Essential Business Process Modeling

State Machines and Activity Diagrams

With its proud history and redoubtable theoretical credentials, the state machine (like the pi-calculus and the Petri net) is a guiding light to the practice of process design. As you will see, the representation of a process as a state diagram is markedly different from its more intuitive flow-chart implementation, and—being based on a more rigorous, more expressive system—is often clearer and more compact

State diagrams have a rich history. Alan Turing, the father of computer science, used the concept to build a model of computability. (See the sidebar "A Noble History...“) State machine theorists Mealy, Moore, and Harel expanded the subject. Mealy and Moore built similar models whose essential differences—Mealy held that actions are performed in transitions, Moore that they are performed in states—were reconciled in Harel’s later work on state charts. Harel allowed actions in both states and transitions, and enhanced the previously flat models with nested states and concurrent states. The UML state diagram, one of the leading contemporary approaches, is heavily influenced by Harel. The UML activity diagram (whose Version 2.0 specification, interestingly, uses Petri net token semantics), though essentially a flow charting technique, is in some respects a special case of the UML state diagram approach. Figure 3-13 depicts these historical milestones.

Figure 3-13. State machine family tree

A Noble History: The State Machine and the Origins of Computer ...