Sunday, July 23, 2017

What is a Component? | An Object-Oriented View | The Traditional View | A Process-Related View

What is a Component?

  • A component is a modular building block for computer software. 
  • As per OMG Unified Modeling Language Specification defines 
  • A component is a modular, deployable, and replaceable part of a system that encapsulates implementation and exposes a set of interfaces. 
  • The true meaning of the term component will differ depending on the point of view of the software engineer who uses it. Based on that, 
Three different views of Components
  • Three important views of what a component is and how it is used as design modeling proceeds. 
    • (1) An Object-Oriented View
    • (2) The Traditional View 
    • (3) A Process-Related View

(1) An Object-Oriented View

  • In the context of object-oriented software engineering, a component contains a set of collaborating classes. 
  • Each class within a component has been fully elaborated to include all attributes and operations that are relevant to its implementation. 
  • As part of the design elaboration, all interfaces that enable the classes to communicate and collaborate with other design classes must also be defined.
  • To accomplish this, you begin with the requirements model and elaborate analysis classes (for components that relate to the problem domain)
An Object-Oriented View

Explanation of Figure : 
  • To illustrate this process of design elaboration, consider software to be built for a sophisticated print shop. 
  • The overall objective of the software is to collect the customer’s requirements at the front counter, cost a print job, and then pass the job on to an automated production facility. 
  • During requirements engineering, an analysis class called PrintJob was derived. The attributes and operations defined during analysis are noted at the top of Figure. 
  • During architectural design, PrintJob is defined as a component within the software architecture and is represented using the shorthand UML notation shown in the middle right of the figure.
  • Note that PrintJob has two interfaces, computeJob, which provides job costing capability, and initiateJob, which passes the job along to the production facility. These are represented using the “lollipop” symbols shown to the left of the component box. 
  • Component-level design begins at this point. The details of the component PrintJob must be elaborated to provide sufficient information to guide implementation. The original analysis class is elaborated to flesh out all attributes and operations required to implement the class as the component PrintJob.

(2) The Traditional View

  • In the context of traditional software engineering, a component is a functional element of a program that incorporates 
    • Processing logic
    • The internal data structures that are required to implement the processing logic, 
    • An interface that enables the component to be invoked and data to be passed to it. 
  • A traditional component, also called a module, resides within the software architecture and serves one of three important roles: 
    • (1) A control component that coordinates the invocation of all other problem domain components, 
    • (2) A problem domain component that implements a complete or partial function that is required by the customer,
    • (3) An infrastructure component that is responsible for functions that support the processing required in the problem domain.
The Traditional View
The Traditional View

(3) A Process-Related View

  • Assume that the component is being designed from scratch or used an existing components for new development. 
  • That is, you have to create a new component based on specifications derived from the requirements model. 
  • There is, of course, another approach. 
  • The software engineering community has emphasized the need to build systems that make use of existing software components or design patterns. 
  • In essence, a catalog of proven design or code-level components is made available to you as design work proceeds. 
  • As the software architecture is developed, you choose components or design patterns from the catalog and use them to populate the architecture. 
  • Because these components have been created with reusability in mind, a complete description of their interface, the function(s) they perform, and the communication and collaboration they require are all available to you.

Saturday, July 22, 2017

Architectural Mapping Using Data Flow | Transform Mapping

Architectural Mapping Using Data Flow

  • A mapping technique, called structured design, is often characterized as a data flow-oriented design method because it provides a convenient transition from a data flow diagram to software architecture. 
  • The transition from information flow to program structure is accomplished as part of a six step process: 
    • (1) The type of information flow is established, 
    • (2) Flow boundaries are indicated,
    • (3) The DFD is mapped into the program structure, 
    • (4) Control hierarchy is defined, 
    • (5) The resultant structure is refined using design measures. 
    • (6) The architectural description is refined and elaborated.
  • Example of data flow mapping, a step-by-step “transform” mapping for a small part of the SafeHome security function. 
  • In order to perform the mapping,
  • The type of information flow must be determined. It is called transform flow and exhibits a linear quality. 
  • Data flows into the system along an incoming flow path where it is transformed from an external world representation into internalized form. Once it has been internalized, it is processed at a transform center. 
  • Finally, it flows out of the system along an outgoing flow path that transforms the data into external world form.

 Transform Mapping

  • Transform mapping is a set of design steps that allows a DFD with transform flow characteristics to be mapped into a specific architectural style. 
  • To illustrate this approach, we again consider the SafeHome security function. 
  • To map these data flow diagrams into a software architecture, you would initiate the following design steps: 
    • Step 1. Review the fundamental system model.
    • Step 2. Review and refine data flow diagrams for the software. 
    • Step 3. Determine whether the DFD has transform or transaction flow characteristics. 
    • Step 4. Isolate the transform center by specifying incoming and outgoing flow boundaries. 
    • Step 5. Perform “first-level factoring.”
    • Step 6. Perform “second-level factoring.” 
  • Step 7. Refine the first-iteration architecture using design heuristic for improved software quality. 

