Low Level Design workbook: OOP fundamentals, SOLID principles, design patterns, and worked OOD interview problems.http://localhost:4321/http://localhost:4321/low-level-design/airline-management-system-design/http://localhost:4321/low-level-design/airline-management-system-design/Aircraft, flights, schedules, crew, bookings, seats, itineraries. Multi-aggregate transactional design at scale.Sat, 30 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studyairlinestate-patternstrategy-patternhttp://localhost:4321/low-level-design/architectural-design-patterns-overview/http://localhost:4321/low-level-design/architectural-design-patterns-overview/MVC, MVVM, layered, microservices, event-driven, hexagonal. The patterns that organise whole systems, not just classes.Sat, 30 May 2026 00:00:00 GMTIntermediateconceptDesign Patternsarchitecturemvclayeredmicroserviceshexagonalhttp://localhost:4321/low-level-design/car-rental-system-design/http://localhost:4321/low-level-design/car-rental-system-design/Fleets, reservations, pickup/return, insurance, billing. State machine on the rental and a Strategy for tiered pricing.Sat, 30 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studycar-rentalstate-patternstrategy-patterndecoratorhttp://localhost:4321/low-level-design/espncricinfo-design/http://localhost:4321/low-level-design/espncricinfo-design/Tournaments, matches, innings, overs, balls, scorecards. Ball-by-ball event stream plus Observer for live-score subscribers.Sat, 30 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studycricketobserver-patternstrategy-patternhttp://localhost:4321/low-level-design/facebook-design/http://localhost:4321/low-level-design/facebook-design/Users, friendships, posts, reactions, comments, groups, pages. Friendship state machine plus Observer for notifications.Sat, 30 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studyfacebookobserver-patternstate-patternhttp://localhost:4321/low-level-design/jigsaw-puzzle-design/http://localhost:4321/low-level-design/jigsaw-puzzle-design/Puzzle, piece, solver. The lightest case study — focus is the class model and a recursive backtracking solver.Sat, 30 May 2026 00:00:00 GMTFoundationalsystemCase Studiesoodcase-studyjigsawbacktrackingcompositehttp://localhost:4321/low-level-design/online-stock-brokerage-system-design/http://localhost:4321/low-level-design/online-stock-brokerage-system-design/Accounts, portfolios, market/limit/stop orders, matching, transactions. Order state plus price-time matching strategy.Sat, 30 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studybrokeragestate-patternstrategy-patternhttp://localhost:4321/low-level-design/online-blackjack-game-design/http://localhost:4321/low-level-design/online-blackjack-game-design/Player, dealer, deck, hand, round. State machine on the round and a Strategy for dealer policy — the cleanest cards-game OOD.Sat, 30 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyblackjackstate-patternstrategy-patternhttp://localhost:4321/low-level-design/restaurant-management-system-design/http://localhost:4321/low-level-design/restaurant-management-system-design/Tables, reservations, orders, menu, bill, waiter, chef. Order state machine plus table-assignment Strategy.Sat, 30 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyrestaurantstate-patternstrategy-patternhttp://localhost:4321/low-level-design/stack-overflow-design/http://localhost:4321/low-level-design/stack-overflow-design/Questions, answers, votes, comments, badges, tags. Reputation as derived state; moderation as a workflow.Sat, 30 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studystack-overflowstrategy-patterncommand-patternhttp://localhost:4321/low-level-design/abstract-factory-pattern/http://localhost:4321/low-level-design/abstract-factory-pattern/Families of related objects without specifying concrete classes. When one factory isn't enough.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatterncreationalabstract-factoryfamilieshttp://localhost:4321/low-level-design/abstraction/http://localhost:4321/low-level-design/abstraction/Hide *how* behind *what*. Interfaces and abstract classes as the language-level expression of the idea.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsoopabstractioninterfacesabstract-classesdiphttp://localhost:4321/low-level-design/activity-diagrams/http://localhost:4321/low-level-design/activity-diagrams/Branching, merging, swim-lanes. When the state machine isn't a single object's lifecycle but a multi-actor workflow.