System Modelling

Usability

is assessed by considering human factors, overall aesthetics, consistency, and documentation

Agile methods and MDA

The developers of MDA claim that it is intended to support an iterative approach to development and so can be used within agile methods. The notion of extensive up-front modeling contradicts the fundamental ideas in the agile manifesto and I suspect that few agile developers feel comfortable with model-driven engineering. If transformations can be completely automated and a complete program generated from a PIM, then, in principle, MDA could be used in an agile development process as no separate coding would be required.

7. Conduct component-level design.

- Specify all algorithms at a relatively low level of abstraction. - Refine the interface of each component. - Define component-level data structures. - Review each component and correct all errors uncovered.

4. Create a set of design classes or components

- Translate analysis class description into a design class. - Check each design class against design criteria; consider inheritance issues. - Define methods and messages associated with each design class. - Evaluate and select design patterns for a design class or a subsystem - Review design classes and revise as required.

Object class aggregation models

An aggregation model shows how classes that are collections are composed of other classes. Aggregation models are similar to the part-of relationship in semantic data models.

modularity

In almost all instances, you should break the design into many modules, hoping to make understanding easier and as a consequence, reduce the cost required to build the software.

What is a Data Object?

a representation of almost any composite information that must be understood by software • The description of the data object incorporates the data object and all of its attributes. • A data object encapsulates data only—there is no reference within a data object to operations that act on the data.

UML diagram types

activity use case sequence class state

model is...

an abstract view of a system that ignores system details

Supportability

combines the ability to extend the program (extensibility), adaptability, serviceability —these three attributes represent a more common term, maintainability—and in addition, testability, compatibility, configurability, the ease with which a system can be installed, and the ease with which problems can be localized.

Coupling

degree to which subsystems are dependent on each other. This is sometimes illustrated by the number of relations (communications) between subsystems or (or on the module level between modules within subsystems).

Instance form

deployment diagrams identifying specific named hardware configurations are developed during the latter stages of design

Descriptor form

deployment diagrams show the computing environment but does not indicate configuration details

Models of the new system

help explain the proposed requirements to other system stakeholders. Engineers use these models to discuss design proposals and to document the system for implementation.

State

how the system reacts to internal and external events.

Cohesion

is an indication of the relative functional strength of a module. - A cohesive module performs a single task, requiring little interaction with other components in other parts of a program. Stated simply, a cohesive module should (ideally) do just one thing.

Use case

show the interactions between a system and its environment

Two related approaches to interaction modeling

• 1. Use case modeling • 2. Sequence diagrams

Functional Independence

• Functional independence is achieved by developing modules with "single-minded" function and an "aversion" to excessive interaction with other modules.

Executable UML

The fundamental notion behind model-driven engineering is that completely automated transformation of models to code should be possible.

Platform specific models (PSM)

These are transformations of the platform-independent model with a separate PSM for each application platform. In principle, there may be layers of PSM, with each layer adding some platform-specific detail.

A computation independent model (CIM)

These model the important domain abstractions used in a system. CIMs are sometimes called domain models.

A platform independent model (PIM)

These model the operation of the system without reference to its implementation. The PIM is usually described using UML models that show the static system structure and how it responds to external and internal events.

Usage of model-driven engineering Pros

- Allows systems to be considered at higher levels of abstraction - Generating code automatically means that it is cheaper to adapt systems to new platforms.

Architectural elements

- Application domain - Analysis classes, their relationships, collaborations and behaviors are transformed into design realizations - Patterns and "styles"

3. Partition the analysis model into design subsystems and allocate these subsystems within the architecture

- Be certain that each subsystem is functionally cohesive. - Design subsystem interfaces. - Allocate analysis classes or functions to each subsystem.

Behavioural models stimuli types

- Data : Some data arrives that has to be processed by the system. - Events : Some event happens that triggers system processing. Events may have associated data, although this is not always the case.

Data elements

- Data model --> data structures - Data model --> database architecture

Two types of behavioural model are..

- Data processing models that show how data is processed as it moves through the system; - State machine models that show the systems response to events.

Business systems-> Data flow mode

- Driven by data - Relatively little external event processing

Real-time systems -> State machine

- Event-driven - with minimal data processing

Good software design should exhibit:

- Firmness: A program should not have any bugs that inhibit its function. - Commodity: A program should be suitable for the purposes for which it was intended. - Delight: The experience of using the program should be pleasurable one.

Usage of model-driven engineering Cons

- Models for abstraction and not necessarily right for implementation. - Savings from generating code may be outweighed by the costs of developing translators for new platforms.

technical reviews (TRs) roles

- Review leaders: plan, set agendas, runs the meeting - Recorder: take notes - Producer: whose work is being reviewed

6. Design the user interface

- Review results of task analysis. - Specify action sequence based on user scenarios. - Create behavioral model of the interface. - Define interface objects, control mechanisms. - Review the interface design and revise as required

Objectives of System Design

- defines architecture and Subsystem design • Identifying subsystems/modules (manageable parts) • Identifying architecture (hardware/software) • Data management / mapping • Access control, flow control (sequencing operations) - That describes boundary use cases: • Initialization, termination, configuration, exception handling

Interface design

- defines how software elements, hardware elements, and end-users communicate

Architectural design

- defines relationships among the major software structural elements

Architectural elements

- information about the application domain for the software to be built; - specific requirements model elements such as data flow diagrams or analysis classes, their relationships and collaborations for the problem at hand

Data/Class design

- transforms analysis classes into implementation classes and data structures

Component-level design

- transforms structural elements into procedural descriptions of software components

Use of graphical models

As a means of facilitating discussion about an existing or proposed system Incomplete and incorrect models are OK as their role is to support discussion. As a way of documenting an existing system Models should be an accurate representation of the system but need not be complete. As a detailed system description that can be used to generate a system implementation Models have to be both correct and complete.