Tuesday, July 18, 2017

Assessing Alternative Architectural Design | An Architecture Trade-Off Analysis Method | Architectural Complexity | Architectural Description Language | An Architectural Design Method | Deriving Program Architecture

Assessing Alternative Architectural Design

  • Design results in a number of architectural alternatives that are each assessed to determine which is the most appropriate for the problem to be solved. 
  • Two different approaches for the assessment of alternative architectural designs. 
    • (1) The first method uses an iterative method to assess design trade-offs.
    • (2)The second approach applies a pseudo-quantitative technique for assessing design quality.

An Architecture Trade-Off Analysis Method

  • The Software Engineering Institute (SEI) has developed an architecture trade-off analysis method (ATAM) that establishes an iterative evaluation process for software architectures. 
  • The design analysis activities that follow are performed iteratively. 
  • 1. Collect scenarios : A set of use cases is developed to represent the system from the user’s point of view. 
  • 2. Elicit (Bring out) requirements, constraints, and environment description. This information is determined as part of requirements engineering and is used to be certain that all stakeholder concerns have been addressed. 
  • 3. Describe the architectural styles/patterns that have been chosen to address the scenarios and requirements. 
  • The architectural style(s) should be described using one of the following architectural views… 
    • Module view for analysis of work assignments with components and the degree to which information hiding has been achieved. 
    • Process view for analysis of system performance. 
    • Data flow view for analysis of the degree to which the architecture meets functional requirements.
  • 4. Evaluate quality attributes : Quality attributes for architectural design assessment include reliability, performance, security, maintainability, flexibility, testability, portability, reusability, and interoperability. 
  • 5.Identify the sensitivity of quality attributes to various architectural attributes for a specific architectural style. This can be accomplished by making small changes in the architecture and determining how sensitive a quality attribute, say performance, is to the change. Any attributes that are significantly affected by variationin the architecture are termed sensitivity points.. 
  • 6. Critique (Assess) candidate architectures (developed in step 3) using the sensitivity analysis conducted in step 5. 
  • The Software Engineering Institute (SEI) describes this approach in the following manner 
    • Once the architectural sensitivity points have been determined, finding trade-off points is simply the identification of architectural elements to which multiple attributes are sensitive. For example, the performance of a client-server architecture might be highly sensitive to the number of servers (performance increases, within some range, by increasing the number of servers). . . . The number of servers, then, is a trade-off point with respect to this architecture.

Architectural Complexity

  • A useful technique for assessing the overall complexity of a proposed architecture is to consider dependencies between components within the architecture. 
  • These dependencies are driven by information/control flow within the system. Zhao suggests three types of dependencies:
    • Sharing dependencies represent dependence relationships among consumers who use the same resource or producers who produce for the same consumers. 
    • For example, for two components u and v, if u and v refer to the same global data, then there exists a shared dependence relationship between u and v. 
    • Flow dependencies represent dependence relationships between producers and consumers of resources 
    • Constrained dependencies represent constraints on the relative flow of control among a set of activities. For example, for two components u and v, u and v cannot execute at the same time (mutual exclusion), then there exists a constrained dependence relationship between u and v.

Architectural Description Language

  • Introduction:
  • The architect of a house has a set of standardized tools and notation that allow the design to be represented in an unambiguous, understandable fashion. 
  • The software architect can draw on Unified Modeling Language (UML) notation, other diagrammatic forms, and a few related tools, there is a need for a more formal approach to the specification of an architectural design. 
  • Architectural description language (ADL) provides a semantics and syntax for describing a software architecture. 
  • Hofmann and his colleagues suggest that 
    • An ADL should provide the designer with the ability to decompose architectural components, 
    • Compose individual components into larger architectural blocks, 
    • Represent interfaces (connection mechanisms) between components. 
  • Once descriptive, language based techniques for architectural design have been established, it is more likely that effective assessment methods for architectures will be established as the design evolves.

An Architectural Design Method

An Architectural Design Method

Deriving Program Architecture

Deriving Program Architecture

Wednesday, July 12, 2017

Architectural Design | Representing the System in Context | Defining Archetypes | Refining the Architecture into Components | Describing Instantiations of the System

Architectural Design

  • Introduction : 
  • As architectural design begins, the software to be developed must be put into context— 
  • That is, the design should define the external entities (other systems, devices, people) that the software interacts with and the nature of the interaction. 
  • This information can generally be acquired from the requirements model and all other information gathered during requirements engineering. 
  • Once context is modeled and all external software interfaces have been described, you can identify a set of architectural archetypes.
  • An archetype is an abstraction (similar to a class) that represents one element of system behavior. 
  • The set of archetypes provides a collection of abstractions that must be modeled architecturally if the system is to be constructed, but the archetypes themselves do not provide enough implementation detail. 
  • Therefore, the designer specifies the structure of the system by defining and refining software components that implement each archetype. 
  • This process continues iteratively until a complete architectural structure has been derived..