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsumlactivity-diagramworkflowstate-machinelldhttp://localhost:4321/low-level-design/adapter-pattern/http://localhost:4321/low-level-design/adapter-pattern/Make incompatible interfaces work together. Object adapter vs class adapter, and the legacy-integration use case.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatternstructuraladapterwrapperlegacy-integrationhttp://localhost:4321/low-level-design/amazon-locker-service/http://localhost:4321/low-level-design/amazon-locker-service/Lockers as a generalised parking lot for packages. Size matching, expiry, access codes, partner-delivery vs customer-pickup.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyamazon-lockerstate-patternstrategy-patternhttp://localhost:4321/low-level-design/approaching-ood-interviews/http://localhost:4321/low-level-design/approaching-ood-interviews/The 45-minute structure: clarify scope, identify entities, draw the class diagram, narrate a flow, defend trade-offs. The meta-script.Fri, 29 May 2026 00:00:00 GMTFoundationalexercisePractice & Interviewinterviewoodmetatime-managementdecompositionhttp://localhost:4321/low-level-design/atm-system-design/http://localhost:4321/low-level-design/atm-system-design/Cards, accounts, transactions, the State pattern on the session. The textbook OOD interview problem that's almost too clean.Fri, 29 May 2026 00:00:00 GMTFoundationalsystemCase Studiesoodcase-studyatmstate-patternstrategy-patternhttp://localhost:4321/low-level-design/builder-pattern/http://localhost:4321/low-level-design/builder-pattern/Step-by-step construction of complex objects, fluent API, telescoping-constructor problem solved.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatterncreationalbuilderfluent-apihttp://localhost:4321/low-level-design/chess-game-design/http://localhost:4321/low-level-design/chess-game-design/Pieces with movement rules, Board, Move, Game state. The cleanest polymorphism problem in the OOD canon.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studychesscommand-patternstate-patternhttp://localhost:4321/low-level-design/class-diagrams/http://localhost:4321/low-level-design/class-diagrams/Boxes, lines, multiplicities. Association vs aggregation vs composition vs inheritance — what each arrow actually claims.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsumlclass-diagramrelationshipsmultiplicitylldhttp://localhost:4321/low-level-design/command-pattern/http://localhost:4321/low-level-design/command-pattern/Encapsulate a request as an object so you can parameterise, queue, log, and undo it. The basis for almost every editor's history.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatternbehavioralcommandundoqueuehttp://localhost:4321/low-level-design/composite-pattern/http://localhost:4321/low-level-design/composite-pattern/Treat individual objects and compositions uniformly. The recursive tree-of-things pattern that shows up everywhere.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatternstructuralcompositetreerecursionhttp://localhost:4321/low-level-design/dependency-inversion-principle/http://localhost:4321/low-level-design/dependency-inversion-principle/Depend on abstractions, not concretions. Where DIP, IoC, and constructor injection are the same idea wearing three names.Fri, 29 May 2026 00:00:00 GMTIntermediateconceptSOLID Principlessoliddipabstractiondependency-injectionhttp://localhost:4321/low-level-design/elevator-system-design/http://localhost:4321/low-level-design/elevator-system-design/Multi-car bank, request scheduling, the State pattern on the car itself. The runner-up to Parking Lot in interview frequency.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyelevatorstate-patternstrategy-patternhttp://localhost:4321/low-level-design/decorator-pattern/http://localhost:4321/low-level-design/decorator-pattern/Wrap an object to add behaviour without subclassing. The classic IO-stream example and how to avoid wrapper explosion.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatternstructuraldecoratorwrappercompositionhttp://localhost:4321/low-level-design/encapsulation/http://localhost:4321/low-level-design/encapsulation/Bundle state with behaviour; hide state behind methods. The basis for invariant enforcement and the SRP setup.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsoopencapsulationinvariantsinformation-hidingsrphttp://localhost:4321/low-level-design/facade-pattern/http://localhost:4321/low-level-design/facade-pattern/A single entry point hiding a complex subsystem. The architectural cost of skipping it.