CBSE 2014Component specification

Bibliography• Main bibliography:

– Sommerville: Software Engineering, 8th Edition• Chapter 19.3 Component Composition

– Ivica Crnkovic, Magnus Larsson (Eds.): Building reliable component-based systems

• Chapter 2: Specification of Software Components

• Chapter 6: Semantic Integrity in Component Based Development

• Other readings:– B.Meyer: Applying Design by Contract– Mary Shaw, Truth vs Knowledge: The Difference Between What

a Component Does and What We Know It Does

Review: Component modelsComponents must obey to common conventions or standards !Only in this way they will be able to connect and communicate to each other

Review: Component Frameworks

Component Component Component

Component platform (component framework)

Operating System Middleware

Hardware

Platform Services: allow components written according to the model to communicate; locating, linking, replacing components

Horizontal Services: application-independent services used by different components. Concurrency, security, transaction management, Resource management

Component composition

• Composition involves integrating components with each other and with the component infrastructure.

• The ways in which components are integrated with this infrastructure are specific for each component model

• In the discussion of this chapter, we assume that the components to be subject to composition belong to the same component model, thus they can be integrated

• Component composition is the process of assembling the components to create a system.

• Normally you have to write ‘glue code’ to compose components.

Types of composition

a) Sequential composition where the composed components are executed in sequence. This involves composing the provides interfaces of each component.– Glue code: calls component A, collects result, then calls component B

with that result as parameter

b) Hierarchical composition where one component calls on the services of another. The provides interface of one component is composed with the requires interface of another.

c) Additive composition where the interfaces of two components are put together to create a new component.

Types of composition

(a)

A A

B B

A B

(b) (c)

Figure 19.9 from [Sommerville]

Component incompatibility

• When you design components specially for composition, interfaces are designed to be compatible

• When the components are developed independently for reuse, interfaces may be incompatible:

• Interface incompatibility (Syntactic incompatibility)

• Semantic incompatibility

• We recall that only components belonging to same component model are considered (thus integration is physically possible)

Example

• Expression Component – Multiplier Component• Interface incompatibility: The expression component requires

an operation multiply, while the multiplier component provides an operation named do_op

• Semantic Incompatibility: The multiplier component implements the operation by repeated addition, thus it works if the second number is a positive integer. The expression component needs to multiply any integers (positive and negative).

Interface incompatibility

• Situations of Interface incompatibility:– Parameter incompatibility where operations have the same name but

are of different types.– Operation incompatibility where the names of operations in the

composed interfaces are different.– Operation incompleteness where the provides interface of one

component is a subset of the requires interface of another

• Interface incompatibility is addressed by writing adaptors

• Adaptors address the problem of component incompatibility by reconciling the interfaces of the components that are composed.

• Different types of adaptor may be required depending on the type of composition.

Semantic incompatibility

• Component composition assumes you can tell from the component documentation whether the interfaces are compatible

– Semantic compatibility: the meaning of parameters is right

• You have to rely on component documentation to decide if interfaces that are syntactically compatible are actually compatible.

Component specification

• There should be no difference between:– What a component does

– What we know it does

• The only way we get to know what a component does is from its component specification

• Levels of a component specification:– Syntax: includes specifications on the programming language

level.

– Semantic: functional contracts

– Non-functional: deals with quality of service.

Component specification levels

• Levels of a component specification:– Syntax: includes specifications on the programming Syntax: includes specifications on the programming

language level.language level.– Semantic: functional contracts– Non-functional: deals with quality of service.

Components and Interfaces

• A component provides:

– The implementation of a set of named interfaces, or types, each interface being a set of named operations

• The following diagram is a UML metamodel

– This model allows an interface to be implemented by several different components, and an operation to be part of several different interfaces

Metamodel of the concepts used in syntactic specification of software components

Figure 2.1 from [Crnkovic]

Example: component SpellChecker

«comp spec»SpellChecker

ISpellCheck

ICustomSpellCheck

Implementation as a COM component:• Uses an IDL

IDL Example

interface ISpellCheck : IUnknown{

HRESULT check([in] BSTR *word, [out] bool *correct);}; interface ICustomSpellCheck : IUnknown{

