Template — The 13-Part LLD Answer
In one line
The LLD analogue: requirements → actors → use cases → entities → responsibilities → relationships → interfaces → SOLID → patterns → extensibility → concurrency → code → tests.
Key ideas
- 01
In plain words: the same idea as the HLD recipe card, but for code-level problems like Parking Lot or Splitwise. 13 steps, always in the same order, so you never freeze up staring at a blank whiteboard.
- 02
1 · Requirements — written, with edge cases; the 3.1 requirement drill.
- 03
2 · Actors — who uses it (parking attendant, driver, system).
- 04
3 · Use cases — the 4–6 core flows in one line each.
- 05
4 · Entities — nouns with identity + state (3.2).
- 06
5 · Responsibilities — which class owns which behavior (3.3).
- 07
6 · Relationships — multiplicities + ownership: 1:1/1:N/N:N, composition/aggregation (1.3).
- 08
7 · Interfaces — the seams every extension plugs into (1.4/3.4).
- 09
8 · SOLID — the violations your design explicitly avoids (1.2).
- 10
9 · Patterns — the pattern(s) chosen for a stated design problem (Phase 2).
- 11
10 · Extensibility — 'what changes when a new X arrives' walkthrough (3.4).
- 12
11 · Concurrency — shared-state audit + the atomic operation choice (Phase 5).
- 13
12 · Code — production-quality implementation, not a script.
- 14
13 · Test cases — unit tests for the invariants + edge cases; the 3.3 testability payoff.
Code & diagrams
The LLD checklist — same idea, different lens.
1. Requirements
2. Actors
3. Use cases
4. Entities
5. Responsibilities
6. Relationships (multiplicity + ownership)
7. Interfaces (the seams)
8. SOLID (violations avoided)
9. Patterns (chosen for a problem, not a vibe)
10. Extensibility (what changes for a new variation)
11. Concurrency (shared-state audit + atomic ops)
12. Code (production quality)
13. Test cases (invariants + edges)Explain without notes
Run the 13 parts on 'design splitwise' without notes: entities → responsibilities → concurrency → tests.
Practice
Two timed rounds: Parking Lot (20 min) and Notification System (30 min) — grade yourself against the checklist.
Trade-offs
- ↔
Skipping concurrency or tests on an LLD is the classic way to lose the round at 4+ YOE.
Completion checklist
I can deliver the 13-part LLD from memory on any prompt.