SwadhyAI Notes

Low-Level Design Notes

Learn low-level design as its own subject: domain modeling, GRASP, SOLID, interview patterns, in-process concurrency, and Java machine-coding case studies.

Lessons

• What is LLD?
• HLD vs LLD
• LLD interview approach
• Requirements to classes
• Interview framework and rubric
• Domain modeling fundamentals
• Entity vs Value Object
• Identity, equality, and object comparison
• Common value objects
• Association, aggregation, composition
• Object ownership and lifecycle
• Designing collections inside domain objects
• Entity vs Service responsibilities
• Rich domain model vs anemic model
• Put behavior where the data lives
• Composition vs inheritance
• Encapsulation in object design
• Interfaces and contracts
• SOLID principles
• Single Responsibility
• Open-Closed
• Liskov Substitution
• Interface Segregation
• Dependency Inversion
• GRASP principles overview
• Information Expert
• Creator
• Controller
• Protected Variations
• Low coupling and high cohesion
• DRY, KISS, and YAGNI
• Law of Demeter
• Use cases and system operations
• Assigning responsibilities
• Object collaboration
• Sequence diagrams
• State machines
• Modeling object lifecycle
• Valid and invalid state transitions
• State machine vs status enum
• Finding changing parts of the design
• When to use a pattern
• Factory Method
• Abstract Factory
• Builder
• Prototype
• Singleton — and when not to
• Adapter
• Facade
• Adapter vs Facade
• Bridge
• Composite
• Decorator
• Flyweight
• Proxy
• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State pattern
• Strategy
• Template Method
• Visitor
• Null Object
• Specification pattern
• Registry pattern
• Policy object
• Dependency injection
• Designing a class API
• Commands vs queries
• Evolving an API without breaking callers
• Collection API design
• Null, Optional, empty collections
• Request/response vs domain objects
• Errors and domain exceptions
• Validation at the boundary
• DTO vs domain model
• Idempotency in object APIs
• Keeping persistence out of the domain
• Repository abstraction
• Where transactions begin and end
• Domain model vs persistence model
• Race conditions in object design
• Atomicity and memory visibility
• Avoiding shared state
• Optimistic vs pessimistic concurrency
• Safely changing object state
• Thread-safe object design
• Immutability as a concurrency tool
• Locks and monitors
• Producer-consumer
• Executors and thread pools
• Concurrent collections
• Parking Lot
• Library Management
• Elevator System
• Splitwise
• Snake and Ladder
• Tic-Tac-Toe
• Chess
• ATM
• Vending Machine
• Movie Ticket Booking
• Ride matching (in-process)
• LRU Cache
• Rate limiter (in-process)
• Pub/Sub (in-process)
• Task Scheduler
• Logging Framework
• Notification Service
• In-memory file system
• In-memory key-value store
• URL shortener (machine coding)
• Meeting / Calendar Scheduler
• Inventory / Order System
• Rule Engine
• Designing for extension
• Testing an LLD design
• Designing code that is easy to test
• Stub, Fake, Mock
• Clock, randomness, UUIDs and tests
• Designing around dependencies
• Refactoring toward a better design
• Design smells
• From smells to patterns
• Package and class organization