# LLD Practice

> A 30-day sprint through the canonical low-level-design problems. Patterns first, then applied "design X" exercises. End able to walk through a parking lot, ticketing system, or game design with implementation and tradeoff notes.

Canonical: https://learn.significanthobbies.com/curriculum/roadmaps/lld-practice

A 30-day sprint through the canonical low-level-design problems. Patterns first, then applied "design X" exercises. End able to walk through a parking lot, ticketing system, or game design with implementation and tradeoff notes.

- Horizon: 30d
- Outcome: Build the design-rounds muscle: model state and behaviour explicitly, justify every class boundary.
- Tracks: System Design

Work each milestone as a claim that needs evidence. Predict the mechanism and
failure mode first, study the linked concepts, complete the drills without
copying an answer, and preserve the resulting code, measurements, or design.

## Milestone 1: Days 1-7 — Modelling foundations

State, behaviour, extension points. The vocabulary every LLD problem reaches for.

### Concepts

- [Object Modeling](https://learn.significanthobbies.com/curriculum/concepts/object-modeling) — Identifying classes, attributes, relationships.
- [State Machines](https://learn.significanthobbies.com/curriculum/concepts/state-management) — States, transitions, guards.
- [Factory & Creational](https://learn.significanthobbies.com/curriculum/concepts/factory-creational) — Abstract factory, builder, singleton.
- [Strategy Pattern](https://learn.significanthobbies.com/curriculum/concepts/strategy-pattern) — Interchangeable algorithms.



## Milestone 2: Days 8-18 — Patterns in practice

The handful of patterns that earn their weight in real interviews.

### Concepts

- [Observer Pattern](https://learn.significanthobbies.com/curriculum/concepts/observer-pattern) — Pub/sub, event propagation.
- [Decorator & Structural](https://learn.significanthobbies.com/curriculum/concepts/decorator-structural) — Decorator, adapter, composite.
- [Command & Chain](https://learn.significanthobbies.com/curriculum/concepts/command-chain) — Command, undo, chain of responsibility.



## Milestone 3: Days 19-30 — Applied designs

Take the patterns into full design problems with concurrency and inventory edge cases.

### Concepts

- [Concurrency Design](https://learn.significanthobbies.com/curriculum/concepts/concurrency-design) — Thread-safety, locks, producer-consumer.
- [Booking & Inventory](https://learn.significanthobbies.com/curriculum/concepts/booking-inventory) — Reservation, hold-confirm, optimistic locking.
- [Game/Simulation Design](https://learn.significanthobbies.com/curriculum/concepts/game-design) — Turn engines, board state, rules.