Low-Level Design (LLD)

The four OOP pillars and the five SOLID principles, each with the one-sentence test an interviewer is listening for and a concrete code smell it fixes.

The creational/structural/behavioral patterns worth knowing cold, each with its one-line intent and the situation that calls for it — plus Strategy and Observer in code.

The 'did you use design patterns?' question, answered with real examples — the source registry, the middleware chain, the alert worker, the pricing engine, and graceful-degradation strategies.

REST resource modeling, which verbs are idempotent and why it matters, status codes, cursor vs offset pagination, versioning, and idempotency keys for safe retries.

The second tier of must-know patterns — Factory, Builder, Adapter, Decorator, Composite, Command, Chain of Responsibility — each with the problem it kills and where you already use it.

The canonical LLD coding question: O(1) get and put with a hash map + doubly linked list. Full implementation, the why, and the follow-ups (thread-safety, TTL, LFU).

The classic object-modeling round. How to scope it, the entities and their relationships, where Strategy fits for spot assignment, and the extensibility follow-ups.

The state-machine classic: elevator states, the SCAN scheduling insight, Strategy for dispatch, and the multi-elevator follow-ups.

Expense modeling with Strategy for split types, the balance-sheet invariant, and the debt-simplification algorithm everyone gets asked.

Movie ticket booking: the entity ladder from City to Seat, seat-lock with TTL, and the double-booking race — the LLD where concurrency IS the question.

The gentle classic that tests one big idea: a Book is not a BookCopy. Catalog vs inventory, loans, reservations and fines.

The state-machine showcase — card, PIN, menu, dispense — plus the question that matters: debit first or dispense first?

The system every company actually builds: multi-channel delivery, templates, preferences, retries, dedup — and the patterns earning their keep together.

The retry round payments teams love: a retry state machine, idempotency across attempts, exponential backoff with jitter, non-retryable vs transient failures, and a dead-letter queue — with clean OOP boundaries.

The object design under the messaging giant: conversations, the message delivery state machine, local-first storage, and sync after offline.

The trip as a state machine, fare calculation as composable strategies, matching behind an interface — the class design beneath the geospatial giant.