Advance Software Engineering

Software Design• Software design deals with transforming the customer

requirements, as described in the SRS document, into a form (a set of documents) that is suitable for implementation in a programming language.

• A good software design is seldom arrived by using a single step procedure but rather through several iterations through a series of steps.

• Design activities can be broadly classified into two important parts: • Preliminary (or high-level) design and • Detailed design.

Preliminary and Detailed design activities

• High-level design means identification of different modules and the control relationships among them and the definition of the interfaces among these modules.

• The outcome of high-level design is called the program structure or software architecture.

Preliminary and detailed design activities

• During detailed design, the data structure and the algorithms of the different modules are designed.

• The outcome of the detailed design stage is usually known as the module-specification document.

Cohesion

• Most researchers and engineers agree that a good software design implies clean decomposition of the problem into modules, and the neat arrangement of these modules in a hierarchy.

• Cohesion is a measure of functional strength of a module.

• The primary characteristics of neat module decomposition are high cohesion and low coupling.

• A module having high cohesion and low coupling is said to be functionally independent of other modules.

Classification of cohesion

Classification of cohesion• Coincidental ( Random) cohesion: A module is said to have coincidental

cohesion, if it performs a set of tasks that relate to each other very loosely, if at all. It is likely that the functions have been put in the module out of pure coincidence without any thought or design.

• Logical cohesion: A module is said to be logically cohesive, if all elements of the module perform similar operations, e.g. error handling, data input, data output, etc. An example of logical cohesion is the case where a set of print functions generating different output reports are arranged into a single module.

• Temporal cohesion: When a module contains functions that are related by the fact that all the functions must be executed in the same time span, the module is said to exhibit temporal cohesion. The set of functions responsible for initialization, start-up, shutdown of some process, etc. exhibit temporal cohesion.

• Procedural cohesion: A module is said to possess procedural cohesion, if the set of functions of the module are all part of a procedure (algorithm) in which certain sequence of steps have to be carried out for achieving an objective, e.g. the algorithm for decoding a message.

• Communicational cohesion: A module is said to have communicational cohesion, if all functions of the module refer to or update the same data structure.

• Sequential cohesion: A module is said to possess sequential cohesion, if the elements of a module form the parts of sequence, where the output from one element of the sequence is input to the next. For example, in a TPS, the get-input, validate-input, sort-input functions are grouped into one module.

• Functional cohesion: Functional cohesion is said to exist, if different elements of a module cooperate to achieve a single function. For example, a module containing all the functions required to manage employees’ pay-roll exhibits functional cohesion.

Coupling

• Coupling between two modules is a measure of the degree of interdependence or interaction between the two modules.

• A module having high cohesion and low coupling is said to be functionally independent of other modules.

• If two modules interchange large amounts of data, then they are highly interdependent. The degree of coupling between two modules depends on their interface complexity.

Classification of Coupling

Classification of Coupling

• Data coupling: Two modules are data coupled, if they communicate through a parameter. An example is an elementary data item passed as a parameter between two modules, e.g. an integer, a float, a character, etc. This data item should be problem related and not used for the control purpose.

• Stamp coupling: Two modules are stamp coupled, if they communicate using a data item such as a record in PASCAL or a structure in C.

Classification of Coupling

• Control coupling: Control coupling exists between two modules, if data from one module is used to direct the order of instructions execution in another.

• Common coupling: Two modules are common coupled, if they share data through some global data items.

• Content coupling: Content coupling exists between two modules, if they share code, e.g. a branch from one module into another module.

Classification of Design• Functional Oriented Design • Class Oriented Design

Function-oriented design

• A system is viewed as something that performs a set of functions. • Starting at this high-level view of the system, each function is

successively refined into more detailed functions.

• For example, consider a function create-new-library-member which essentially creates the record for a new member, assigns a unique membership number to him, and prints a bill towards his membership charge. This function may consist of the following sub-functions:

• assign-membership-number • create-member-record • print-bill

Function-oriented design

• Each of these sub-functions may be split into more detailed sub functions and so on.

• The system state is centralized and shared among different functions, e.g. data such as member-records is available for reference and updation to several functions such as

• • create-new-member • • delete-member • • update-member-record

Object-oriented design

• In the object-oriented design approach, the system is viewed as collection of objects (i.e. entities).

• The state is decentralized among the objects and each object manages its own state information.

• For example, in a Library Automation Software, each library member may be a separate object with its own data and functions to operate on these data.

Object-oriented design

• In fact, the functions defined for one object cannot refer or change data of other objects.

• Objects have their own internal data which define their state.

• Similar objects constitute a class. In other words, each object is a member of some class. Classes may inherit features from super class.

• Conceptually, objects communicate by message passing.

Object-oriented design approach

• Unlike function-oriented design methods, in OOD, the basic abstraction are not real-world functions such as sort, display, track, etc, but real-world entities such as employee, picture, machine, radar system, etc.

