Why this matters
- Customer inserts card and enters PIN to start authenticated session
- Scope — what "Session lifecycle" covers inside ATM System and what it does not
- Scope — what "Dispenser chain" covers inside ATM System and what it does not
- Scope — what "Concurrency" covers inside ATM System and what it does not
- Machine problems test State, Strategy, and facade coordination.
Client / API
FacadeSingle entry point
EntitiesDomain objects
State / Strategy
CampusOS context
CampusOS is the fictional smart-campus platform used across this track. Treat this module as a real product slice:
Context
- Belongs to CampusOS — same naming and design vocabulary as parking, library, and booking modules
- Scope is one JVM — persistence is in-memory or behind a repository interface
- Goal is a class diagram + core Java that an interviewer can follow in 35–45 minutes
- Extension paragraph at the end links to System Design when scale exceeds one process
Requirements snapshot
Functional requirements
- Customer inserts card and enters PIN to start authenticated session
- Check balance, withdraw cash, deposit (optional), transfer between accounts
- Cash dispenser tracks note counts per denomination
- Session timeout ends authenticated context
- Failed PIN attempts lock card after threshold
- Print receipt for each successful transaction
Non-functional requirements
- Single JVM, in-process design for this LLD article
- Core flows testable with in-memory repositories
- Extension points for new rules without editing central god classes
- Clear public API on facade/service layer
- CampusOS extension links to System Design when scale demands distribution
Overview
This CampusOS module designs ATM System as a production-ready in-process service. Read requirements point-by-point before drawing classes.
Requirements
Points to cover
- Customer inserts card and enters PIN to start authenticated session
- Check balance, withdraw cash, deposit (optional), transfer between accounts
- Cash dispenser tracks note counts per denomination
- Session timeout ends authenticated context
- Failed PIN attempts lock card after threshold
- Print receipt for each successful transaction
Session lifecycle
Points to cover
- Scope — what "Session lifecycle" covers inside ATM System and what it does not
- Classes — name 2–4 classes/interfaces responsible for this slice
- Flow — step-by-step: trigger → validation → state change → side effect
- Edge cases — invalid input, concurrent access, empty state, timeout
- Pattern — Strategy, State, Observer, or Facade if it fits naturally
- Test hook — one scenario an interviewer will ask you to walk through
Dispenser chain
Points to cover
- Scope — what "Dispenser chain" covers inside ATM System and what it does not
- Classes — name 2–4 classes/interfaces responsible for this slice
- Flow — step-by-step: trigger → validation → state change → side effect
- Edge cases — invalid input, concurrent access, empty state, timeout
- Pattern — Strategy, State, Observer, or Facade if it fits naturally
- Test hook — one scenario an interviewer will ask you to walk through
Concurrency
Points to cover
- Scope — what "Concurrency" covers inside ATM System and what it does not
- Classes — name 2–4 classes/interfaces responsible for this slice
- Flow — step-by-step: trigger → validation → state change → side effect
- Edge cases — invalid input, concurrent access, empty state, timeout
- Pattern — Strategy, State, Observer, or Facade if it fits naturally
- Test hook — one scenario an interviewer will ask you to walk through
// CampusOS — AtmSystem facade (keep thin)
public final class AtmSystemService {
private final AtmSystemRepository repository;
public AtmSystemService(AtmSystemRepository repository) {
this.repository = repository;
}
/** Orchestrate: validate → domain rules → persist → return result */
public Result handle(Request request) {
request.validate();
var domain = DomainModel.from(request);
domain.applyRules();
return repository.save(domain);
}
}
Quick recall
Everything you need if you only revisit this box.
- Requirements — review this slice before mock interviews.
- Session lifecycle — review this slice before mock interviews.
- Dispenser chain — review this slice before mock interviews.
- Concurrency — review this slice before mock interviews.
CampusOS extension
When this outgrows one JVM
- Shard inventory or state by campus ID or geographic region
- Push notifications, payments, and search to dedicated services (System Design track)
- Use message queues for async side effects (email, analytics)
- The class diagram you drew here becomes one box on the HLD architecture diagram
Test yourself
Answer these before moving on — recall is what makes it stick.