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatternstructuralfacadesubsystemapi-designhttp://localhost:4321/low-level-design/factory-method-pattern/http://localhost:4321/low-level-design/factory-method-pattern/Defer instantiation to a method subclasses override. The seam between knowing what to make and knowing how to make it.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatterncreationalfactory-methodpolymorphismhttp://localhost:4321/low-level-design/food-delivery-design-exercise/http://localhost:4321/low-level-design/food-delivery-design-exercise/Open-ended practice prompt: restaurants, orders, delivery agents, the three-sided state machine in a different shell.Fri, 29 May 2026 00:00:00 GMTIntermediateexercisePractice & Interviewexerciseoodfood-deliverystate-machinehttp://localhost:4321/low-level-design/interface-segregation-principle/http://localhost:4321/low-level-design/interface-segregation-principle/No client should be forced to depend on methods it doesn't use. The case for many small interfaces over one fat one.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptSOLID Principlessolidispinterfacescohesionhttp://localhost:4321/low-level-design/hotel-management-system/http://localhost:4321/low-level-design/hotel-management-system/Rooms, bookings, services, billing. Multi-aggregate transactional design; the closest OOD prompt to a real product.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyhotelstate-patternstrategy-patternhttp://localhost:4321/low-level-design/inheritance-and-generalization/http://localhost:4321/low-level-design/inheritance-and-generalization/IS-A relationships, when to reach for them, and the cost of getting it wrong. Composition as the more common right answer.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsoopinheritancegeneralizationcompositionlsphttp://localhost:4321/low-level-design/library-management-system/http://localhost:4321/low-level-design/library-management-system/Members, books, copies, borrowing, fines. The bread-and-butter OOD problem; the prompt that filters on basic discipline.Fri, 29 May 2026 00:00:00 GMTFoundationalsystemCase Studiesoodcase-studylibrarystate-patternstrategy-patternhttp://localhost:4321/low-level-design/linkedin-design/http://localhost:4321/low-level-design/linkedin-design/Members, connections, feeds, messaging. Where OOD touches the system-design boundary and you must hold the line.Fri, 29 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studysocial-networkobserver-patterncomposite-patternhttp://localhost:4321/low-level-design/liskov-substitution-principle/http://localhost:4321/low-level-design/liskov-substitution-principle/Subtypes must honour their parent's contract. The Square-Rectangle problem and why it isn't pedantry.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptSOLID Principlessolidlspinheritancecontractshttp://localhost:4321/low-level-design/meeting-scheduler-design/http://localhost:4321/low-level-design/meeting-scheduler-design/Calendars, rooms, attendees, time-zone arithmetic. The interval-management problem with first-class side-effects.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studymeeting-schedulerstrategy-patternobserver-patternhttp://localhost:4321/low-level-design/observer-pattern/http://localhost:4321/low-level-design/observer-pattern/Publish-subscribe for objects. Push vs pull, the leaky-listener problem, and modern event-bus alternatives.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatternbehavioralobserverpub-subeventshttp://localhost:4321/low-level-design/online-shopping-system/http://localhost:4321/low-level-design/online-shopping-system/Catalog, cart, orders, payments, shipping. The biggest single OOD prompt; the discipline is *what you cut*.Fri, 29 May 2026 00:00:00 GMTAdvancedsystemCase Studiesoodcase-studye-commercestate-patterncommand-patternhttp://localhost:4321/low-level-design/movie-ticket-booking-system/http://localhost:4321/low-level-design/movie-ticket-booking-system/Cinemas, screens, shows, seat-holds, payments. The concurrency-with-holds problem in a recognisable shell.Fri, 29 May 2026 00:00:00 GMTIntermediatesystemCase Studiesoodcase-studyticket-bookingstate-patternstrategy-patternhttp://localhost:4321/low-level-design/oop-fundamentals/http://localhost:4321/low-level-design/oop-fundamentals/Encapsulation, abstraction, inheritance, polymorphism. The vocabulary every LLD interview assumes you have internalised.