Requirements

Let's refine our design for an online auction system. Imagine you're building a platform where users can list items for sale and other users can bid on those items. The core challenge lies in managing the auction lifecycle, ensuring fairness, and handling concurrent bids efficiently. We want a robust system that can support different auction types, bidding strategies, and notification mechanisms. Consider aspects like: * **Auction Types:** Dutch auctions, Penny auctions, and standard English auctions. * **Bidding Strategies:** Automated bidding (proxy bidding), and the need to prevent bid sniping. * **Notification Systems:** Real-time updates for users involved in an auction. * **Concurrency:** Handling multiple users bidding on the same item at the same time to prevent race conditions and ensure the highest bid wins. # Requirements ## Functional Requirements * **User Management:** * Users should be able to register and log in. * Users should have profiles with information like name, contact details, and bidding history. * **Auction Creation:** * Users should be able to create auctions, specifying: * Item details (name, description, images). * Starting price. * Auction duration. * Auction type (English, Dutch, Penny). * The system should support different auction types, each with its specific rules (e.g., declining price in Dutch auctions, participation fees in Penny auctions). * **Bidding:** * Users should be able to place bids on active auctions. * The system should validate bids to ensure they meet the auction's requirements (e.g., minimum increment, above current highest bid). * The system should support proxy bidding (automatic bidding up to a user-defined maximum). * The system must handle concurrent bids correctly, ensuring only one bid is accepted at a time. * **Auction Lifecycle:** * Auctions should automatically start and end at the specified times. * The system should determine the winner at the end of the auction. * The system should handle edge cases like no bids being placed. * The system should notify the seller and the winner when an auction ends. * **Notifications:** * Users should receive notifications about: * Outbids on auctions they are participating in. * Auction end results (winning or losing). * Other relevant auction events. * **Reporting:** * Admin users should be able to generate reports on auction activity, user behavior, and system performance. ## Non-Functional Requirements * **Thread Safety:** * The system must be thread-safe to handle concurrent bids without data corruption or race conditions. This is critical for ensuring fairness and data integrity, especially within the bidding process. * **Extensibility:** * The system should be designed to easily support new auction types, bidding strategies, and notification methods without requiring major code changes. Use interfaces and abstract classes extensively. * **Modularity:** * The system should be composed of loosely coupled modules to improve maintainability and testability. Each module should have a well-defined responsibility. * **Testability:** * The system should be designed to be easily testable using unit tests and integration tests. Consider dependency injection and mocking frameworks. * **Scalability:** * While this is an LLD problem, consider that the design should be scalable to handle a large number of concurrent users and auctions. Think about how data structures are used and potential bottlenecks. * **Maintainability:** * Code should be clean, well-documented, and easy to understand. ## Core Entities * **User:** Represents a user of the auction system, with attributes like ID, username, password, contact information, and bidding history. * **Item:** Represents an item being auctioned, with attributes like ID, name, description, and images. * **Auction:** Represents an auction, with attributes like ID, item, seller, starting price, duration, auction type, current highest bid, and list of bids. * **Bid:** Represents a bid placed on an auction, with attributes like ID, user, auction, bid amount, and timestamp. * **AuctionType (Interface/Abstract Class):** Defines the behavior of different auction types (English, Dutch, Penny). Implementations would handle the specific rules for each type. * **BiddingStrategy (Interface/Abstract Class):** Defines the bidding strategy to be used (e.g., standard, proxy). Implementations would handle the logic for placing bids according to the chosen strategy. * **NotificationService (Interface):** Defines the interface for sending notifications to users. Implementations could use different notification methods (e.g., email, SMS, push notifications).

Design an Online Auction.

Requirements

Interview Simulation

Functional Requirements

💡 Interview Tip

Non-Functional Requirements

Core Entity Identification

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