How to use this reference
- Work through topics top to bottom — classic problem walkthroughs assume you know SOLID and the core patterns from earlier topics.
- This is class-level design (objects, interfaces, patterns), not distributed system architecture — keep that boundary clear in interviews.
- Optional practice tasks are whiteboard sketches or small local implementations — the skill is articulating trade-offs, not passing hidden tests.
- Use any language you interview in; examples assume OOP (Java/C#/Kotlin/Python with classes) but principles transfer.
The Reference
- 1
The principles behind maintainable class design — SOLID, composition over inheritance, and crisp boundaries between objects.
- 2
The ~10 patterns that actually show up in LLD loops — not all 23 GoF patterns, just the ones worth drilling.
- 3
How to run the 45-minute round — clarify, API, classes, trade-offs — without jumping to code too early or over-designing.
- 4
Problems that test entity modeling, state transitions, and allocation logic — the bread and butter of OOD interview loops.
- 5
The building blocks senior backend loops love — caches, rate limiters, schedulers, and the ordered/streaming structures underneath them — where data structures meet class design.
- 6
Making designs interview-ready — clean public surfaces, plugin extensibility, and defensive error handling at the class level.