The LLD method in eight steps
Requirements to interfaces to code in 60 minutes. The eight steps, the patterns that actually show up in security components, and the mistakes that cost the most points.
● Must know for interviewsFREELesson·18 min read·Beginner → Advanced
methodOOPSOLIDpatterns
01Eight steps
✓What you will learn
- Run the eight-step method under time pressure
- Choose interfaces first, implementations second
- Use the four patterns that recur in security components
- Write tests that prove the security property, not just the happy path
- Clarify (5 min): inputs, outputs, scale, what is out. For security components: who calls this, with what trust, and what is the failure mode (fail open or closed)?
- Nouns and verbs (3 min): underline them in the requirements. Nouns become types; verbs become methods.
- Interfaces (7 min): define the public surface before any class body.
evaluate(request): Decision. Interviewers judge this most. - Core model (10 min): the entities and their relationships. Keep value objects immutable.
- Behaviour (15 min): implement the main flow. Use a pattern only when a requirement demands it.
- Extension points (5 min): where would a new rule, format or backend plug in? That is where Strategy or a registry belongs.
- Tests (10 min): the happy path, the security property (deny by default, no bypass), and one edge case.
- Walkthrough (5 min): trace one request through the code out loud.
| Pattern | Shows up as | Example here |
|---|---|---|
| Strategy | Pluggable algorithm behind one interface | Hash algorithm, rate-limit algorithm, parser per format |
| Chain of responsibility | Ordered handlers, first match or all | Policy rules, log pipeline stages |
| Builder / factory | Constructing a validated object | A policy from JSON, a parsed event |
| Observer / event | Side effects decoupled from decisions | Audit log on every decision, alert on reuse detection |
!Patterns on demand only
A Factory with one product, a Singleton for a config object, an abstract base class with one subclass: all of these cost points. Introduce a pattern when the second implementation arrives or a requirement names it.
“How to say it in the room
"I'll start with the interface the caller sees, because that's the contract. Then the model, then the behaviour, and I'll add an extension point only where the requirements say something will vary."
Finished this one?
Tick it and it shows as done on the roadmap and the course.