Selecting with multiple interfaces
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This document discusses three levels (signature, semantic, and protocol) at which component interfaces and services can be specified and substituted. At the signature level, substitutability is determined by whether all services of one component are offered by another. At the protocol level, substitutability requires messages be consistently accepted and ordered. At the semantic level, components must conform behaviorally as well as syntactically to allow substitution. The document also defines equivalence and interoperation between interfaces or components at each of the three levels. Equivalence means two components can substitute each other, while interoperation means components can successfully interact in their defined roles.
Selecting with multiple interfaces
- 2. Component capabilities and usages are specified by interfaces. A service abstraction, that defines the operations that the service supports, independently from any particular implementation
- 3. • Signature level : The interfaces describe just the names of the methods, the types of their arguments, and the return values—i.e., the signature. • Semantic level : deals with the “meaning” of operations—i.e, the behavior. • Protocol level : deals just with the components’ service access protocols—i.e., the partial order in which components expect their methods to be called, and the order in which they invoke other methods.
- 4. Substitutability - refers to the ability of a component to replace another so that clients of the first one remain unaware of the change. This operator defines a partial order between components and is usually noted by “≤”. With it, given an application and two components C and D, D ≤ C means that we can replace C with D in the application with the warranty that the application will continue working without problems.
- 5. At the signature level, substitutability D ≤ C roughly a matter of checking that all services offered by C are also offered by D. At the protocol level, we need to check two main issues: (a) that all messages accepted by C are also accepted by D, and that outgoing messages of D are a subset of the outgoing messages of C;and (b) we need to test that the relative order among the incoming and outgoing messages of both components are consistent. Finally, semantic level substitutability is known in this context as “behavioural subtyping”. Here, the behaviour refers to the specification on how the object methods manipulate the object attributes, and behavioural types are defined as an extension of object signature types that associate behaviour to signatures and to identify subtypes that conform to their supertypes not only syntactically, but also semantically.
- 6. Equivalence Relation Between interfaces, and say that two interfaces R1 and R2 are equivalent (R1 ≡ R2) iff C1 ≤ C2 and C2 ≤ C1
- 7. Interoperation (noted by “∞”) At the signature level this means that all exchanged messages are understood by each other, and at the protocol level that their protocols match in each role they share. At the semantic level, it implies that the behaviour provided by a component should be accordant to the behaviour expected from its client component, as has been the basis for the “design by contract” development discipline.