Finite State Machines

A finite state machine (fsm) diagram, also called a statechart diagram, is a directed graph. The nodes represent internal states of some abstract machine. The arrows represent state transitions. A state transition is usually triggered by some event, such as receiving a signal, or timing out. At any given moment of time only one state in an FSM is considered to be the active state. One state is usually designated as the start state. Zero or more states may be designated as final or accept states.

Here's a simple process model:

In this diagram states represent actions.

Note the start and final states.

The "done" label is a timing event.

An unlabeled transition is triggered by the completion of the source action state.

Transitions

A transition is enabled if its source state is active and if any additional enabling condition is true. Only an enabled transition can be executed. Executing a transition changes the active state from the source state of the transition to its destination state. In addition, some actions may be performed.

The (optional) label of a transition arrow has the form:

trigger[guard]/effect

where

trigger = the event causing the transition to execute
guard = an additional enabling condition
effect = actions performed by executing the transition

Trigger events can be:

signal events
call events
timing events
change events

For example: