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Spring state machine - demo

7.3.2021

This demo application presents Spring State Machine. State machine is an ancient design pattern, but it’s widely used and sometimes it can be handy.

Spring state machine demo on github

Abstract

This application implements a simple auction. If market is open, cusomers can send offers. At end of business minutes it will evaluate the results. Best price will win. In case of equal offers it will use the ‘First come, first served’ rule.

Two actors are defined: user and administrator. User can write price, nothing more. User offers are accepted when market is open.

There is scheduled start in every five minutes, with two minutes long trading window, but Administrator can start and stop the market manually. In addition to that administrator is able to get finished deals.

State Machine

Application’s state machine is available on pictur below. Implementation is in StateMachineConfig class. Do not forget @EnableStateMachine annotation in main Spring Boot class. (SmDemoApplication)

State machine

Basic Spring State machine elements: State, Event, Transition, Guard, Action, Fork and Join Some of them are obvious. Transition is the most important, because it defines the acceptable changes and behaviours. Conditions are supported by Guards, more tasks are supported by Fork and Join.

// example Transiton definition
transition.withExternal()
        .source(AppState.RUNNING)
        .target(AppState.COMPLETED)
        .event(AppEvent.COMPLETE)
        .action(valamiAction)

Actions can be attached to Transitions or States. It implements standalone functionality.

It is worth to mention stateContext.getExtendedState().getVariables() Map. All data in this map are available in state machine context from different sources. Now it stores just bargain startup timestamp, but it has much more possibilities.

I would recommend to take a look at Spring State Machine dokumentation .

A quick summary about our application states.

  • INIT : just bootstrap
  • WAITING : base camp state :)
  • RUNNING : market is open, offers accepted
  • COMPLETED : minimum one valid offer arrived
  • FAILED : no offer arrived

State Machine is available just through StateMachineManager class. It will be used by Dependency Injection.

State Machine test is not always trivial. For state transition tests there is Listener, which is able to save the states in a list. After that it’s easy to validate the changes.

Security

We need to differentiate user and administrator, so we need authentication and authorization. Just for this demo BasicAuth was used. It is not recommended in production!

When Spring Boot security was added to dependency list, it has been activated automatically. It can be surprise sometimes.

SecurityConfig class describe all security related configuration. Spring Boot has default configs for everything (here), however if it is not the wanted behaviour, then do not forget disable auto configuration.

@SpringBootApplication(exclude = {
		SecurityAutoConfiguration.class,
		ManagementWebSecurityAutoConfiguration.class
})

Based on this article it’s not impossible to create a working setup.

Secure page testing is not obvious, SecurityConfigTest implementation can be good entry point. If secure access is not needed, then it can be disabled:

@AutoConfigureMockMvc(addFilters = false)

Web UI

It is immediately visible, that Frontend is not a strenght in this app. But a minimal UI is still better for a demo compare to Swagger REST API.

VueJS is a comfortable tool for small web applications, it can cover our minimal REST requests.

Current state is continously polled in every five seconds. It is not a real reactive solution, but this is not the focus now. offer page uses the Javascript’s scheduler with Vue created() function.

var app = new Vue({
    el: '#app',
    data() { 
      ...
      timer: ''
    },
    methods: {
      ...
      getCurrentState: function() {...}
    },
    created () {
        this.getCurrentState();
        this.timer = setInterval(this.getCurrentState, 5000);
    }
});

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Screenshots

index page offer page admin page

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