What is Software Specification?

3. Software Specification

3.1 Uses of specification,

3.2 Classification of specification,

3.3 Verification of specifications,

3.4 Operational and descriptive specifications, building and using specifications.

Software specification

In broad term, specification means definition. Software specification is used in different stages of the software development for different purpose. In general term, software specification is the agreement or contract between producer of the service and consumer of the service or the implementer and the user. All the desirable qualities and requirements are specified in software specification document. There are different types of specifications such as requirement specification (which is the agreement between end user and system developer), design specification (is an agreement between system designer and implementer) and module specification (is an agreement between programmer using module and programmer implementing module.

Uses of specification

In every stages of the software development specification document is created for different purpose. The major uses of specification document are:

1. A statement of user requirement.

The primary purpose of any software product is to fulfill user requirements. During the phase of requirement specification of software development, all the requirements of the user are specified in a specification document. This document is used to design new system as well as to remove misunderstandings and ambiguity in the existing system.

2. A statement of the interface between the machine and controlled environment.

Computers interact with the external environment by receiving inputs from the sensors of a controlled plant or commands from a user and provide output. A flawed specification or a misunderstanding between software engineer and expert in the given domain may result serious undesirable effect. It is therefore necessary to specify the interface between the machine and the controlled environment by describing precisely inputs, outputs and expected relationships including specification of time constraints that the controlled should satisfy. For such purpose, finely refined specification document is required.

3. A statement of the requirements for the implementation.

Specifications are used as a reference point while the product is being implemented. The ultimate goal of the implementation is to build a product that meets the specifications. Thus the implementation uses the specifications during the design phase to make design decision and during the verification activity to check that the implementation compiles with the specification.

4. A reference point during product maintenance.

Specification is used in maintenance of the software product. There are several kinds of maintenance such as:

Adaptive maintenance:

Adaptive maintenance occurs because of changes in the requirements. Among such changes are the modifications of the functionality of the product. The new implementation must be checked against the specifications for correctness.

Corrective maintenance

In corrective maintenance only the implementation is changed. Thus the specification is required to check whether the new implementation corrects the errors contained in the previous version of the product.

Perfective maintenance

In perfective maintenance, the users may wish to restructure the design of a product in an attempt to gain an improvement in performance and inclusion of new functions or the modification of an existing function. Specification is required to understand the impact of the change clearly and to accomplish the change reliably.

Classification of specification

Specifications are classified as:

Formal Specification

Specifications that are written by using notations that has precise syntax and meaning is known as formal specification. The textual design notations (TDN) and graphical design notations (GDN) are used to write a formal specification.

Informal Specification

Specifications that are written in a natural language are known as informal specification. They can also use figures, tables and other notations to provide more clear structure and understanding. Informal specification can also be structured in a standardized way.

Operational Specification

The operational specification describes the system by describing its desired behavior by providing a model implementation of the system. For example:

Specification of a geometric figure E:

E can be drawn as follows:

1.Select two points P1 and P2 on a plane

2.Get a string of a certain length and fix its ends to P1 and P2

3.Position a pencil as shown in next figure

4.Move the pen clockwise, keeping the string tightly stretched, until you reach the point where you started drawing

This is an operational specification

Fig: implementing the specification of a circle

Another example of operational specification is sorting of an array:

“Let „a‟ be an array of n elements. The result of its sorting is an array b of n elements such that the first element of b is the minimum of a (if several elements of a have the same value, any one of them is acceptable); the second element of b is the minimum of the array of n-1 elements obtained from a by removing its minimum element; and so on until all n elements of a have been removed.”

Descriptive Specification

Declarative specifications try to state the desired properties of the system in a purely declarative fashion and the behavior is described in terms of properties of the system. Descriptive specifications are more abstract then operational specifications. They focus on essential properties of the system without modeling the behavior of any implementation. The descriptive specification of the above curve drawing example is:

Geometric figure E is describe by the following equation

ax2 + by2 + c = 0 where a, b, and c are suitable constants

Verification of specifications

Specification is used as a reference document for an implementation. So before using it for implementation, it must be verified in order to assess the correctness. Following are general ways of verifying a specification:

- Observing dynamic behavior of specified system in order to check whether it conforms to the intuitive understanding of the behavior of the ideal system. (simulation, prototyping, testing)

- Analyze properties of the specified system that can be deduced from the specification.

- Verify all three aspect of the specifications:

o Functional

It is necessary to see the functional part of the specification to verify the specification.

o Consistency

To verify the specification, it is necessary to see the consistency of the specification.

o Completeness

Verify whether the specifications is complete or not (internally or externally). The specification is internally complete, if it defines any new concept or terminology it uses in the specification and externally complete, if it include all the requirements.

Operational specification

Operational specification uses formal and semi-formal notations. They are

Data Flow Diagram (DFD):

• A semi-formal operational specification

• System viewed as collection of data manipulated by “functions”

• Data can be persistent

– they are stored in data repositories

The function symbol The data f low symbol The data store symbol The input dev ice symbol The output dev ice symbol

• Data can flow

– they are represented by data flows

• DFDs have a graphical notation

– bubbles represent functions

– arcs represent data flows

– open boxes represent persistent store

– closed boxes represent I/O interaction

UML use-case diagrams

• Define functions on basis of actors and actions

Librarian Catalogue member card + book request membership OK book request book available book borrowed time Customer Customer Librarian Catalogue 1: member card + book request 2: membership OK 3: book request 4: book available 5: book borrowed

UML sequence diagrams

• Describe how objects interact by exchanging messages

• Provide a dynamic view

UML collaboration diagrams

• Give object interactions and their order

• Equivalent to sequence diagrams

Declarative specifications

Descriptive specification also uses semi-formal and informal notations such as ER-Diagram, logic specifications algebraic specifications.

ER-Diagram

• Often used as a complement to DFD to describe conceptual data models

• Based on entities, relationships, attributes

STUDENTCLASSENROLLED_INNAMESEXAGESUBJECTCOURSE_IDMAX_ENROLLMENT

Logic specifications

Declarative specification uses the logical specification such as first order theory.

Examples of first-order theory (FOT) formulas:

• x > y and y > z implies x > z

• x = y º y = x

• for all x, y, z (x > y and y > z implies x > z)

• x + 1 < x – 1

• for all x (exists y (y = x + z))

• x > 3 or x < -6

Algebraic specifications

Algebraic specifications use algebraic expressions to write the declarative specification. For example:

• A system for strings, with operations for

– creating new, empty strings (operation new)

– concatenating strings (operation append)

– adding a new character at the end of a string (operation add)

– checking the length of a given string (operation length)

– checking whether a string is empty (operation isEmpty)

– checking whether two strings are equal (operation equal)

• algebra String Spececification introduces

sorts String, Char, Nat, Bool;

operations

new: ()® String;

append: String, String ® String;

add: String, Char ® String;

length: String ® Nat;

isEmpty: String ® Bool;

equal: String, String ® Bool