It is impossible to persist a StateMachine using normal java serialization as object graph is too rich and contains too much dependencies into other Spring context classes. StateMachineContext is a runtime representation of a state machine which can be used to restore an existing machine into a state represented by a particular StateMachineContext object.

Building a StateMachineContext and then restoring a state machine from it has always been a little bit of a black magic if done manually. Interface StateMachinePersister aims to ease these operations by providing persist and restore methods. Default implementation of this interface is DefaultStateMachinePersister

Usage of a StateMachinePersister is easy to demonstrate by following a snippets from tests. We start by creating to two similar configs for a state machine machine1 and machine2. We could build different machines for this demonstration using various other ways but this serves a purpose for this case.

@Configuration
@EnableStateMachine(name = "machine1")
static class Config1 extends Config {
}

@Configuration
@EnableStateMachine(name = "machine2")
static class Config2 extends Config {
}

static class Config extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states) throws Exception {
        states
            .withStates()
                .initial("S1")
                .state("S1")
                .state("S2");
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions) throws Exception {
        transitions
            .withExternal()
                .source("S1")
                .target("S2")
                .event("E1");
    }
}

As we’re using a StateMachinePersist we simply create an in-memory implementation.

	Note
	In-memory sample is just for demonstration purposes, use a real persistent storage implementations.

static class InMemoryStateMachinePersist implements StateMachinePersist<String, String, String> {

    private final HashMap<String, StateMachineContext<String, String>> contexts = new HashMap<>();

    @Override
    public void write(StateMachineContext<String, String> context, String contextObj) throws Exception {
        contexts.put(contextObj, context);
    }

    @Override
    public StateMachineContext<String, String> read(String contextObj) throws Exception {
        return contexts.get(contextObj);
    }
}

After we have instantiated two different machines we can transfer machine1 into state S2 via event E1, then persist it and restore machine2.

InMemoryStateMachinePersist stateMachinePersist = new InMemoryStateMachinePersist();
StateMachinePersister<String, String, String> persister = new DefaultStateMachinePersister<>(stateMachinePersist);

StateMachine<String, String> stateMachine1 = context.getBean("machine1", StateMachine.class);
StateMachine<String, String> stateMachine2 = context.getBean("machine2", StateMachine.class);
stateMachine1.start();

stateMachine1.sendEvent("E1");
assertThat(stateMachine1.getState().getIds(), contains("S2"));

persister.persist(stateMachine1, "myid");
persister.restore(stateMachine2, "myid");
assertThat(stateMachine2.getState().getIds(), contains("S2"));

Support for persisting State Machine into Redis is done via RepositoryStateMachinePersist which implements StateMachinePersist. Specific implementation is a RedisStateMachineContextRepository which uses kryo serialization to persist a StateMachineContext into Redis.

For StateMachinePersister we have a redis related RedisStateMachinePersister implementation which takes an instance of a StateMachinePersist and uses String as its context object.

	Tip
	Check sample Chapter 45, Event Service for detailed usage.

RedisStateMachineContextRepository will need a RedisConnectionFactory for it to work and we recommend a JedisConnectionFactory for it as seeing from above example.