). * Randomly assign users to different coupon variants. * Track the performance of each coupon variant. * Analyze A/B test results to determine the best-performing coupon strategy. ## Non-Functional Requirements 1. **Extensibility:** The system should be easily extensible to support new coupon types, application scenarios, and validation rules without modifying existing code. This implies adherence to the Open/Closed Principle. 2. **Maintainability:** The code should be well-structured, modular, and easy to understand, making it easy to maintain and debug. SOLID principles are crucial here. 3. **Testability:** The system should be designed to be easily testable, with clear separation of concerns and well-defined interfaces. Unit tests, integration tests, and end-to-end tests should be feasible. 4. **Thread Safety:** The coupon application logic should be thread-safe to handle concurrent requests from multiple users without data corruption or race conditions. Consider scenarios where multiple users are applying coupons simultaneously. 5. **Performance:** The coupon application logic should be efficient and performant, especially when dealing with a large number of coupons and users. Avoid N+1 queries and optimize data access patterns. 6. **Modularity:** The system should be composed of loosely coupled modules with well-defined interfaces, making it easier to develop, test, and deploy individual components independently. 7. **Scalability:** The system should be able to handle a large number of coupons and users without significant performance degradation. Consider using caching mechanisms to improve performance. (Caching at the class level, *not* a distributed cache). ## Core Entities 1. **Coupon:** Represents a coupon or promo code. * Attributes: `couponId`, `code`, `type`, `discountValue`, `startDate`, `endDate`, `usageLimit`, `usesRemaining`, `target`, `conditions`, `exclusive`. * Methods: `isValid()`, `applyDiscount()`, `recordUsage()`. 2. **DiscountStrategy (Interface):** Defines the strategy for calculating the discount based on the coupon type. Implementations would be `PercentageDiscountStrategy`, `FixedAmountDiscountStrategy`, `FreeShippingDiscountStrategy`. 3. **Target:** Represents the scope to which the coupon applies (e.g., entire order, specific products, categories). Can be an interface or an enum with relevant data. 4. **Condition (Interface):** Represents a condition that must be met for the coupon to be valid (e.g., minimum order value, user group restriction, specific product purchase). Implementations would be `MinimumOrderValueCondition`, `UserGroupCondition`, `ProductCondition`. 5. **Order:** Represents a customer order. * Attributes: `orderId`, `items`, `totalAmount`, `userId`. * Methods: `addItem()`, `removeItem()`, `calculateTotal()`, `applyCoupon()`. 6. **CartItem:** Represents an item in the customer's cart. * Attributes: `productId`, `quantity`, `price`. 7. **CouponValidator (Interface):** Defines methods for validating if a coupon is applicable to a given order and user. Implementations could include `DateValidator`, `UsageLimitValidator`, `ProductValidator`, etc. This promotes the Single Responsibility Principle. 8. **CouponService:** Orchestrates the coupon application process, including validation and discount calculation. 9. **ABTest:** Represents an A/B test configuration for a coupon. * Attributes: `testId`, `couponVariants`, `trafficAllocation`. * Methods: `assignUserToVariant()`, `trackPerformance()`. 10. **Locking Strategy (Interface):** Defines a strategy for thread safety. Implementations can be Optimistic or Pessimistic locking using `java.util.concurrent.locks`.

Design a generic coupon/promocode system.

Requirements

Interview Simulation

Functional Requirements

💡 Interview Tip

Non-Functional Requirements

Core Entity Identification

Premium Content