Representing the System in Context

  • At the architectural design level, a software architect uses an architectural context diagram (ACD) to model the manner in which software interacts with entities external to its boundaries. 
  • The generic structure of the architectural context diagram is illustrated in Figure.

Representing the System in Context

  • In figure, systems that interoperate with the target system (the system for which an architectural design is to be developed) are represented as
    • Super ordinate systems : those systems that use the target system as part of some higher-level processing scheme. 
    • Subordinate systems—those systems that are used by the target system and provide data or processing that are necessary to complete target system functionality. 
    • Peer-level systems—those systems that interact on a peer-to- peer basis (i.e., information is either produced or consumed by the peers and the target system. 
    • Actors—entities (people, devices) that interact with the target system by producing or consuming information. 
  • Each of these external entities communicates with the target system through an interface (the small shaded rectangles).

Defining Archetypes

  • An archetype is a class or pattern that represents a core abstraction that is critical to the design of an architecture for the target system. 
  • In general, a relatively small set of archetypes is required to design even relatively complex systems. 
  • Archetypes are the abstract building blocks of an architectural design.
  • In many cases, archetypes can be derived by examining the analysis classes defined as part of the requirements model. 
  • An archetype is a generic, idealized model of a person, object, or concept from which similar instances are derived, copied, patterned, or emulated. 
  • For example, an archetype for a car: wheels, doors, seats, engine In software engineering
  • As per in previous figure : The SafeHome home security function, you might define the following archetypes : 
    • Node : Represents a cohesive collection of input and output elements of the home security function.
    • For example a node might be included of (1) various sensors and (2) a variety of alarm (output) indicators. 
    • Detector : An abstraction that covers all sensing equipment that feeds information into the target system. 
    • Indicator. An abstraction that represents all mechanisms (e.g., alarm siren, flashing lights, bell) for indicating that an alarm condition is occurring. 
    • Controller. An abstraction that describes the mechanism that allows the arming (Supporting) or disarming of a node. If controllers reside on a network, they have the ability to communicate with one another.

 Refining the Architecture into Components

  • As the software architecture is refined into components. 
  • Analysis classes represent entities within the application (business) domain that must be addressed within the software architecture. 
  • In some cases (e.g., a graphical user interface), a complete subsystem architecture with many components must be designed. 
  • For Example : The SafeHome home security function example, you might define the set of top-level components that address the following functionality: 
  • External communication management — coordinates communication of the security function with external entities such as other Internet-based systems and external alarm notification. 
  • Control panel processing— manages all control panel functionality. 
  • Detector management — coordinates access to all detectors attached to the system. 
  • Alarm processing — verifies and acts on all alarm conditions
  • The overall architectural structure (represented as a UML component diagram) is in the following Figure.
Refining the Architecture into Components

Describing Instantiations of the System

  • The architectural design that has been modeled to this point is still relatively high level. 
  • The context of the system has been represented 
  • Archetypes that indicate the important abstractions within the problem domain have been defined, 
  • The overall structure of the system is apparent, and the major software components have been identified. 
  • However, further refinement is still necessary. 
  • To accomplish this, an actual instantiation of the architecture is developed.It means, again it simplify by more details. 
  • The figure demonstrates this concept.
Describing Instantiations of the System

Architectural Pattern

Architectural Pattern

  • As the requirements model is developed, you’ll notice that the software must addressa number of broadproblems that span the entire application. 
  • For example, the requirements model for virtually every e-commerce application is faced with the following problem:
    • How do we offer a broad array of goods to a broad array of customers? 
    • Allow those customers to purchase our goods online?
  • The requirements model also defines a context in which this question must be answered. 
  • For example, an e-commerce business that sells golf equipment to consumers will operate in a different context than an e-commerce business that sells high-priced industrialequipment to medium and large corporations.
  • In addition, a set of limitations and constraints may affect the way in which you addressthe problem to be solved. 
  • Architectural patterns address an application-specific problem within a specific context and under a set of limitations and constraints. 
  • The pattern proposes an architectural solution that can serve as the basis for architectural design…

Tuesday, July 11, 2017

Architectural Styles | Taxonomy of Architectural Styles | Data-Centered Architecture | Data Flow architectures | Call and return architectures | Object-oriented architectures | Layered architectures

