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4.3 Interfaces

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  1. 4.3 Interfaces

  2. Interfaces • Define the component’s access points • Will normally have multiple interfaces • Standard naming schemes needed to reference interfaces • Standardisation of message formats also needed

  3. Interfaces (2) • Two types of interfaces • Provided interfaces • Required interfaces

  4. Interfaces (3) Print service Required Interfaces Provided Interfaces Print GetPDFile GetQueue Remove PrinterInt Transfer Register Unregister

  5. Interfaces (4) • Are the means by which components connect • Is a set of named operations that can be invoked by clients

  6. Interfaces (5) • Dual role of the interface specification • Providers implementing the interface • Clients using the interface

  7. Interfaces (6) • Component specification • A component can be specified separate from their interfaces • A component specification includes • All the interfaces that the component should support • Component-specific properties • Multiple component implementation can conform to such specification

  8. Interfaces (7) • Versions • Immutable interfaces • Multiple versions are supported by multiple interfaces • Constrained changes • Rules define the kind of changes that are valid to retain backward compatibility

  9. Interfaces (8) • Contracts • Interface specifications can be viewed as contracts between clients and providers • Contract state • what the client needs to do to use the interface • What the provider has to implement to meet the services define in the interface

  10. Interfaces (9) • A popular contractual specification method • The client meets a precondition before calling the operation • The provider meets a postcondition before returning to the client

  11. Interfaces (10) • When implementation is changed • Contracts should be maintained • At most, the provider most required less and provide more

  12. Interfaces (11) • Non-functional requirements • A contract should include all essential functional and non-functional aspects • Specifying complexity bounds • Might include platform specific performance figures

  13. Abstraction levels • Meyer identifies 5 abstraction levels: • Functional abstraction • Casual abstraction • Data abstraction • Cluster abstraction • System abstraction

  14. Some more terms • Component model [Szyperski] • Defines the rules of composition

  15. Some more terms (2) • Component Model [Wallnau] • “Is the set of component types, their interfaces, and, additionally, a specification of the allowable patterns of interaction among component types”

  16. Some more terms (3) • Component framework • “Is a set of interfaces and rules of interaction that govern the interaction of components ‘plugged into’ them.” • Is a reusable architectural design targeting a specific domain whereby desired architectural properties and invariants are enforced

  17. 4.4 Callbacks and contracts

  18. Callbacks and contracts • A callback is a procedure that is passed to a library at one point Other party Library Client Client installs callback Third party calls library Library invokes callback Callback queries library Callback returns Library returns

  19. Callbacks and contracts (2) • Common feature in procedural libraries that have to handle asynchronous events • Windowing libraries use a callback to notify a particular window’s client code when the user resized the window

  20. Callbacks and contracts (3) • A library can make part of its state observable by exporting variables or inspection functions • The relative order in which callbacks are performed can be observed • However, such ordering is not specified by normal pre- and postconditions

  21. 4.5 Polymorphism

  22. Polymorphism • Polymorphism is • The ability of something to appear in multiple forms, depending on the context

  23. Polymorphism • Same interface • Used by large numbers of different clients • Supported by a large number of different providers • Keep balance of what is required from providers and clients

  24. Polymorphism • If an interface requires too little from providers • it is useless • If an interface requires too much from providers or clients • it is difficult to implement

  25. Polymorphism • A carefully crafted interface requires no more than is essential • Clients and providers are always free to overfulfill their contract

  26. Polymorphism • Pre- and post conditions are usually specified using predicates • Which define • The permitted input and output values • The permitted values of an internal variable • Operations that should be executed • Example: • len<max and 0<=position<=max

  27. Polymorphism • When is it legal to substitute one service provider for another? • When a provider satisfies the contract

  28. Polymorphism • Ideally, contracts should be explicitly specified as part of an interface spec • It would be desirable to have a compiler to check contract compatibility • In reality, it is far from attainable: • Formalising contracts is difficult and expensive • Automatic checking still part of research

  29. Polymorphism • Checking can be done at: • compiling-time • Load-time • Run-time better

  30. Polymorphism • Type • Group of values of related semantics • Basic types, such as INTEGER and REAL, can be understood as sets of values • In case of objects and interfaces, type is a set of objects implementing a certain interface

  31. Polymorphism • Types as simplified contracts • The types of the in-out parameters form part of: • an operation’s preconditions • An operation’s postconditions • However, other part of the pre and postconditions are not part of a type • Successful type checking does not ensure contracts are not violated

  32. Polymorphism • Subtyping • Named subtyping (interface inheritance) • Base interfaces are named • Structural subtyping • Base interface are not named • Operations are repeated • Is a superset of operations defined by another type

  33. Polymorphism • There is polymorphism • When a variable can refer to objects of multiple subtypes • Example: • The variable of type View can refer to objects of type View, TextView, GraphicsView, etc

  34. Architecture not enough to achieve qualities

  35. Quality scenarios: Examples

  36. Quality scenarios: Examples

  37. Open Distributed Processing • Openness required to overcome the problems of • Complexity • Heterogeneity • ODP’s main function: • distributed services transparently located and accessed • regardless of the underlying platform

  38. Open Distributed Processing • An open system [Coulouris]: • is one that can be extended and implemented in different ways; • for this the key interfaces must be made public

  39. We require: • To standardise well-defined interfaces • ODP can achieve: • Interoperability • Portability • Middleware is an effort to meet ODP’s principles

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