What is Low Level Design in system design?

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Low Level Design (LLD) is the second phase of system design that takes the architectural blueprint from High Level Design and expands it into detailed, implementation-ready specifications. While HLD answers "what" the system should do, LLD answers "how" each component will do it. LLD specifies class structures, database schemas, API endpoints, algorithm implementations, error handling, and the precise interactions between modules. This phase produces documents that developers can directly use to write code, ensuring consistency and reducing ambiguity during implementation. LLD focuses on a single component or module at a time. For each component defined in the HLD, LLD defines its internal structure, the data models it uses, its public interfaces, and how it handles edge cases and errors. Common outputs include Unified Modeling Language (UML) diagrams (class diagrams, sequence diagrams, activity diagrams), database schema designs, API specifications, and pseudocode for complex algorithms. LLD also addresses concurrency, transaction boundaries, and performance optimizations at the code level. **Class Diagrams**: UML diagrams showing classes, their attributes, methods, relationships (inheritance, composition, aggregation), and visibility levels (public, private, protected). **Sequence Diagrams**: Detailed interaction flows between objects, showing method calls, return values, and timing for specific use cases. **Database Schema**: Complete table definitions with columns, data types, primary and foreign keys, indexes, constraints, and relationships. **API Specifications**: REST endpoints with HTTP methods, request/response payload structures, authentication requirements, error codes, and status codes. **Algorithm Design**: Pseudocode or flowcharts for complex business logic, sorting, searching, or data processing algorithms. **Error Handling Strategy**: How the component handles failures, exceptions, retry logic, circuit breakers, and fallback mechanisms. **Concurrency Model**: Locking strategies, transaction isolation levels, optimistic vs pessimistic concurrency control, and deadlock prevention. **Data Validation**: Input validation rules, sanitization requirements, and boundary condition handling. ## Low Level Design: Cart Service ### 1. Class Design ```java public class CartService { private final CartRepository cartRepository; private final ProductClient productClient; private final RedisCache cartCache; public Cart getCart(String userId) { ... } public Cart addItem(String userId, CartItem item) { ... } public Cart removeItem(String userId, String itemId) { ... } public Cart updateQuantity(String userId, String itemId, int quantity) { ... } public void clearCart(String userId) { ... } } public class Cart { private String userId; private List items; private LocalDateTime lastUpdated; private BigDecimal totalPrice; // getters, setters, business logic } public class CartItem { private String productId; private String name; private int quantity; private BigDecimal unitPrice; // getters, setters } ``` ### 2. API Specification | Endpoint | Method | Request | Response | Status Codes | |----------|--------|---------|----------|--------------| | `/cart/{userId}` | GET | - | Cart object | 200 OK, 404 Not Found | | `/cart/{userId}/items` | POST | CartItem | Cart object | 201 Created, 400 Bad Request | | `/cart/{userId}/items/{itemId}` | DELETE | - | Cart object | 200 OK, 404 Not Found | | `/cart/{userId}/items/{itemId}` | PUT | { quantity } | Cart object | 200 OK, 400 Bad Request | ### 3. Database Schema ```sql CREATE TABLE carts ( user_id VARCHAR(36) PRIMARY KEY, total_price DECIMAL(10,2) NOT NULL DEFAULT 0, last_updated TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE cart_items ( id VARCHAR(36) PRIMARY KEY, user_id VARCHAR(36) NOT NULL, product_id VARCHAR(36) NOT NULL, name VARCHAR(255) NOT NULL, quantity INT NOT NULL CHECK (quantity > 0), unit_price DECIMAL(10,2) NOT NULL, FOREIGN KEY (user_id) REFERENCES carts(user_id) ); CREATE INDEX idx_cart_items_user_id ON cart_items(user_id); ``` ### 4. Sequence Diagram (Add Item to Cart) ``` User -> Controller: POST /cart/123/items Controller -> CartService: addItem(userId, item) CartService -> Redis: GET cart:123 CartService -> ProductClient: getProduct(productId) ProductClient -> ProductService: [REST call] ProductService -> CartService: { price, name } CartService -> CartService: calculate total CartService -> Redis: SET cart:123 (with TTL) CartService -> Database: UPSERT cart_item CartService -> User: Cart object ``` ### 5. Error Handling - **Product Not Found**: Return 400 with message "Invalid product ID" - **Cart Not Found**: Return 404 with message "Cart not found" - **Quantity <= 0**: Return 400 with message "Quantity must be positive" - **Inventory Check Failure**: Return 409 with message "Insufficient inventory" ##

What is Low Level Design in system design?

Low Level Design (LLD) is the detailed design phase that translates the high-level architecture into implementation-ready specifications, focusing on individual components, data structures, algorithms, and interface contracts.

What is Low Level Design in system design?

Low Level Design (LLD) is the detailed design phase that translates the high-level architecture into implementation-ready specifications, focusing on individual components, data structures, algorithms, and interface contracts.

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