Component Based Development

Component Based Development

• Component-based software engineering (CBSE) is a process that emphasizes the design and construction of computer-based systems using reusable software “components.” 
• Clements describes CBSE in the following way… 
• A number of questions arise like …. 
• Is it possible to construct complex systems by assembling them from a catalog of reusable software components?
• Can this be accomplished in a cost- and time-effective manner? 
• Can appropriate incentives be established to encourage software engineers to reuse rather than reinvent? 
• Is management willing to incur the added expense associated with creating reusable software components? 
• Increasingly, the answer to each of these questions is “yes.”
• Domain Engineering 
• The objective of domain engineering is to identify, construct, catalog, and disseminate (distribute) a set of software components that have applicability to existing and future software in a particular application domain. 
• The overall goal is to establish mechanisms that enable software engineers to share these components—to reuse them—during work on new and existing systems. 
• It provides the library of reusable components that are required for component based SE. 
• Domain engineering includes three major activities 
• Analysis,
• Construction,
• Dissemination [Distribution]
• Component Qualification :
• Component qualification ensures that a candidate component will perform the function required, will properly “fit” into the architectural style specified for the system, and will exhibit the quality characteristics (e.g., performance, reliability, usability) that are required for the application. 
• The following factors are considered during component qualification : 
• Application programming interface (API). 
• Development and integration tools required by the component. 
• Run-time requirements, including resource usage (e.g., memory or storage), 
• timing or speed, and network protocol. 
• Service requirements, including operating system interfaces and support from other components. 
• Security features, including access controls and authentication protocol. 
• Exception handling
• Component Adaptation : 
• Domain engineering creates a library of components that can be easily integrated into an application architecture. 
• The implication of “easy integration” is that 
• (1) Consistent methods of resource management have been implemented for all components in the library, 
• (2) Common activities such as data management exist for all components, 
• (3) Interfaces within the architecture and with the external environment have been implemented in a consistent manner.
• Component Composition
• The component composition task assembles qualified, adapted, and engineered components to populate the architecture established for an application. 
• A library of components must be available. 
• Components should have a consistent structure. 
• A standard should exist, e.g., 
• OMG/CORBA   [Object Mgt Group / Common Object request broker architecture] 
• Microsoft COM and .Net   [Com – Common Object Model] 
• Sun JavaBeans

Designing Traditional Components

Designing Traditional Components

• Introduction : Traditional components are designed based on different constructs like 
• Sequence,
• Condition, 
• Repetition. 
• Sequence implements processing steps that are essential in the specification of any algorithm. 
• Condition provides the facility for selected processing based on some logical occurrence. 
• Repetition allows for looping. 
• These three constructs are fundamental to structured programming— an important component-level design technique.
• The use of the structured constructs reduces program complexity and thereby enhances readability, testability, and maintainability. 
• The use of a limited number of logical constructs also contributes to a human understanding process that psychologists call chunking.
• Graphical Design Notation 
• A picture is worth a thousand words,” but it’s rather important to know which picture and which 1000 words. 
• There is no question that graphical tools, such as the UML activity diagram or the flowchart, provide useful pictorial patterns that readily depict procedural detail. 
• The activity diagram allows you to represent sequence, condition, and repetition— all elements of structured programming. 
• It is a descendent of an earlier pictorial design representation (still used widely) called a flowchart. 
• A flowchart, like an activity diagram, is quite simple pictorially. A box is used to indicate a processing step. A diamond represents a logical condition, and arrows show the flow of control

• Tabular Design Notation 
• In many software applications, a module may be required to evaluate a complex combination of conditions and select appropriate actions based on these conditions.
• Decision tables provide a notation that translates actions and conditions into a tabular form. 
• The table is difficult to misinterpret and may even be used as a machine-readable input to a table-driven algorithm.

• Program Design Language 
• Program design language (PDL), also called structured English or pseudocode, 
• It incorporates the logical structure of a programming language with the free-form expressive ability of a natural language (e.g., English). 
• Narrative text (e.g., English) is embedded within a programming language-like syntax. 
• Automated tools can be used to enhance the application of PDL.
• A basic PDL syntax should include constructs for 
• Component definition, 
• Interface description, 
• Data declaration, 
• Block structuring, 
• Condition constructs, 
• Repetition constructs, 
• Input-output (I/O) constructs. 
• It should be noted that PDL can be extended to include keywords.