HRESULT add([in] BSTR *word);HRESULT remove([in] BSTR *word);

}; library SpellCheckerLib{

coclass SpellChecker{

[default] interface ISpellCheck;interface ICustomSpellCheck;

};};

Uses of Syntactic Specification

• The primary uses of syntactic specifications are:– Type checking (static of dynamic) of client code. – Base for interoperability between independently developed

components and applications. • Interoperability may be achieved in different ways:

– Binary format for interfaces– IDL to programming language mappings

• An important aspect of interface specifications is how they relate to substitution and evolution of components

Substitution

• Substituting a component Y for a component X is said to be safe if:– All systems that work with X will also work with Y

• From a syntactic viewpoint, a component can safely be replaced if:– The new component implements at least the same interfaces

as the older components, or– The interface of the new component is a subtype of the

interface of the old component.

Forms of syntactic specification

• All component models use syntactic specification of interfaces:– Programming language – IDL

• Examples – Microsoft’s Component Object Model (COM) – Common Object Request Broker Architecture (CORBA) – JavaBeans

Component specification levels

• Levels of a component specification:– Syntax: includes specifications on the programming

language level.– Semantics: functional contractsSemantics: functional contracts– Non-functional: deals with quality of service.

Contracts

Contracts for the semantic specification of components are a kind of Meyer’s Design by Contract:

“Applying Design by Contract,” B. Meyer, IEEE Computer, pp. 40-51, October 1992.

Design-by-contract background

• A Client-Server Design

• Server Objects– Provides services for client objects to use– The object whose methods are being invoked

• Client Object– Consumes the services offered by the supplier object– The object that invokes the methods of the supplier object

• Contract– A set of benefits and obligations that are mutually agreed upon by the

client and supplier

– In practice, specified by the supplier object

– Clients implicitly accept the contract by using objects of the supplier class

• Good contracts are always in writing!

Contracts in real life - Example

Table 1 from [Meyer]

Contract with a courier delivery service:

Party Obligations Benefits

Client

Provide package of no more than 5 kgs, each

dimension no more than 2 meters; pay 100 francs

Get package delivered to recipient in 4 hours or less

Supplier Deliver package to recipient in 4 hours and less

No need to do deal with deliveries too big, too heavy

or unpaid

What is a Contract?

• A contract between a client and a supplier protects both sides– It protects the client by specifying how much should be done to get the

benefit. The client is entitled to receive a certain result.

– It protects the supplier by specifying how little is acceptable. The supplier must not be liable for failing to carry out tasks outside of the specified scope.

• If a party fulfills its obligations it is entitled to its benefits– No Hidden Clauses Rule: no requirement other than the obligations

written in the contract can be imposed on a party to obtain the benefits

Contracts for softwareExample: add node to tree

• Contract is between:– Client = caller– Supplier = called routine

• Example: – A class Tree describing tree nodes– A routine put_child for adding a new child to the tree node Current– The routine put_child receives a reference to an existing node object

and adds it as a child of Current • Problem: a reference is either void or attached to an object. • Question: who should check if new is void ? • Approaches:

– Defensive programming: include as many checks as possible, even if redundant with checks done by caller -> leads to increased complexity, which leads to probability of errors

– Contracts

Table 2 from [Meyer]

Contracts for softwareExample: add node to tree

Table 2 from [Meyer]

Informal description of contract::

Party Obligations Benefits

Client Use as argument a reference, new, to an existing node object

Get updated tree where Current has one more child

– the new node.

Supplier Insert node new as child of Current

No need to do anything if new is void

Contracts for softwareExample: add node to tree

• Contracts for software are expressed through:– preconditions - the obligations of the client – postconditions - properties that are ensured in return by the

execution of the function call

• Example:Operation put_child (new:Node);

-- add new to the children of Current

Precondition: new is not voidPostcondition: new’s parent is Current and the children-count of current has been increased by 1

Weak and strong contracts“Who has to check all the conditions?”