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsoopencapsulationabstractioninheritancepolymorphismhttp://localhost:4321/low-level-design/ooad-and-uml-overview/http://localhost:4321/low-level-design/ooad-and-uml-overview/Object-oriented analysis and design as the discipline; UML as the notation. The four diagrams the LLD round actually expects.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsooadumlmodellinglldinterviewshttp://localhost:4321/low-level-design/open-closed-principle/http://localhost:4321/low-level-design/open-closed-principle/Open to extension, closed to modification. The strategy-pattern motivation, said as a rule.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptSOLID Principlessolidocpextensionpolymorphismhttp://localhost:4321/low-level-design/polymorphism/http://localhost:4321/low-level-design/polymorphism/Subtype, parametric, and ad-hoc. The procedural form of the Open-Closed Principle. The instanceof smell.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsooppolymorphismdispatchgenericsocphttp://localhost:4321/low-level-design/parking-lot-design/http://localhost:4321/low-level-design/parking-lot-design/The canonical OOD interview problem. Multi-floor lot, spot types, tickets, payment. The first hour of every LLD prep.Fri, 29 May 2026 00:00:00 GMTFoundationalsystemCase Studiesoodcase-studyparking-lotstate-patternstrategy-patternhttp://localhost:4321/low-level-design/prototype-pattern/http://localhost:4321/low-level-design/prototype-pattern/Clone an existing object instead of constructing from scratch. Shallow vs deep copy and when each is correct.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatterncreationalprototypecloningdeep-copyhttp://localhost:4321/low-level-design/ride-sharing-design-exercise/http://localhost:4321/low-level-design/ride-sharing-design-exercise/Open-ended practice prompt: matching, surge pricing, the three-sided state machine. Work it under 45 minutes.Fri, 29 May 2026 00:00:00 GMTIntermediateexercisePractice & Interviewexerciseoodride-sharingstate-machinehttp://localhost:4321/low-level-design/sequence-diagrams/http://localhost:4321/low-level-design/sequence-diagrams/Lifelines, messages, activations. The narration the interviewer is reading along with as you talk through a flow.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsumlsequence-diagramlifelinemessagelldhttp://localhost:4321/low-level-design/singleton-pattern/http://localhost:4321/low-level-design/singleton-pattern/One instance, global access point. The threading caveats, the why-everyone-says-don't, and when it's still right.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatterncreationalsingletonthreadinghttp://localhost:4321/low-level-design/single-responsibility-principle/http://localhost:4321/low-level-design/single-responsibility-principle/A class should have one and only one reason to change. The smell, the refactor, the cost of misapplying it.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptSOLID Principlessolidsrpcohesionrefactoringhttp://localhost:4321/low-level-design/state-pattern/http://localhost:4321/low-level-design/state-pattern/An object's behaviour changes with its internal state — represented as a class hierarchy. The state-machine pattern in disguise.Fri, 29 May 2026 00:00:00 GMTIntermediatepatternDesign Patternspatternbehavioralstatestate-machinepolymorphismhttp://localhost:4321/low-level-design/use-case-diagrams/http://localhost:4321/low-level-design/use-case-diagrams/Actors, use cases, includes, extends. The two-minute artefact that proves you scoped the problem before drawing classes.Fri, 29 May 2026 00:00:00 GMTFoundationalconceptFoundationsumluse-caseactorsscopinglldhttp://localhost:4321/low-level-design/strategy-pattern/http://localhost:4321/low-level-design/strategy-pattern/Encapsulate interchangeable algorithms behind a common interface. The dual of the Open-Closed Principle in practice.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatternbehavioralstrategyalgorithmscompositionhttp://localhost:4321/low-level-design/template-method-pattern/http://localhost:4321/low-level-design/template-method-pattern/The skeleton of an algorithm in the base class; specifics in the subclass hooks. When inheritance is the right answer.Fri, 29 May 2026 00:00:00 GMTFoundationalpatternDesign Patternspatternbehavioraltemplate-methodinheritancehookshttp://localhost:4321/low-level-design/vending-machine-design/http://localhost:4321/low-level-design/vending-machine-design/Inventory, coin handling, the State pattern on the machine. The smallest realistic state-machine LLD problem.Fri, 29 May 2026 00:00:00 GMTFoundationalsystemCase Studiesoodcase-studyvending-machinestate-patternstrategy-pattern