Architectural Styles

  • Introduction : 
  • For example (For understanding purpose)
  • When a builder uses the phrase “center hall colonial” to describe a house, most people familiar with houses in a general image of what the house will look like and what the floor plan is likely to be. 
  • The builder has used an architectural style as a descriptive mechanism to differentiate the house from other styles (e.g., A-frame, raised ranch, Cape Cod). 
  • The architectural style is also a template for construction. 
  • Further details of the house must be defined, its final dimensions must be specified, customized features may be added, building materials are to be determined, but the style—a “center hall colonial”—guides the builder in his work. 
  • software architecture represents a structure in which some collection of entities (often called components) is connected by a set of defined relationships (often called connectors). 
  • Both components and connectors are associated with a set of properties that allow the designer to differentiate the types of components and connectors that can be used.
  • Introduction: 
  • The software that is built for computer-based systems also exhibits one of many architectural styles.
  • Each style describes a system category that encompasses 
    • (1) A set of components (e.g., a database, computational modules) that perform a function required by a system; 
    • (2) A set of connectors that enable “communication, coordination and cooperation” among components;
    • (3) Constraints that define how components can be integrated to form the system;
    • (4) Semantic models that enable a designer to understand the overall properties of a system by analyzing the known properties of its constituent parts.

Taxonomy of Architectural Styles

  • Introduction:
  • Millions of computer-based systems have been created over the past 60 years, the vast majority can be categorized into one of a relatively small number of architectural styles 
  • The following are the Taxonomy (Classification) of Architectural Style
    • (1) Data-centered architectures 
    • (2) Data flow architectures 
    • (3) Call and return architectures 
    • (4) Object-oriented architectures
    • (5) Layered architectures

Data-Centered Architecture

Data-Centered Architecture

  • Introduction:
  • A data store (e.g., a file or database) resides at the center of this architecture and is accessed frequently by other components that 
    • update, add, delete, or otherwise modify data within the store. 
  • Figure in slide, illustrates a typical data-centered style. Client software accesses a central repository (Storage area).
  • Data-centered architectures promote inerrability. That is, existing components can be changed and new client components added to the architecture without concern about other clients 
  • (because the client components operate independently). 
  • In addition, data can be passed among clients using the black-board mechanism (i.e., the blackboard component serves to coordinate the transfer of information between clients). 
  • Client components independently execute processes.

Data Flow architectures

  • This architecture is applied when input data are to be transformed through a series of computational or manipulative components into output data. 
  • A pipe-and-filter pattern (in Figure) has a set of components, called filters, connected by pipes that transmit data from one component to the next.
  • Each filter works independently of those components upstream and downstream, is designed to expect data input of a certain form, and produces data output (to the next filter) of a specified form. 
  • However, the filter does not require knowledge of the workings of its neighboring filters. 
  • If the data flow degenerates into a single line of transforms, it is termed batch sequential. This structure accepts a batch of data and then applies a series of sequential components (filters) to transform it
batch sequential

pipes filters

An example of the pipe and filter architecture

An example of the pipe and filter architecture

Call and return architectures

  • This architectural style enables you to achieve a program structure that is relatively easy to modify and scale.
  • A number of substyles exist within this category…
  • Main program/subprogram architectures. This classic program structure decomposes function into a control hierarchy where a “main” program” invokes a number of program components that in turn may invoke still other components. an architecture of this type.
  • Remote procedure call architectures. The components of a main program/subprogram architecture are distributed across multiple computers on a network.
Main program/subprogram architectures

Object-oriented architectures

  • The components of a system encapsulate data and the operations that must be applied to manipulate the data.
  • Communication and coordination between components are accomplished via message passing…

Layered architectures

Layered architectures
  • The basic structure of a layered architecture is illustrated in Figure.
  • A number of different layers are defined, each accomplishing operations that progressively become closer to the machine instruction set.
  • At the outer layer, components service user interface operations.
  • At the inner layer, components perform operating system interfacing.
  • Intermediate layers provide utility services and application software functions…

Monday, July 10, 2017

Architectural design | What is Architecture? | Difference between Architectural & Design

Introduction of Architectural design

  • Architectural design represents
    • The structure of data
    • Program components ,that are required to build a computerbased system.
  • Architectural design considers the architectural style that the system will take, 
    • The structure and properties of the components that constitute the system, 
    • The interrelationships that occur among all architectural components of a system.

What is Architecture?

  • Bass, Clements, and Kazman define this term in the following way:
  • The software architecture of a program or computing system is the structure or structures of the system, which comprise (include) software components, the externally visible properties of those components, and the relationships among them. 
  • For example : It is something like building a home and prepare a model of it. Finally, it is an art. 
  • The architecture is not the operational software. Rather, it is a representation that enables you to 
    • (1) Analyze the effectiveness of the design in meeting its stated requirements, 
    • (2) Consider architectural alternatives at a stage when making design changes is still relatively easy, 
    • (3) Reduce the risks associated with the construction of the software.
  • The definition emphasizes the role of “software components” in any architectural representation. 
  • In the context of architectural design, a software component can be something as simple as a 
    • Program module or an object-oriented class, 
    • Database
    • Middleware
  • The properties of components are those characteristics that are necessary for an understanding of how the components interact with other components. 
  • At the architectural level, internal properties (e.g., details of an algorithm) are not specified. 
  • The relationships between components can be as simple as a procedure call from one module to another or as complex as a database access protocol.

