LLD Handbook — SDE2 / SDE3 Interview Preparation
Target audience: Backend engineers with 6+ years of experience interviewing for SDE2/SDE3 roles at Amazon, Google, Meta, Microsoft, and other top product companies.
This handbook is built for interview preparation, not for teaching OOP from scratch. If you are still learning what encapsulation means, this is the wrong document. If you have shipped production systems, led design reviews, and want a structured way to sharpen your LLD interview performance — keep reading.
What This Handbook Gives You
- A repeatable delivery framework you can apply to any LLD problem in 45 minutes.
- Principle-level fluency — SOLID, cohesion/coupling, composition-over-inheritance — with the exact wording interviewers respond to.
- Design patterns filtered to the 10 that show up, with crisp when to use and when to avoid.
- Concurrency literacy — enough depth to discuss thread safety, coordination, and backpressure when the interviewer probes.
- 14 fully-worked problems at SDE2/SDE3 depth, each with clarifying questions, tradeoffs, concurrency notes, follow-ups, and an explicit how does SDE3 beat SDE2 on this problem breakdown.
How to Use It
- 2 days out: Read Section 1 and one problem in Section 3. Get the framework into muscle memory.
- Week of: Work through 3-4 problems a day. Speak the clarifying-question phase out loud. Time yourself — target 40 minutes per problem.
- Morning of: Skim the Delivery Framework and the SDE2 vs SDE3 table. Nothing else.
Sections
Section 1: Fundamentals
| File | Focus |
|---|---|
| 01 — Interview Introduction | What LLD tests. SDE2 vs SDE3 bar. What "good" looks like. |
| 02 — Delivery Framework | 8-step framework with exact phrases you can reuse. |
| 03 — Design Principles | SOLID, DRY, KISS, YAGNI, cohesion/coupling, composition, immutability. |
| 04 — OOP Concepts | Senior-level OOP — interfaces vs abstract, aggregation vs composition, DI. |
| 05 — Design Patterns | Strategy, Factory, Singleton, Observer, Builder, Adapter, Decorator, State, Command, Template. |
Section 2: Concurrency
| File | Focus |
|---|---|
| 06 — Correctness | Threads, shared state, locks, CAS, deadlock, reentrancy, idempotency. |
| 07 — Coordination | Semaphores, producer/consumer, blocking queue, condition vars, thread pools, futures. |
| 08 — Scarcity & Resilience | Rate limiting, connection pools, backpressure, retries, circuit breaker. |
Section 3: Problem Breakdowns
Core Problems — gateway problems; miss these and nothing else matters.
| # | Problem | Why it's asked |
|---|---|---|
| 10 | Tic Tac Toe | Warm-up. Tests clean abstractions without scale pressure. |
| 11 | Connect Four | Reuses TTT's chassis with a twist (gravity). Good for extensibility talk. |
| 12 | Amazon Locker | Multi-entity, assignment strategy, state transitions, expiry. |
| 13 | Elevator | Dispatch algorithm, state machine, multi-car coordination. |
| 14 | Parking Lot | The classic. Strategy for assignment, pricing, ticketing. |
Real Interview Problems — what actually shows up in loops.
| # | Problem | Why it's asked |
|---|---|---|
| 15 | File System | Composite pattern, path parsing, tree traversal. |
| 16 | Movie Ticket Booking | Concurrency under contention — seat reservation is the whole problem. |
| 17 | Rate Limiter | Strategy pattern, token bucket vs sliding window, distributed awareness. |
| 18 | Inventory Management | Reservations, reorder thresholds, event-driven design. |
Senior-Level Bonus Problems — the depth-probing extras.
| # | Problem | Why it's asked |
|---|---|---|
| 19 | Notification Service | Multi-channel fan-out, templates, retries, rate limits. |
| 20 | Food Delivery System | Multi-actor state machines, location, assignment. |
| 21 | Meeting Scheduler | Interval logic, conflict detection, recurrence. |
| 22 | Cab Booking System | Matching, pricing/surge, driver-rider state transitions. |
| 23 | Cache with Eviction Policies | Pluggable eviction (LRU/LFU/TTL/FIFO), concurrency, observability. |
Scope — What This Handbook is Not
- Not an OOP textbook. Principles are covered but assumed mostly internalized.
- Not a language reference. Code is TypeScript for consistency; translate to your preferred language.
- Not HLD. System design at the distributed-system level is a separate discipline. Some LLD problems touch scale; full HLD is out of scope.
- Not behavioral prep. For Amazon LP prep, see Amazon Prep.