Component Level Design for WebApps

Component Level Design for WebApps

• Introduction : A WebApp component is 
• (1) A well-defined cohesive (Interrelated) function that manipulates content or provides computational or data processing for an end user 
• (2) A cohesive package of content and functionality that provides the end user with some required capability. 
• Therefore, component-level design for WebApps often incorporates elements of 
• Content design 
• Functional design.
• Content Design at the Component Level 
• Content design at the component level focuses on content objects and the manner in which they may be packaged for presentation to a WebApp end user. 
• The formality of content design at the component level should be tuned to the characteristics of the WebApp to be built. 
• However, as the size and complexity (of the WebApp, content objects, and their interrelationships) grows, it may be necessary to organize content in a way that allows easier reference and design. 
• For example, consider a Web-based video surveillance capability within SafeHomeAssured.com. Among many capabilities, the user can select and control any of the cameras represented as part of a floor plan, require video-capture thumbnail images from all the cameras, and display streaming video from any one camera.
•  Functional Design at the Component Level 
• Modern Web applications deliver increasingly sophisticated processing functions that 
• (1) Perform localized processing to generate content and navigation capability in a dynamic fashion, 
• (2) Provide computation or data processing capability that is appropriate for the WebApp’s business domain, 
• (3) Provide sophisticated database query and access, 
• (4) Establish data interfaces with external corporate systems. 
• To achieve these (and many other) capabilities, you will design and construct WebApp functional components that are similar in form to software components for conventional software.

Conducting Component Level Design

Conducting Component Level Design

• Introduction
• Component-level design is elaborative in nature.
• It transforms information from requirements and architectural models into a design representation that provides sufficient detail to guide the construction (coding and testing) activity. 
• The following steps represent a typical task set for component-level design, when it is applied for an object-oriented system. 
•  Step 1. Identify all design classes that correspond to the problem domain.
• Using the requirements and architectural model, each analysis class and architectural component is elaborated… 
• Step 2. Identify all design classes that correspond to the infrastructure domain. 
• These classes are not described in the requirements model and are often missing from the architecture model, but they must be described at this point. 
• Classes and components in this category include
• GUI components (often available as reusable components), 
• Operating system components, 
• Object and data management components
• Step 3. Elaborate all design classes that are not acquired (Obtain) as reusable components.
• Elaboration requires that all interfaces, attributes, and operations necessary to implement the class be described in detail. 
• Design heuristics (e.g., component cohesion and coupling).
• Step 3 (a) . Specify message details when classes or components collaborate. 
• The requirements model makes use of a collaboration diagram to show how analysis classes collaborate with one another. 
• Step 3 (b). Identify appropriate interfaces for each component.
• Within the context of component-level design, a UML interface is “a group of externally visible (i.e., public) operations.
• The interface contains no internal structure, it has no attributes, no associations . . .
• Step 3 (c). Elaborate attributes and define data types and data structures required to implement them.
• In general, data structures and types used to define attributes are defined within the context of the programming language that is to be used for implementation. 
• UML defines an attribute’s data type using the following syntax: 
• name : type-expression initial-value {property string} 
• where name is the attribute name, type expression is the data type, initial value is the value that the attribute takes when an object is created, and property-string defines a property or characteristic of the attribute. 
• Step 3 (d). Describe processing flow within each operation in detail.
• This may be accomplished using a programming language-based pseudocode or with a UML activity diagram. 
• Each software component is elaborated through a number of iterations that apply the stepwise refinement concept .
• Step 4. Describe persistent data sources (databases and files) and identify the classes required to manage them.
• Databases and files normally transcend the design description of an individual component. In most cases, these persistent data stores are initially specified as part of architectural design. 
• Step 5. Develop and elaborate behavioral representations for a class or component. 
• UML state diagrams were used as part of the requirements model to represent the externally observable behavior of the system. 
• Step 6. Elaborate deployment diagrams to provide additional implementation detail. Deployment diagrams are used as part of architectural. 
• Step 7. Refactor every component-level design representation and always consider alternatives.