Difference between Architectural & Design

  • Introduction
  • For example (For understanding purpose)
  • When a builder uses the phrase “center hall colonial” to describe a house, most people familiar with houses in a general image of what the house will look like and what the floor plan is likely to be. 
  • The builder has used an architectural style as a descriptive mechanism to differentiate the house from other styles (e.g., A-frame, raised ranch, Cape Cod). 
  • The architectural style is also a template for construction. 
  • Further details of the house must be defined, its final dimensions must be specified, customized features may be added, building materials are to be determined, but the style—a “center hall colonial”—guides the builder in his work. 
  • software architecture represents a structure in which some collection of entities (often called components) is connected by a set of defined relationships (often called connectors). 
  • Both components and connectors are associated with a set of properties that allow the designer to differentiate the types of components and connectors that can be used.

Thursday, July 6, 2017

Design Model | Dimensions of the Design Model | Data Design Elements | Architectural Design Elements | Interface Design Elements | Component-Level Design Elements | Deployment-Level Design Elements

Introduction of Design Model

  • The design model can be viewed in two different dimensions.
    • (Horizontally) The process dimension
      • It indicates the evolution of the parts of the design model as each design task is executed.
    • (Vertically) The abstraction dimension 
      • It  represents the level of detail as each element of the analysis model is transformed into the design model and then iteratively refined. 
  • The elements of the design model use many of the same UML diagrams that were used in the analysis model.
  • The difference is that these diagrams are
    • Refined and elaborated as part of design; 
    • More implementation-specific detail is provided, 
    • Architectural structure and style, components that reside within the architecture, 
    • Interfaces between the components and with the outside world are all emphasized.

Design Model Introduction

Design Model Introduction

Dimensions of the Design Model

Dimensions of the Design Model

Data Design Elements

  • Customer’s/ User’s View:
    • Data Architecting (Creates a model of data that is represented at a high level of abstraction). (Build Architecture of Data) 
  • Program Component Level: The design of Data structure & algorithms. 
  • Application Level: Translate Data Model into a database. 
  • Business Level: Data warehouse(Reporting & Analysis of DB) & Data mining(Analysis). 
  • At last it means creation of Data Dictionary.

Architectural Design Elements

  • Provides an overall view of the software product(Similar like Floor Plan of house) 
  • The architectural model [Sha96] is derived from three sources:
    • (1) Information about the application domain for the software to be built; 
    • (2) Specific requirements model elements such as data flow diagrams or analysis classes, their relationships and collaborations for the problem at hand; 
    • (3) The availability of architectural styles (Chapter 9) and patterns
  • Difference: An architectural style is a conceptual way of how the system will be created / will work. 
  • An architectural pattern describes a solution for implementing a style at the level of subsystems or modules and their relationships.

Interface Design Elements

  • The interface design elements for software represent information flows into and out of the system and how it is communicated among the components defined as part of the architecture. 
  • For example : A set of detailed drawings (and specifications) for the doors, windows, and external utilities of a house. These drawings describe the size and shape of doors and windows, the manner in which they operate, the way in which utility connections (e.g., water, electrical, gas, telephone) come into the house and are distributed among the rooms depicted in the floor plan.
  • There are three important elements of interface design: 
    • (1) The user interface (UI); 
    • (2) External interfaces to other systems, devices, networks, or other producers or consumers of information; 
    • (3) Internal interfaces between various design components. 
  • UI design (increasingly called usability design) is a major software engineering action
  • Usability design incorporates 
    • Visual elements (e.g., layout, color, graphics, interaction mechanisms), 
    • Ergonomic elements (e.g., information layout and placement, metaphors, UI navigation), 
    • Technical elements (e.g., UI patterns, reusable components). 
    • In general, the UI is a unique subsystem within the overall application architecture.
Interface Design Elements

  • The design of external interfaces requires perfect information about the entity to which information is sent or received. 
  • The design of external interfaces should incorporate error checking and (when necessary) appropriate security features 
  • For example, the SafeHome security function makes use of a control panel that allows a homeowner to control certain aspects of the security function. 
  • In an advanced version of the system, control panel functions may be implemented via a wireless PDA or mobile phone. 
  • The ControlPanel class (in Figure ) provides the behavior associated with a keypad, and therefore, it must implement the operations readKeyStroke () and decodeKey (). 
  • If these operations are to be provided to other classes (in this case, WirelessPDA and MobilePhone), it is useful to define an interface as shown in the figure. 
  • The interface, named KeyPad, is shown as an <<interface>> stereotype or as a small, labeled circle connected to the class with a line. The interface is defined with no attributes and the set of operations that are necessary to achieve the behavior of a keypad.

Component-Level Design Elements

  • The component-level design for software fully describes the internal detail of each software component. 
  • Component elements (detailed drawing of each room, wiring, place of switches…) 
    • Internal details of each software component
      • Data structures, 
      • algorithmic details, 
      • interface to access component operation (behavior).

