Design an Order State Machine

Placed → paid → shipped → delivered with guarded transitions.

Expert15 min read

Read these first

Design a Deck of Cards

Why this matters

Scope — what "States" covers inside Order State Machine and what it does not
Scope — what "Transitions" covers inside Order State Machine and what it does not
Scope — what "Side effects" covers inside Order State Machine and what it does not
Scope — what "CampusOS store" covers inside Order State Machine and what it does not
Games and architecture patterns signal staff+ depth.

Idle

Moving

DoorOpen

Idle

State pattern keeps door logic separate from movement logic.

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

States: PLACED, PAID, SHIPPED, DELIVERED, CANCELLED
Legal transitions only — illegal transition throws
Side effects on transition: charge, notify warehouse
Cancel allowed only before SHIPPED

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 Order State Machine as a production-ready in-process service. Read requirements point-by-point before drawing classes.

States

Points to cover

Scope — what "States" covers inside Order State Machine 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

Transitions

Points to cover

Scope — what "Transitions" covers inside Order State Machine 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

Side effects

Points to cover

Scope — what "Side effects" covers inside Order State Machine 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 store

Points to cover

Scope — what "CampusOS store" covers inside Order State Machine 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

Java

// CampusOS — OrderStateMachine facade (keep thin)
public final class OrderStateMachineService {
    private final OrderStateMachineRepository repository;

    public OrderStateMachineService(OrderStateMachineRepository 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.

States — review this slice before mock interviews.
Transitions — review this slice before mock interviews.
Side effects — review this slice before mock interviews.
CampusOS store — 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.