• Postconditions – specify the exit conditions guaranteed by an operation at its end provided the

precondition was satisfied at the entry in the operation– The outcome when the precondition was not satisfied is explicitly left

undefined [Meyer]

• Strong contract:– the precondition specifies conditions for success

– postconditions need to specify only the outcome in the well-defined situations– Back-end-components usually have strong contracts

• Weak contract: – the precondition is incomplete, the component must be able to filter out

invalid uses– The postconditions will specify also the outcome of the invalid uses – Front-end-components (such as GUI-components) usually have weak

contracts

Contracts for components

Contracts for components are expressed through:• Pre-conditions• Post-conditions• Invariants

A Pre-condition

• Is an assertion that the component assumes to be fulfilled before an operation is invoked.

• Is a predicate over the operation’s input parameters and this component’s state

A Post-condition

• Is an assertion that the component guarantees will hold just after an operation has been invoked, provided the operation’s pre-conditions were true when it was invoked.

• Is a predicate over both input and output parameters as well as the state just before the invocation and the state just after

An Invariant

• Is a predicate over the interface’s state model that will always hold

• A set of invariants may be associated with an interface.

Metamodel of the concepts used in semantic specification of software components

Figure 2.2 from [Crnkovic]

Semantic specification of components

• Semantic specification of a component comprises:– Specify component interfaces– For each interface, specify:

• Model of state and Invariants

• Operations with pre- and post-conditions

• The model allows that different interfaces act on the same state model

– Inter-interface constraints

Note that state models and operation semantics are associated with interfaces rather than with a component !

Component specificationExample: component SpellChecker

«comp spec»SpellChecker

ISpellCheck

ICustomSpellCheck

Specifying a component that provides interfaces

Interface specification diagram

Example: Interface ISpellCheck

• State: words• Operations:

– check (in word:String, out correct:Boolean):HRESULT;• Pre: the word to be checked is non-empty string

• Post: if the return value indicates success, then the value of correct is true if word was a member of words and false otherwise

Interface specification diagram Example: ICustomSpellCheck

• State: words

• Operations:– add (in word:String):HRESULT;

• Pre: the word to be added is non-empty string• Post: if the return value indicates success, then word has been added to words

– remove(in word:String):HRESULT;• Pre: the word to be removed is non-empty string• Post: if the return value indicates success, then word has been removed from

words

Inter-interface Constraints

• The component specification can be completed by the specification of its inter-interface constraints:

• Relationships between the models of state associated with the different interfaces

context SpellChecker

ISpellCheck::words = ICustomSpellCheck::words

Uses of Semantic Specification

• Tool support for component developers

• Tool support for developers of component-based applications

Substitution extended with semantics

• Substituting a component Y for a component X is said to be safe if:– All systems that work with X will also work with Y

• From a semantic viewpoint, a component can safely be replaced if:– A client that satisfies the preconditions for X must always satisfy the

preconditions specified for Y– A client that can rely on postconditions ensured by X can also be

ensured it can rely on Y

• Conditions for component Y:– Interfaces of Y can have weaker preconditions on operations– Interfaces of Y can have stronger postconditions on operations– State models of X and Y need not be identical

Levels of Formalism for Semantic Specifications

• The levels of formalism, in an increasing order of formalism: – No semantics

– Intuitive semantics

– Structured semantics

– Executable semantics

– Formal semantics

An Example

• Component RandomAccess– controlls the access to random access file of a record type R

– records of a fixed size

– access to the file is by record number, numbers start from 0.

– It is assumed that the file is continuous, thus record numbers go up to the current maximum number, called the high water mark

• Operations:– addRecord

– getRecord

– delRecord

– getHighWaterMark

The contract

• Operation getRecord – retrieves a record with a given number

• The precondition :– the single input parameter of the operation is the number of the record

concerned, which must exist in the file.

• The post-condition: – If an unrecoverable system error occurs (file system error) the operation

indicates a failure • Weak part of the contract: client does not have to check file status before

– the result of the operation is the required data record of type R.• Strong part of the contract: assumes that record number is always correctly

given

public R getRecord(int number) throws IOException

Level 0: No Semantics

• The following definition of the operation getRecord illustrates how a purely syntactic specification would be given:

public R getRecord(int number) throws IOException

Level 1: Intuitive Semantics

• Plain text, unstructured description and comments about a component and its parts

• An intuitive specification of the operation getRecord:

The operation getRecord retrieves a record by its number, returning the record requested. If an error occurs, such as a disk read error or a file system error, the an I/O error is returned.

