State Machine Tutorial

A state machine is a computational model that can be in exactly one of a finite number of states at any given time.

• States: Define the status of the system at a point in time.
• Events: Trigger transitions between states.
• Transitions: Define rules for moving from one state to another.

A state machine can be formally defined as a 5-tuple:

M = (Q, Σ, δ, q₀, F)

• Q: Set of all states
  Q = {DRAFT, REVIEW, APPROVED, REJECTED}
• Σ: Set of input events
  Σ = {SUBMIT, APPROVE, REJECT, EDIT}
• δ: Transition function
  δ(DRAFT, SUBMIT) → REVIEW
δ(REVIEW, APPROVE) → APPROVED
δ(REVIEW, REJECT) → REJECTED
δ(REJECTED, EDIT) → DRAFT
• q₀: Initial state
  q₀ = DRAFT
• F: Final states
  F = {APPROVED, REJECTED}

1. Add spring-statemachine-core as a dependency.
2. Use enums to define states and events.
3. Configure transitions using StateMachineConfigurerAdapter.

Java Example:

@Configuration
  @EnableStateMachine
  public class StateMachineConfig extends StateMachineConfigurerAdapter {
  @Override
  public void configure(StateMachineStateConfigurer states) throws Exception {
    states.withStates()
      .initial(State.DRAFT)
      .state(State.REVIEW)
      .end(State.APPROVED)
      .end(State.REJECTED);
  }

  @Override
  public void configure(StateMachineTransitionConfigurer transitions) throws Exception {
    transitions
      .withExternal().source(State.DRAFT).target(State.REVIEW).event(Event.SUBMIT)
      .and()
      .withExternal().source(State.REVIEW).target(State.APPROVED).event(Event.APPROVE)
      .and()
      .withExternal().source(State.REVIEW).target(State.REJECTED).event(Event.REJECT)
      .and()
      .withExternal().source(State.REJECTED).target(State.DRAFT).event(Event.EDIT);
  }
}

With Mono:

Mono.just(Event.SUBMIT)
  .doOnNext(event -> stateMachine.sendEvent(event))
  .subscribe();

With Flux:

Flux.just(Event.SUBMIT, Event.APPROVE)
  .delayElements(Duration.ofSeconds(1))
  .doOnNext(event -> stateMachine.sendEvent(event))
  .subscribe();

.withExternal()
  .source(State.DRAFT)
  .target(State.REVIEW)
  .event(Event.SUBMIT)
  .guard(ctx -> isValid())
  .action(ctx -> log.info("Transitioned to REVIEW"));