# Low Level Design
All Low Level Design notes →1 — Parking Lot
Models a multi-level parking lot with heterogeneous vehicle and spot types, exercising strategy-based spot allocation and a clean split between lot, floor, and spot entities.
10 — ATM
Models an ATM's cash withdrawal, deposit, and balance-inquiry flows, exercising the state pattern across card-inserted, PIN-entry, and transaction states plus the greedy cash-dispensing algorithm.
11 — Vending Machine
Models a vending machine's product inventory, coin/payment handling, and dispensing logic, exercising the state pattern across idle, selection, payment, and dispense states.
12 — Coffee Machine
Models a coffee machine with multiple beverage recipes and shared ingredient inventories, exercising the recipe/ingredient composition model and low-stock/refill handling.
13 — Cricbuzz
Models live cricket match scoring, commentary, and scorecards, exercising the observer pattern for pushing real-time score updates to subscribed clients.
14 — Amazon Locker
Models a package-locker system for deliveries and pickups, exercising locker-size-to-package-size allocation strategy and the notification flow for one-time pickup codes.
15 — Cab Booking
Models a ride-hailing service matching riders to nearby drivers, exercising the driver-matching/dispatch strategy and dynamic surge-pricing calculation.
16 — Food Delivery
Models restaurants, menus, orders, and delivery-partner assignment for a food-delivery platform, exercising order-state-machine design and partner-assignment strategy.
17 — Notification Service
Models a multi-channel notification system spanning email, SMS, and push, exercising the strategy pattern for channel selection and template-based message rendering.
18 — Cache (LRU/LFU)
Implements an in-memory cache with fixed capacity, exercising O(1) get/put eviction design via a hash map paired with a doubly linked list for LRU or frequency buckets for LFU.
19 — Rate Limiter
Designs an API rate limiter enforcing per-client request quotas, exercising the tradeoffs between token-bucket, sliding-window, and fixed-window algorithms.
2 — Elevator System
Designs a multi-elevator dispatch system for a building, exercising the SCAN/LOOK scheduling algorithm choice and the state machine governing elevator direction and door control.
20 — Logging Framework
Designs a pluggable logging library with configurable levels, formatters, and appenders, exercising the chain-of-responsibility pattern for log-level filtering and multi-destination output.
21 — File System
Models a hierarchical in-memory file system with files and directories, exercising the composite pattern for uniform file/directory traversal and path resolution.
22 — Linux `find`
Implements a simplified version of the Unix find command over a directory tree, exercising predicate composition for filter chaining by name, type, size, and depth during traversal.
23 — Kafka-like Queue
Models a simplified publish-subscribe message queue with partitions and consumer groups, exercising partition-assignment strategy and offset-tracking for at-least-once delivery.
24 — Pub/Sub System
Models a generic publish-subscribe messaging system decoupling publishers from subscribers, exercising the observer pattern and topic-based routing/fan-out design.
3 — Library Management System
Models book catalog, members, and lending/reservation workflows for a library, exercising due-date and fine-calculation logic and the relationship between books, copies, and holds.
4 — Hotel Booking System
Models room inventory, reservations, and pricing across a hotel chain, exercising availability search and overlapping-date conflict resolution for bookings.
5 — Movie Ticket Booking
Models cinemas, shows, and seat inventory for booking movie tickets, exercising the concurrent seat-locking strategy needed to prevent double booking during checkout.
6 — Splitwise
Models shared expenses and running balances among a group of users, exercising the debt-simplification algorithm that minimizes the number of settlement transactions.
7 — Snake and Ladder
Models the classic board game with dice, snakes, and ladders for multiple players, exercising the board's jump-mapping design and the turn-based game-loop control flow.
8 — Chess
Models a full chess board, pieces, and move validation, exercising the polymorphic per-piece move-rule design and check/checkmate detection.
9 — Tic Tac Toe
Models a simple two-player grid game, exercising win-condition detection and a pluggable player strategy for human versus AI opponents.