Component-Level Design Guidelines | Cohesion | Coupling

Component-Level Design Guidelines

• Introduction : 
• In addition to the principles , a set of pragmatic (Practical) design guidelines can be applied as component-level design proceeds. 
• These guidelines apply to 
• Components, 
• Interfaces 
• Dependencies and inheritance 
• Components 
• Naming conventions should be established for components that are specified as part of the architectural model and then refined and elaborated as part of the component-level model
• Interfaces 
• Interfaces provide important information about communication and collaboration.
• Dependencies and Inheritance 
• it is a good idea to model dependencies from left to right and inheritance from bottom (derived classes) to top (base classes).

Cohesion

• Introduction : As per context of component-level design for 
• Conventional view:
• the “single-mindedness” of a module
• OO view (Object Oriented View):
• cohesion implies that a component or class encapsulates only attributes and operations that are closely related to one another and to the class or component itself
• Levels of cohesion OR different types of cohesion
• Functional
• Layer
• Communicational

(1) Functional

• Exhibited primarily by operations, this level of cohesion occurs when a component performs a targeted computation and then returns a result.

(2) Layer

• Exhibited by packages, components, and classes, this type of cohesion occurs when a higher layer accesses the services of a lower layer, but lower layers do not access higher layers.
• For example, the SafeHome security function requirement to make an outgoing phone call if an alarm is sensed. It might be possible to define a set of layered packages as shown in Figure.

(3) Communicational

• All operations that access the same data are defined within one class. In general, such classes focus only on the data for accessing and storing it.
• Classes and components that exhibit functional, layer, and communicational cohesion are relatively easy to implement, test, and maintain.
• It is important to note, however, that pragmatic (Practical) design and implementation issues sometimes force you to opt for lower levels of cohesion.

Coupling

• As per 
• Conventional view coupling is : 
• The degree to which a component is connected to other components and to the external world 
• OO view coupling is :
• A qualitative measure of the degree to which classes are connected to one another 
• As classes (and components) become more interdependent, coupling increases. An important objective in component-level design is to keep coupling as low as is possible. 
• Coupling categories  OR Level of coupling 
• Content Coupling 
• Common coupling 
• Control coupling 
• Stamp coupling 
• Data coupling 
• Routine call coupling 
• Type use coupling 
• Inclusion or import coupling 
• External coupling
• Content Coupling 
• It occurs when one component “secretly modifies data that is internal to another component” 
• This violates information hiding—a basic design concept.
• Common coupling 
• It Occurs when a number of components all make use of a global variable. 
• Although this is sometimes necessary (e.g., for establishing default values that are applicable throughout an application)
• Control coupling 
• It Occurs when operation A() invokes operation B() and passes a control flag to B. The control flag then “directs” logical flow within B. 
• Stamp coupling
• It Occurs when ClassB is declared as a type for an argument of an operation of ClassA. Because ClassB is now a part of the definition of ClassA, modifying the system becomes more complex.
• Data coupling
• It Occurs when operations pass long strings of data arguments.
• The “bandwidth” of communication between classes and components grows and the complexity of the interface increases. 
• Testing and maintenance are more difficult. 
• Routine call coupling 
• It Occurs when one operation invokes another.
• This level of coupling is common and is often quite necessary. 
• However, it does increase the connectedness of a system.
• Type use coupling 
• It Occurs, when component A uses a data type defined in component B (e.g., this occurs whenever “a class declares an instance variable or a local variable as having another class for its type”.
• If the type definition changes, every component that uses the definition must also change.
• Inclusion or import coupling
• It  Occurs when component A imports or includes a package or the content of component B. 
• External coupling 
• It Occurs when a component communicates or collaborates with infrastructure components (e.g., operating system functions, database capability, telecommunication functions).
• Although this type of coupling is necessary, it should be limited to a small number of components or classes within a system.

