LLD

The code inside a service: classes and SOLID, layers, APIs and data models, errors, concurrency and tests, ending with a parking lot. 4 modules, about 5 hours of reading. Every lesson ends in something to do.

Object-Oriented Design

OOP ConceptsEncapsulation, abstraction, inheritance and polymorphism as boundary-drawing tools — what each buys you, and where each is misused.Beginner18 min
SOLID PrinciplesFive rules for code that survives change, each explained by the pain it exists to prevent.Intermediate19 min
Dependency InjectionWhy a class should never build its own collaborators, and how one habit makes code testable and swappable.Intermediate16 min
Code Organization & ModularizationWhy folder structure by feature beats folders by type, plus coupling, cohesion and dependency direction.Intermediate17 min

Services, APIs and Data

Service Layer & Repository PatternWhere business logic belongs, why it must not know about SQL, and how that one rule makes code testable.Intermediate17 min
API DesignResources over verbs, status codes that mean something, idempotency, and versioning you will not regret.Intermediate17 min
Data ModelingFinding the entities hiding in a requirements document, getting relationships right, and modelling time correctly.Intermediate17 min
DTOs & ValidationWhy passing database entities straight to your API leaks data, and how validation at the boundary prevents whole bug classes.Intermediate17 min
Database Schema DesignChoosing keys and types, how indexes really work, and why schema mistakes are the most expensive kind.Intermediate18 min
PaginationWhy OFFSET gets slower the deeper you go, how cursors fix it, and the bug that silently skips rows.Intermediate17 min

Production Readiness

Error HandlingFailing at the boundary, catching only what you can act on, and leaving the system in a valid state when something breaks midway.Intermediate19 min
Concurrency & Thread SafetyShared mutable state, race conditions, locks and deadlock — and why the best concurrent code is the code that shares nothing.Advanced17 min
Configuration ManagementSeparating config from code, why secrets never belong in git, and validating settings before your app starts.Intermediate16 min
Logging & MonitoringStructured logs, correlation IDs, the three pillars of observability, and how to alert without waking people for nothing.Intermediate17 min
Rate Limiting & ThrottlingThe four algorithms, why in-memory counters break behind a load balancer, and how to reject requests politely.Intermediate17 min
Security (Auth & Authorization)Who you are versus what you may do, storing passwords correctly, sessions versus tokens, and the bug everyone ships.Intermediate17 min
Testing StrategiesWhat each layer of the pyramid actually buys, faking at the boundary rather than the internals, and why coverage is not correctness.Intermediate18 min

Case Studies

Design a Parking LotThe classic LLD case study worked end to end — entities, class design, allocation and pricing strategies, and the concurrency bug everyone misses.Intermediate20 min

Object-Oriented Design

OOP ConceptsEncapsulation, abstraction, inheritance and polymorphism as boundary-drawing tools — what each buys you, and where each is misused.Beginner18 min

SOLID PrinciplesFive rules for code that survives change, each explained by the pain it exists to prevent.Intermediate19 min

Dependency InjectionWhy a class should never build its own collaborators, and how one habit makes code testable and swappable.Intermediate16 min

Code Organization & ModularizationWhy folder structure by feature beats folders by type, plus coupling, cohesion and dependency direction.Intermediate17 min

Services, APIs and Data

Service Layer & Repository PatternWhere business logic belongs, why it must not know about SQL, and how that one rule makes code testable.Intermediate17 min

API DesignResources over verbs, status codes that mean something, idempotency, and versioning you will not regret.Intermediate17 min

Data ModelingFinding the entities hiding in a requirements document, getting relationships right, and modelling time correctly.Intermediate17 min

DTOs & ValidationWhy passing database entities straight to your API leaks data, and how validation at the boundary prevents whole bug classes.Intermediate17 min

Database Schema DesignChoosing keys and types, how indexes really work, and why schema mistakes are the most expensive kind.Intermediate18 min

PaginationWhy OFFSET gets slower the deeper you go, how cursors fix it, and the bug that silently skips rows.Intermediate17 min

Production Readiness

Error HandlingFailing at the boundary, catching only what you can act on, and leaving the system in a valid state when something breaks midway.Intermediate19 min

Concurrency & Thread SafetyShared mutable state, race conditions, locks and deadlock — and why the best concurrent code is the code that shares nothing.Advanced17 min

Configuration ManagementSeparating config from code, why secrets never belong in git, and validating settings before your app starts.Intermediate16 min

Logging & MonitoringStructured logs, correlation IDs, the three pillars of observability, and how to alert without waking people for nothing.Intermediate17 min

Rate Limiting & ThrottlingThe four algorithms, why in-memory counters break behind a load balancer, and how to reject requests politely.Intermediate17 min

Security (Auth & Authorization)Who you are versus what you may do, storing passwords correctly, sessions versus tokens, and the bug everyone ships.Intermediate17 min

Testing StrategiesWhat each layer of the pyramid actually buys, faking at the boundary rather than the internals, and why coverage is not correctness.Intermediate18 min

Object-Oriented Design

Services, APIs and Data

Production Readiness

Case Studies

Object-Oriented Design

Services, APIs and Data

Production Readiness

Case Studies

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