State Machines

What

• A state machine can be loosely defined as anything with a set of known states

• A system or process, where it can only be in one state at a time

• A collection of states, which when triggered by an event starts a transitions a state from the old state to a new state.

  • As part of this transitioning, actions can take place

• These events can be

  • Times/dates/timers will do something

  • incoming request to the system (http request, gui trigger)

  • Scheduled action

  • listening for some change somewhere (filesytem being updated, a queue)

• When in the new state, it can take some actions

  • ie store the state in the database, inform the user (request/gui update), validate next transition from event (is event legal)

History

• Comes from the idea of Finite Automata invented by McCulloch and Pitts in 1943

• Finite Automata consists of

  • Finite set of states

  • Set of inputs

  • Initial State

  • Final State

  • Transition Function

Why use

• In general, most code is a state machine, using

  • multiple if -else

  • case/switch statements

  • These can get messy and confusing, hard to debug with more states

• Using a FSM can help improve clarity of the flow from state to state

  • define consistent behavior for a finite number of states

  • Application logic is defined for specific states or state transitions

  • Application logic becomes more modular and more precisely defined

Usage

• Message (Event) based applications

• Events get published based on state changes

• UI Applications With Actions triggered by Use - Caps Lock On, Caps Lock Off

• Application behavior changes based on known states

Terminology

• States - The specific state of the state machine. Finite and predetermined values.

  • Frequently defined in an enumeration

• Events - Something that happens to the system - may or may not change the state.

• Actions - The response of the State Machine to events. Can be changing variables, calling a method or changing to a different state

  • Transitions - Type of action which changes state

• Guards - Boolean conditions

• Extended State - State Machine variables (in addition to state)

• https://medium.com/datadriveninvestor/state-machine-design-pattern-why-how-example-through-spring-state-machine-part-1-f13872d68c2d

Examples

• https://github.com/shanwu/finite_state_machine/tree/master/FiniteStateMachine/src/main/java/FiniteStateMachine

• https://www.baeldung.com/java-enum-simple-state-machine

• https://blog.frankel.ch/builder-pattern-finite-state-machine/

• https://medium.com/datadriveninvestor/state-machine-design-pattern-part-2-state-pattern-vs-state-machine-3010dd0fcf28

• https://rclayton.silvrback.com/use-state-machines

• http://www.java2s.com/Code/Java/Collections-Data-Structure/AprogrammableFiniteStateMachineimplementation.htm