Level 2: Structured Semantics

• The semantics is presented in a structured way but needs not be in accordance with any particular syntax or formalism

• A structured specification of the operation getRecord:

getRecord returns a record identified by its number. Parameters: number: the number of the record to retrieve, counted from zero Precondition: number >= 0 and number <= the high water mark Postcondition: the record with the given number is returned, unless a file system error occurs, in which case a file system error is reported and the value returned is undefined.

Level 3: Executable Semantics • The semantics is expressed in a way that can be executed and

controlled by the system during run-time. • The executable specification is included in the implementation

of the component• Limitation: not all conditions can be expressed in an executable

way

Executable specification for getRecord:

getRecord returns a record identified by its number.Parameters:number: the number of the record to retrieve, counted from zeroPrecondition: (0 <= number) && (number <= hwm())Postcondition: throw IOException || (result == record(number))

Examples of executable semantics

• For program implementations:

– Assertions in Java: http://docs.oracle.com/javase/7/docs/technotes/guides/language/assert.html

– MSDN: Code Contracts: http://msdn.microsoft.com/en-us/library/dd264808(v=vs.110).aspx

• For UML models: OCL (Object Constraint Language)

• General rules: – The execution of the assertions should not add functionality !

– Assertions serve to detect coding errors and should not try to handle or compensate for them

Ensuring a Correct Call

• The client code must check the precondition before the call, as illustrated below:

if ((0 <= number) && (number <= theFile.hwm())){ try { record = theFile.getRecord(number); // record == the record requested } catch (IOException e) { /* unrecoverable IO error */ }}

Trapping Offending Calls

• For debugging purposes, the component itself may use the executable precondition to trap offending calls:

public R getRecord(int number) throws IOException{ System.assert((0 <= number) && (number <= hwm())); // the implementation of the method}

(The example here just assumes that the System class contains an assert method)

OCL

• The Object Constraint Language (OCL) is a declarative language for describing rules that apply to UML models

• OCL can be used– to describe constraints

• A constraint is a restriction on one or more values of a model or system.

• A constraint is an expression that evaluates to true or false– as a query language

• Queries are expressions that evaluate to a value (true, false and other values)

• Can be used to define new attributes and operations• OCL expressions are always associated with a UML

model– OCL expressions can be associated with any model element

in UML

OCL Constraints vs. Queries

• Examples of constraints:– Duration of a flight is the same as the difference between the arrival

and departure times– The maximum number of passengers on a flight must be less than

1,001– The origin of a flight must be different than its destination

• Examples of queries:– Return all the departing flights from a given airport– Return all the flights departing from a given airport with a departure

time after 4p.m.– Derive the arrival time by adding the duration of the flight to the

departure time.

Airport

Flight

*

*

departTime: Time/arrivalTime: Timeduration : IntervalmaxNrPassengers: Integer

origin

desti-nation

name: String

arrivingFlights

departingFlights

1

1

Different kinds of OCL constraints

• Class invariant– a constraint that must always be met by all instances of the class

• Precondition of an operation – a constraint that must always be true BEFORE the execution of the

operation

• Postcondition of an operation – a constraint that must always be true AFTER the execution of the

operation

• Constraint context: the element that the constraint restricts – Every OCL expression is bound to a context

– Own context may be denoted by “self”

Example: SpellChecker component

«comp spec»SpellChecker

ISpellCheck

ICustomSpellCheck

Example: OCL Interface Specificationcontext ISpellCheck::check(in word : String, out correct :

Boolean): HRESULTpre:word <> “”post:SUCCEEDED(result) implies correct = words->includes(word) context ICustomSpellCheck::add(in word : String) : HRESULTpre:word <> “”post:SUCCEEDED(result) implies words = words@pre->including (word) context ICustomSpellCheck::remove(in word : String) : HRESULTpre:word <> “”post:SUCCEEDED(result) implies words = words@pre->exluding(word)

Example: OCL Inter-interface constraints

• The component specification is completed by the specification of its inter-interface constraints, an example constraint is formulated in OCL below.

context SpellCheckercontext SpellChecker

ISpellCheck::words = ICustomSpellCheck::wordsISpellCheck::words = ICustomSpellCheck::words

Level 4: Formal Semantics

• With formal semantics programs can be proven to have consistent and sound semantics

• Formal specification languages:– VDM– Z– Lambda

Formal Semantics Example

• The visible state of the random access component is defined in a Z state schema called RandomAccess

• The term records represent all the records in the file and R is the record data type.