Designing Class base Components | Basic Design Principles applicable to component level design | The Open-Closed Principle (OCP) | The Liskov Substitution Principle (LSP) | Dependency Inversion Principle (DIP) | The Interface Segregation Principle (ISP) | The Release Reuse Equivalency Principle (REP) | The Common Closure Principle (CCP) | The Common Reuse Principle (CRP)

Designing Class base Components

• Introduction:
• Component-level design 
• Draws on information developed as part of the requirements model 
• Represented as part of the architectural model. 
• When an object-oriented software engineering approach is chosen, component-level design focuses on the elaboration of problem domain specific classes 
• The definition and refinement of infrastructure classes contained in the requirements model. The detailed description of the attributes, operations, and interfaces used by these classes is the design detail.

Basic Design Principles applicable to component level design

• Introduction : Four basic design principles are applicable to component-level design and have been widely adopted when object-oriented software engineering is applied. 
• The Open-Closed Principle (OCP).  “A module [component] should be open for extension but closed for modification.
• The Liskov Substitution Principle (LSP).  “Subclasses should be substitutable for their base classes. 
• Dependency Inversion Principle (DIP). “Depend on abstractions. Do not depend on concretions.” 
• The Interface Segregation (Separation) Principle (ISP). “Many client-specific interfaces are better than one general purpose interface. 
• The Release Reuse Equivalency Principle (REP). “The granule of reuse is the granule of release.” 
• The Common Closure Principle (CCP). “Classes that change together belong together.” 
• The Common Reuse Principle (CRP). “Classes that aren’t reused together should not be grouped together.”

The Open-Closed Principle (OCP)

• Design should specify the component in a way that allows it to be extended (within the functional domain that it addresses) without the need to make internal (code or logic-level) modifications to the component itself. 
• To accomplish this, you create abstractions that serve as a buffer between the functionality that is likely to be extended and the design class itself.
• For example : assume that the SafeHome security function makes use of a Detector class that must check the status of each type of security sensor. It is likely that as time passes, the number and types of security sensors will grow. 
• If internal processing logic is implemented as a sequence of if-then-else constructs, each addressing a different sensor type, the addition of a new sensor type will require additional internal processing logic (still another if-then-else). This is a violation of OCP. 
• One way to accomplish OCP for the Detector class is illustrated in Figure 1. The sensor interface presents a consistent view of sensors to the detector component. If a new type of sensor is added no change is required for the Detector class (component). The OCP is preserved

The Liskov Substitution Principle (LSP)

• Subclasses should be substitutable for their base classes”.
• It suggests that, a component that uses a base class should continue to function properly if a class derived from the base class is passed to the component instead.
• LSP demands that any class derived from a base class must honor any implied contract between the base class and the components that use it
• In the context of this discussion, a “contract” is a precondition that must be true before the component uses a base class and a postcondition that should be true after the component uses a base class. 
• When you create derived classes, be sure they conform to the pre and postconditions.

Dependency Inversion Principle (DIP)

• Depend on abstractions. Do not depend on concretions”.
• Here abstractions are the place where a design can be extended without great complication. 
• The more a component depends on other concrete components (rather than on abstractions such as an interface), the more difficult it will be to extend.

The Interface Segregation Principle (ISP)

• Many client-specific interfaces are better than one general purpose interface”.
• There are many instances in which multiple client components use the operations provided by a server class. 
• ISP suggests that, you should create a specialized interface to serve each major category of clients. Only those operations that are relevant to a particular category of clients should be specified in the interface for that client. 
• If multiple clients require the same operations, it should be specified in each of the specialized interfaces.

The Release Reuse Equivalency Principle (REP)

• “The granule of reuse is the granule of release”.
• When classes or components are designed for reuse, there is an implicit contract that is established between the developer of the reusable entity and the people who will use it. 
• The developer commits to establish a release control system that supports and maintains older versions of the entity while the users slowly upgrade to the most current version. 
• Rather than addressing each class individually, it is often advisable to group reusable classes into packages that can be managed and controlled as newer versions evolve

The Common Closure Principle (CCP)

• “Classes that change together belong together.” 
• Classes should be packaged cohesively.
• That is, when classes are packaged as part of a design, they should address the same functional or behavioral area. 
• When some characteristic of that area must change, it is likely that only those classes within the package will require modification.

The Common Reuse Principle (CRP)

• Classes that aren’t reused together should not be grouped together”.
• Classes that are reused together should be included within a package.

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)

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.

(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.