Distributed state is probably one of a most complicated concepts of a Spring State Machine. What exactly is a distributed state? A state within a single state machine is naturally really simple to understand but when there is a need to introduce a shared distributed state through a state machines, things will get a little complicated.

For generic configuration support see section Section 10.9, “Configuring Common Settings” and actual usage example see sample Chapter 42, Zookeeper.

Distributed State Machine is implemented via a DistributedStateMachine class which simply wraps an actual instance of a StateMachine. DistributedStateMachine intercepts communication with a StateMachine instance and works with distributed state abstractions handled via interface StateMachineEnsemble. Depending on an actual implementation StateMachinePersist interface may also be used to serialize a StateMachineContext which contains enough information to reset a StateMachine.

While Distributed State Machine is implemented via an abstraction, only one implementation currently exists based on Zookeeper.

Here is a generic example of how Zookeeper based Distributed State Machine would be configured.

@Configuration
@EnableStateMachine
public class Config
        extends StateMachineConfigurerAdapter<String, String> {

    @Override
    public void configure(StateMachineConfigurationConfigurer<String, String> config)
            throws Exception {
        config
            .withDistributed()
                .ensemble(stateMachineEnsemble())
                .and()
            .withConfiguration()
                .autoStartup(true);
    }

    @Override
    public void configure(StateMachineStateConfigurer<String, String> states)
            throws Exception {
        // config states
    }

    @Override
    public void configure(StateMachineTransitionConfigurer<String, String> transitions)
            throws Exception {
        // config transitions
    }

    @Bean
    public StateMachineEnsemble<String, String> stateMachineEnsemble()
            throws Exception {
        return new ZookeeperStateMachineEnsemble<String, String>(curatorClient(), "/zkpath");
    }

    @Bean
    public CuratorFramework curatorClient()
            throws Exception {
        CuratorFramework client = CuratorFrameworkFactory
                .builder()
                .defaultData(new byte[0])
                .connectString("localhost:2181").build();
        client.start();
        return client;
    }

}

Current technical documentation of a Zookeeker based distributed state machine can be found from an appendice Appendix C, Distributed State Machine Technical Paper.