• For example in OOD, an employee pay-roll software is not developed by designing functions such as update-employee-record, get-employee-address, etc. but by designing objects such as employees, departments, etc.

Design Diagrams

• Data Flow diagrams (DFD) • Entity-Relationship diagrams (ERD) • State-Transition diagrams (STD)• Class Diagrams • Decision Table and Tree

Data Flow Diagram

• Basic tool of structured analysis • A DFD identifies the

• transformational processes of a system, • the collections of data or material that the system

manipulates, • and the flows of data or material between processes, stores,

and the outside world. • Data flow modeling takes a functional decomposition

approach to systems analysis, breaking complex problems into progressive levels of detail.

Data Flow (Chemical Tracking System)

Data Flow Diagram

• High-level DFDs provide a holistic, bird's-eye view of the data and processing components in a multistep activity, which complements the precise, detailed view embodied in the SRS.

Context Diagram

• The context is the highest level of abstraction of the DFD. • The context diagram represents the entire system as a

single black-box process, depicted as a circle (a bubble). • It also shows the terminators, or external entities, that

connect to the system and the data or material flows between the system and the terminators.

• Flows on the context diagram often represent complex data structures, which are defined in the data dictionary.

Data Flow (Chemical Tracking System)

Level 0 DFD (Chemical Tracking System)

ERD

• Entities are physical items (including people) or aggregations of data items that are important to the business you're analyzing or to the system you intend to build

• Entities are named as singular nouns and are shown in rectangles in an ERD.

ERD

• Each entity is described by several attributes; individual instances of an entity will have different attribute values.

• Example: the attributes for each chemical include a unique chemical identifier, its formal chemical name, and a graphical representation of its chemical structure.

• The data dictionary contains the detailed definitions of those attributes.

ERD

• The diamonds in the ERD represent relationships, which identify the logical and numeric linkages between pairs of entities.

• Relationships are named in a way that describes the nature of the connections.

ERD Example (Chemical Tracking System)

ERD

• The cardinality, or multiplicity, of each relationship is shown with a number or letter on the lines that connect entities and relationships.

• Example: As each Requester can place multiple requests, there's a one-to-many relationship between Requester and Chemical Request.

ERD

• Other possible cardinalities in the example are as follows:• One-to-one (every Chemical Container is tracked by a single

Container History)• Many-to-many (every Vendor Catalog contains many Chemicals,

and some Chemicals appear in multiple Vendor Catalogs)

State Transition Diagrams (STD)

• All software systems involve a combination of functional behavior, data manipulation, and state changes.

• Real-time systems and process control applications can exist in one of a limited number of states at any given time.

• A state change can take place only when well-defined criteria are satisfied, such as receiving a specific input stimulus under certain conditions.

STD

• The STD contains three types of elements:• Possible system states, shown as rectangles.• Allowed state changes or transitions, shown as arrows connecting

pairs of rectangles.• Events or conditions that cause each transition to take place,

shown as text labels on each transition arrow.

STD

STD

• The STD doesn't show the details of the processing that the system performs; it shows only the possible state changes that result from that processing.

• An STD helps developers understand the intended behavior of the system.

• It's a good way to check whether all the required states and transitions have been correctly and completely described in the functional requirements.

STD Example

Class Diagrams

• A class diagram is a graphical way to depict the classes identified during object-oriented analysis and the relationships among them.

• Class descriptions encompass both attributes (data) and the operations that can be performed on the attributes.

Example Class diagram (Chemical Tracking System)

Class Diagrams

• The attributes associated with the class are shown in the middle portion of its class box

• Those are the properties or data items associated with each object that is a member of the class.

• Operations are shown in the bottom portion of the class box and typically are followed by empty parentheses.

Class Diagram

• The lines that connect the class boxes represent associations between the classes.

• The numbers on the lines show the multiplicity of the association.

Decision Tables and Decision Trees

• Decision tables and decision trees are two alternative techniques for representing what the system should do when complex logic and decisions come into play

• The decision table lists the various values for all the factors that influence the behavior and indicates the expected system action in response to each combination of factors.

Decision Tables

• The factors can be shown either as statements with possible conditions of true and false or as questions with possible answers of yes and no.

• Of course, you can also use decision tables with factors that can have more than two possible values.

Example Decision Table (Chemical Tracking System)

  Requirement Number

Condition 1 2 3 4 5

User is authorized F T T T T

Chemical is available — F T T T

Chemical is hazardous — — F T T

Requester is trained — — — F T

Action

Accept request     X   X

Reject request X X   X  

Example Decision Tree (Chemical Tracking System)

Assignment 1You have been asked to make ERP system for EME College.The system should have the following functionality.

•Add /Delete student•Add/Delete/Edit Student Address•Add/Delete/Edit Student Roll Number•Add/Delete/Edit Student Course•Add/Delete/Edit Student Attendance•Add/Delete/Edit Student Result

Make a SRS Document for the above systemSpecification TechniquesPlanguage