• The variable hwm (for 'high water mark') shows how much of the file is in use.

• The formula expresses the invariant

RandomAccessrecords: N Rhwm: N

i : 0..hwm { records(i) }

Formal Semantics Example Continued

• The file operation is defined as a state schema called getRecord and is illustrated below:

getRecord RandomAccessnumber?: Nrecord!: Rstatus!: {OK, FileSystemError}

number? hwm((status! = OK) record! = records(number?)) (status! = FileSystemError)

Component specification levels

• Levels of a component specification:– Syntax: includes specifications on the programming

language level.– Semantic: functional contracts– Non-functional: deals with quality of service.Non-functional: deals with quality of service.

Quality-of-service specification• What:

• Non-functional (extra-functional) properties (associated more with component implementations rather than interfaces)

• Examples: Reusability, Configurability, Distributeability, Availability, Confidentiality, Integrity, Maintainability, Reliability, Safety, Security, Affordability, Accessibility, Administrability, Understandability, Generality, Operability, Simplicity, Mobility, Nomadicity, Hardware independence, Software independence, Accuracy, Footprint, Responsiveness, Scalability, Schedulability, Timeliness, CPU utilization, Latency, Transaction, Throughput, Concurrency, Efficiency, Flexibility, Changeability, Evolvability, Extensibility, Modifiability, Tailorability, Upgradeability, Expandability, Consistency, Adaptability, Composability, Interoperability, Openness, Heterogenity, Integrability, Audibility, Completeness, , Conciseness, Correctness, Testability, Traceability, Coherence, Analyzability, Modularity, …

• Who:

• Credentials

• Why needed:

• Selection between semantic equivalent components

• Negotiation possibilities

Credentials

•Mary Shaw, Truth vs Knowledge: The Difference Between What a Component Does and What We Know It Does, Proceedings of the 8th International Workshop on Software Specification and Design, pp. 181-185, 1996•http://www.cs.cmu.edu/afs/cs/project/vit/www/paper_abstracts/Credentials.html

•Credential is a triple <Attribute, Value, Credibility>•Attribute: is a description of a property of a component•Value: is a measure of that property•Credibility: is a description of how the measure has been obtained

•A specification technique based on credentials must include:•a set of registered attributes•notations for specifying their value and credibility•provisions for adding new attributes. A technique could specify some attributes as required and others as optional.

Metamodel of the concepts used in extra-functional specification of software components

Figure 2.7 from [Crnkovic]

Summary

• A component has two parts: an interface and some code • In current practice, component specification techniques specify

components only syntactically. • There are many steps towards semantic specifications:

– Executable semantics: (Assertions, Code Contracts, OCL)

– Formal semantics

• Specification of extra-functional properties of components is still an open area of research, and it is uncertain what impact it will have on the future of software component specification

Component specification conclusion

•Software specification in a Conventional software doctrine :•Sufficient and complete: provide everything a user needs to know and may rely on in using the software•Static : written once and frozen•Homogeneous: written in a single notation

•Specification of architectural components:•Incomplete:

•To use an architectural component successfully, information about more than just its functionality is required. •It is not realistic to expect specifications to be complete with respect to all such properties, due to the great effort this would require. (Nor is it realistic to expect that the developer of a component could anticipate all aspects of the component in which its user might be interested.)

•Extensible:• Because we cannot expect software components to be delivered with specifications that are sufficient and complete, and because developers are likely to discover new kinds of dependencies as they attempt to use independently developed components together

•Heterogeneous: •since the diversity of properties that might be of interest is unlikely to be suitably captured by a single notation.