Deployment-Level Design Elements

  • Deployment-level design elements indicate how software functionality and subsystems will be allocated within the physical computing environment that will support the software. 
  • For example, the elements of the SafeHome product are configured to operate within three primary computing environments
    • A home-based PC, 
    • The SafeHome control panel, 
    • Server housed at CPI Corp. (providing Internet-based access to the system).
Deployment-Level Design Elements
  • The diagram shown in Figure is in descriptor form. 
  • This means that the deployment diagram shows the computing environment but does not explicitly indicate configuration details. 
  • For example, the “personal computer” is not further identified. It could be a Mac or a Windows-based PC, a Sun workstation, or a Linux-box.
  • These details are provided when the deployment diagram is revisited in instance form during . 
  • Each instance of the deployment (a specific, named hardware configuration) is identified…

Design Concepts | Abstraction | Architecture | Patterns | Separation | Modularity | Information Hiding | Functional Independence | Refinement | Aspects | Refactoring | Object-Oriented Design Concepts | Design Classes | Cohesion | Coupling

Design Concepts

  • Introduction : A set of fundamental software design concepts have developed over the history of software engineering. 
  • Although the degree of interest in each concept has varied over the years, each has stood the test of time. 
  • Each provides the software designer with a foundation from which more sophisticated design methods can be applied. 
  • The following important software design concepts that span both traditional and object-oriented software development. 
    • (1) Abstraction [data, procedure, control]
    • (2) Architecture [The overall structure of the software] 
    • (3) Patterns [conveys the essence” of a proven design solution] 
    • (4) Separation of Concerns [Divide complex problem in small parts] 
    • (5) Modularity [Small Components] 
    • (6) Information Hiding [controlled interfaces] 
    • (7) Functional Independence
    • (8) Refinement
    • (9) Aspects
    • (10) Refactoring
    • (11) Object-Oriented Design Concepts
    • (12) Design Classes

(1) Abstraction  (Concept / Generalization)

  • Introduction : When you consider a modular solution to any problem, many levels of abstraction can be created. 
    • At the highest level of abstraction, a solution is stated in broad terms using the language of the problem environment. 
    • At lower levels of abstraction, a more detailed description of the solution is provided. 
  • Finally, at the lowest level of abstraction, the solution is stated in a manner that can be directly implemented. 
  • As different levels of abstraction are developed, you work to create both 
    • Procedural abstractions 
    • Data abstractions.
  • A procedural abstraction refers to a sequence of instructions that have a specific and limited function. The name of a procedural abstraction implies these functions, but specific details are suppressed.
  • An example of a procedural abstraction would be the word open for a door. Open implies a long sequence of procedural steps (e.g., walk to the door, reach out and grasp knob, turn knob and pull door, step away from moving door, etc.)
Procedural abstractions

  • A data abstraction is a named collection of data that describes a data object. 
  • In the context of the procedural abstraction open, we can define a data abstraction called door. 
  • A set of attributes that describe the door (e.g., door type, swing direction, opening mechanism, weight, dimensions). 
  • It follows that the procedural abstraction open would make use of information contained in the attributes of the data abstraction door.

(2) Architecture
[The overall structure of the software]

  • Software architecture refers to “the overall structure of the software and the ways in which that structure provides conceptual integrity for a system” [Sha95a]
  • Architecture is the structure or organization of program components (modules), the manner in which these components interact, and the structure of data that are used by the components.
  • The following set of properties that should be specified as part of an architectural design.
    • Structural properties.
    • Extra-functional properties
    • Families of related systems
  • Structural properties : This representation defines the components of a system e.g., modules, objects, filters
    • The manner in which those components are packaged and interact with one another.
    • For example, objects are packaged to encapsulate both data and methods.
  • Extra-functional properties : The architectural design description should address the following.
    • How the design architecture achieves requirements for performance, capacity, reliability, security, adaptability, and other system characteristics.
  • Families of related systems : The architectural design should draw upon repeatable patterns that are commonly encountered in the design of families of similar systems.
    • The design should have the ability to reuse architectural building blocks.

(3) Pattern [”conveys the essence (Fundamental nature) ” of a proven design solution]

  • Design pattern describes a design structure that solves a particular design problem within a specific context.
  • The objective of each design pattern is to provide a description that enables a designer to determine 
    • (1) whether the pattern is applicable to the current work,
    • (2) whether the pattern can be reused (hence, saving design time),
    • (3) whether the pattern can serve as a guide for developing a similar, but functionally or structurally different pattern.

(4) Separation of Concerns
[Divide complex problem in small parts]

  • Separation of concerns is a design concept, that suggests any complex problem can be more easily handled if it is subdivided into pieces that can each be solved and/or optimized independently.
  • A concern is a feature or behavior that is specified as part of the requirements model for the software. By separating concerns into smaller, and therefore more manageable pieces, a problem takes less effort and time to solve. 
  • Separation of concern is manifested (Marked / Visible) in other related design concepts: 
    • Modularity,
    • Aspects, 
    • Functional independence, 
    • Refinement.

