state transition diagram in software engineering

state transition diagram

A State Transition Diagram (STD) is a modeling tool that represents the time-dependent aspects of a system. An STD is a good tool used to model the sequencing of many real-time systems.

state transition diagram

STD models time-dependent behavior. Many complex systems have time-dependent behavior i.e. the sequence in which data will be accessed and functions will be performed. 

For some information transaction systems, the sequence is trivial. However, many real-time systems in engineering applications have complex time-dependent behavior that has to be considered during the system analysis stage.

A stage transition diagram usually has an identified initial stage, although this is not required. What is required, however, is that an STD can only have at most one initial state. The initial state is indicated by an unlabelled transition arrow entering a state and coming from no particular source.

Similarly, an STD usually has an identified final state. However, unlike initial states, there is no restriction on the number of possible final states. A final state is identified by a state that has no transitions leaving it i.e. once the system has entered the state, no other state will be entered.

STDs are similar to DFDs in that STDs may also be leveled or, more accurately, partitioned. That is, one state may be decomposed into several more finely grained states with their own state transitions.

Symbols Used for STDs

State transition diagrams are simple diagrams, as there are only two types of symbols defined, states and transitions.

  • A state is usually denoted either with a rectangle or an oval. Both are acceptable, but they must be consistent within a diagram.
  • The state is labeled with a noun.
  • A transition is denoted by an arrow line connecting one state to another, with the arrow indicating the transition direction.
  • The transition is labeled with a condition, and optionally a corresponding action.
  • A condition is usually a noun phrase, while action is most often a verb phrase.

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