Formal architectural abstractions for interactive software

ion-side and Display-side behaviours of an interactor. Interactors are characterised by the behaviour they exhibit when observed from the abstraction or the display sides. The abstraction-side behaviour of the interactor is observed by interaction at gates Gaout and Gainp. The display-side behaviour is observed at gates Gdinp and Gdout. They are described by the pseudo-LOTOS expressions: A P = hide ( G c ∪ G dinp ∪ G dout ) in P D P = hide ( G c ∪ G ainp ∪ G aout ) in P Abstraction and Display Equivalence. Two interactors P and Q (or two behaviour expressions describing two states of the same interactor) are called abstraction equivalent if their abstraction-side behaviours are observationally equivalent, i.e. AP♠AQ. They will be called display equivalent when their display-side behaviours are observationally equivalent, i.e. DP♠DQ.ion and Display Equivalence. Two interactors P and Q (or two behaviour expressions describing two states of the same interactor) are called abstraction equivalent if their abstraction-side behaviours are observationally equivalent, i.e. AP♠AQ. They will be called display equivalent when their display-side behaviours are observationally equivalent, i.e. DP♠DQ. The meaning of these definitions depends on the equivalence relation ♠ they stipulate. The choice between equivalence relations for processes is a contentious issue, cf. de Nicola (1989), which, in the present context, impinges on the ability of humans to tell apart interactive behaviours and to detect and interpret differences of the display contents. Without wishing to postulate a theory of user cognition, weak observational equivalence is used here as it distinguishes processes only with respect to observable interactions with their environment and its verification is supported by model checking tools, e.g., Fernandez, Garavel, Mounier, Rasse, Rodriguez and Sifakis (1992). These definitions are used in table 1 which is a summative classification of observability and predictability properties results comparable to an earlier classification by Abowd (1992). Consider, for example, the first row of table 2. An interactor is called display predictable, if the similarity of two instances of its display implies that they are also display equivalent. In other words, the display status of the interactor determines its display-side behaviour. A symmetrical definition of result predictability can be written as in row 2 of table 2.