(5) Modularity
[Divide complex problem in small parts]

  • Modularity is the most common manifestation of separation of concerns. Software is divided into separately named and addressable components, sometimes called modules, that are integrated to satisfy problem requirements. 
  • It has been stated that “modularity is the single attribute of software that allows a program to be intellectually manageable” [Mye78].
  • Monolithic software (i.e., a large program composed of a single module) cannot be easily grasped by a software engineer. 
  • You modularize a design (and the resulting program) so that 
    • Development can be more easily planned; 
    • Software increments can be defined and delivered;
    • Changes can be more easily accommodated;
    • Testing and debugging can be conducted more efficiently, 
    • Long-term maintenance can be conducted without serious side effects
Modularity [Divide complex problem in small parts]

(6) Information Hiding[Controlled interfaces]

  • The concept of modularity leads you to a fundamental question: “How do I decompose a software solution to obtain the best set of modules?”
  • The principle of information hiding [Par72] suggests that modules be “characterized by design decisions that (each) hides from all others.” In other words, modules should be specified and designed so that information (algorithms and data) contained within a module is inaccessible to other modules that have no need for such information. 
  • Hiding implies that effective modularity can be achieved by defining a set of independent modules that communicate with one another only that information necessary to achieve software function.
  • The use of information hiding as a design criterion for modular systems provides the greatest benefits when modifications are required during testing and later during software maintenance. Because most data and procedural detail are hidden from other parts of the software

(7) Functional Independence

  • The concept of functional independence is a direct result of separation of concerns, modularity, and the concepts of abstraction and information hiding.
  • Functional independence is achieved by developing modules with “singleminded” function.
  • Functional independence is a key to good design, and design is the key to software quality. 
  • Independence is assessed using two qualitative criteria which evaluate quality of design : 
    • Cohesion
    • Coupling
Cohesion :

  • Cohesion measures how strongly each of the functions are related within a program module.
  • Well structured classes guide to highly cohesive programs. If a certain class is performing a set of highly related functions, that class is said to be cohesive.
  • On the other hand, if a class is performing a bunch of totally unrelated functionalities that means the class is not cohesive at all.
Coupling: Degree of Dependence among Components

  • Coupling measures how much each of the program modules are dependent on the other program modules.
  • Interactions between two objects occur because there is coupling.
  • Loosely-coupled programs are high in flexibility and extensibility.
  • Strong coupling is never good because one object can be highly dependent on some other object.
Difference of Cohesion - Coupling

Difference of Cohesion - Coupling

(8) Refinement
(Modification / Improvement)

  • Stepwise refinement is a top-down design strategy originally proposed by Niklaus Wirth. 
  • Refinement is actually a process of elaboration. You begin with a statement of function (or description of information) that is defined at a high level of abstraction. That is, the statement describes function or information conceptually but provides no information about the internal workings of the function or the internal structure of the information. 
  • You then elaborate on the original statement, providing more and more detail as each successive refinement (elaboration) occurs.
Refinement (Modification / Improvement), Stepwise Refinement
  • Abstraction and refinement are complementary concepts. 
  • Abstraction enables you to specify procedure and data internally but suppress the need for “outsiders” to have knowledge of low-level details. 
  • Refinement helps you to reveal (disclose) low-level details as design progresses. Both concepts allow you to create a complete design model as the design develops.

(9) Aspects
( Features / phases )

  • A representation of a cross-cutting concern that must be accommodated as refinement and modularization occur.
  • For example : (Module A cross cut Module B means B cannot be satisfied without considering A).

( Features / phases )

  • Refactoring is a reorganization technique that simplifies the design (or code) of a component without changing its function or behavior.
  • Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure.
  • When software is refactored, the existing design is examined for redundancy, unused design elements, inefficient or unnecessary algorithms, poorly constructed or inappropriate data structures, or any other design failure that can be corrected to yield a better design.

(11) Object-Oriented Design Concepts 

  • The object-oriented (OO) paradigm is widely used in modern software engineering. 
  • OO design concepts such as classes and objects, inheritance, messages, and polymorphism, among others are the different features of oo paradigm.

