Low-Level Design for MAANG Interviews

A book-shaped table of contents for LLD interview prep: OOP fundamentals through SOLID, design principles, UML, design patterns, dependency management, reliability, concurrency, domain modeling, API design, testing, refactoring, classic interview problems, advanced architecture, and performance — cross-linking Object-Oriented Programming and Patterns instead of duplicating them.

1 — Interface Design

Defines how a service's public surface is shaped so consumers depend on a stable contract rather than on internal implementation details.

2 — DTOs

Explains why data transfer objects decouple wire formats from domain models, so internal refactors don't ripple into API consumers.

3 — Validation Layers

Distinguishes syntactic, semantic, and business-rule validation so each concern is enforced at the layer best suited to catch it.

4 — Mapping Objects

Covers translating between domain models and DTOs at the API boundary without leaking persistence details or business logic across it.