(12) Design Class 

  • Refine analysis classes by providing detail needed to implement the classes and implement a software infrastructure the support the business solution 
  • Five types of design classes can be developed to support the design architecture…..
  • User interface classes – abstractions needed for human-computer interaction (HCI) 
  • Business domain classes – refinements of earlier analysis classes 
  • Process classes – implement lower level business abstractions required to fully manage the business domain classes. 
  • Persistent classes – represents data stores that will persist beyond software execution 
  • System classes – implement software management and control functions…

Requirements Modeling for WebApps | Requirements Modeling Input for Web App | The Content Model | Data Tree | The Interaction Model | Sequence Diagram | State Diagram | The Functional Model | Activity Diagram | Navigation Modeling | The Configuration Model

Requirements Modeling for WebApps

  • Introduction : Web developers are often skeptical (doubtful) when the idea of requirements analysis for WebApps is suggested.
  • “After all,” they argue, “the Web development process must be agile, and analysis is time consuming. It’ll slow us down just when we need to be designing and building the WebApp.”.
  • Requirements analysis does take time, but solving the wrong problem takes even more time.
  • How Much Analysis Is Enough?
    • WebApps is emphasized depends on the following factors:
    • Size and complexity of WebApp increment.
    • Number of stakeholders (analysis can help to identify conflicting requirements coming from different sources).
    • Size of the WebApp team.
    • Analysis can help develop a common understanding of the project.


  • It is a good idea to analyze the problem before beginning design. 
  • It is not true that all analysis must precede all design.
  • In fact, the design of a specific part of the WebApp only demands an analysis of those requirements that affect only that part of the WebApp.
    • For example from SafeHome, you could validly design the overall website structure (layouts, color schemes, etc.) without having analyzed the functional requirements.
    • You only need to analyze that part of the problem that is relevant to the design work for the increment to be delivered. 

Requirements Modeling Input for Web App

  • An agile version of the generic software process can be applied when WebApps are engineered. 
  • The process incorporates a communication activity that identifies
    • Stakeholders and user categories, 
    • The business context, 
    • Defined informational and applicative goals, 
    • General WebApp requirements, 
    • Usage scenarios— 
  • All above information that becomes input to requirements modeling. 
  • This information is represented in the form of natural language descriptions, rough outlines, sketches, and other informal representations. 
  • Analysis takes this information and then produces more rigorous models as an output. 
  • The requirements model provides a detailed indication of the true structure of the problem and provides insight into the shape of the solution…

Requirements Modeling Output for WebApps

  • Introduction : Requirements analysis provides a disciplined mechanism for representing and evaluating WebApp content and function.
  • Each of these characteristics can be represented as a set of models that allow the WebApp requirements to be analyzed in a structured manner. While the specific models depend largely upon the nature of the WebApp,
  • There are five main classes of models
  • Content Model : The full spectrum of content to be provided by the WebApp is identified, including text, graphics and images, video, and audio data.
    • Data modeling can be used to identify and describe each of the data objects.
  • Interaction Model : Describes the manner in which users interact with the WebApp.
    • Use-cases can be developed to provide detailed descriptions of this interaction.
  • Functional Model : The usage scenarios (use-cases) created as part of interaction analysis define the operations that will be applied to WebApp content and imply other processing functions. All operations and functions are described in detail.
  • Navigation Model : Defines the overall navigation strategy for the webApp.
  • Configuration Analysis. The environment and infrastructure in which the WebApp resides are described in detail.

The Content Model 

  • Content objects are extracted from use-cases
    • examine the scenario description for direct and indirect references to content 
  •  Attributes of each content object are identified 
  • The relationships among content objects and/or the hierarchy of content maintained by a WebApp
    • Relationships—entity-relationship diagram or UML 
    • Hierarchy—data tree or UML

Data Tree

Data Tree

The Interaction Model

  • Composed of four elements: 
    • use-cases 
    • sequence diagrams
    • state diagrams
    • a user interface prototype

Sequence Diagram

Sequence Diagram

State Diagram

State Diagram

The Functional Model

  • The functional model addresses two processing elements of the WebApp
    • user observable functionality that is delivered by the WebApp to end-users 
    • the operations contained within analysis classes that implement behaviors associated with the class. 
  • An activity diagram can be used to represent processing flow

Activity Diagram

Activity Diagram

Navigation Modeling-I 

  • Introduction : Navigation modeling considers how each user category will navigate from one WebApp element (e.g., content category will navigate from one WebApp element (e.g., content object) to another.The mechanics of navigation are defined as part of design. At this stage, you should focus on overall navigation requirements. 
  • The following questions should be considered… 
  • Should certain elements be easier to reach (require fewer navigation steps) than others? What is the priority for presentation?
  • Should certain elements be emphasized to force users to navigate in their direction? 
  • How should navigation errors be handled? 
  • Should navigation be accomplished via links, via search-based access,or by some other means?
  • Should a navigation log be maintained for users?

Navigation Modeling-II 

  • Should a full navigation map or menu (as opposed to a single “back” link or directed pointer) be available at every point in a “back” link or directed pointer) be available at every point in a user’s interaction?
  • For which user category should optimal navigation be designed?
  • How should links external to the WebApp be handled? overlaying the existing browser window? as a new browser window? as a separate frame?

The Configuration Model

  • Server-side
    • Server hardware and operating system environment must be specified
    • Interoperability considerations on the server-side must be considered
    • Appropriate interfaces, communication protocols and related collaborative information must be specified 
  • Client-side
    • Browser configuration issues must be identified
    • Testing requirements should be defined