Class diagrams

Class diagrams are used when developing an object- oriented system model to show the classes in a system and the associations between these classes. An object class can be thought of as a general definition of one kind of system object. An association is a link between classes that indicates that there is some relationship between these classes. When you are developing models during the early stages of the software engineering process, objects represent something in the real world, such as a patient, a prescription, doctor, etc.

Design Model Elements

Data elements Architectural elements Interface elements Component elements deployment elements

What is a Relationship?

Data objects are connected to one another in different ways. • The relationships owns and insured to drive define the relevant connections between person and car. • Several instances of a relationship can exist • Objects can be related in many different ways

why high cohesion is desirable

Encapsulation (information hiding) Localization of functions Reusability increases

SW Quality Attributes in Design (FURPS)

Functionality Usability Reliability Performance Supportability

Generalization

Generalization is an everyday technique that we use to manage complexity. Rather than learn the detailed characteristics of every entity that we experience, we place these entities in more general classes (animals, cars, houses, etc.) and learn the characteristics of these classes. This allows us to infer that different members of these classes have some common characteristics e.g. squirrels and rats are rodents In modeling systems, it is often useful to examine the classes in a system to see if there is scope for generalization. If changes are proposed, then you do not have to look at all classes in the system to see if they are affected by the change.

Why low coupling is desirable

Simplifies interfaces between subsystems/modules Reduces the number of interfaces between subsystems Reduces dependency between modules of a subsystem or subsystems of a system

System boundaries

System boundaries are established to define what is inside and what is outside the system. They show other systems that are used or depend on the system being developed. The position of the system boundary has a profound effect on the system requirements. Defining a system boundary is a political judgment There may be pressures to develop system boundaries that increase / decrease the influence or workload of different parts of an organization.

What is system modelling?

System modeling is the process of developing abstract models of a system, with each model presenting a different view or perspective of that system.

Models of the existing system

They help clarify what the existing system does and can be used as a basis for discussing its strengths and weaknesses. These then lead to requirements for the new system.

Features of executable UML

To create an executable subset of UML, the number of model types has therefore been dramatically reduced to these 3 key types: Domain models that identify the principal concerns in a system. They are defined using UML class diagrams and include objects, attributes and associations. Class models in which classes are defined, along with their attributes and operations. State models in which a state diagram is associated with each class and is used to describe the life cycle of the class. The dynamic behavior of the system may be specified declaratively using the object constraint language (OCL), or may be expressed using UML's action language.

refactoring

When software is refactored, the existing design is examined for - redundancy - unuseddesignelements - inefficient or unnecessary algorithms - poorly constructed or inappropriate data structures - or any other design failure that can be corrected to yield a better design.

Boundary classes

are developed during design to create the interface (e.g., interactive screen or printed reports) that the user sees and interacts with as the software is used. - Boundary classes are designed with the responsibility of managing the way entity objects are represented to users.

Coupling

is an indication of the relative interdependence among modules. - Coupling depends on the interface complexity between modules, the point at which entry or reference is made to a module, and what data pass across the interface.

Functionality

is assessed by evaluating the feature set and capabilities of the program, the generality of the functions that are delivered, and the security of the overall system.

Reliability

is evaluated by measuring the frequency and severity of failure, the accuracy of output results, the mean-time-to-failure (MTTF), the ability to recover from failure, and the predictability of the program.

Performance

is measured by considering processing speed, response time, resource consumption, throughput, and efficiency.

Detailed requirements are often best represented as ____ instead of natural language

models. - More precise - Often graphical - May have CASE tool support • Importantly, models are abstractions of a system, and so leave out non-essential details

Use case modelling

mostly used to model interactions between a system and external actors (users or other systems).

Context models

show how a system that is being modeled is positioned in an environment with other systems and processes.

Sequence

show interactions between actors and the system and between system components

Activity diagrams

show the activities involved in a process or in data processing .

Controller classes are designed to manage

the creation or update of entity objects; the instantiation of boundary objects as they obtain information from entity objects; complex communication between sets of objects; validation of data communicated between objects or between the user and the application.

design and quality

the design must implement all of the explicit requirements • the design must be a readable, understandable guide -the design should provide a complete picture of the software • A design should be modular • A design should contain distinct representations of data, architecture, interfaces, and components. • A design should lead to data structures that are appropriate for the classes • A design should lead to components that exhibit independent functional characteristics. • A design should lead to interfaces that reduce the complexity • A design should be derived using a repeatable method • A design should be represented using a notation that effectively communicates its meaning.

Architecture is early

the set of earliest design decisions - hardest to change - most critical to get right

Interface elements

the user interface (UI) external interfaces to other systems, devices, networks or other producers or consumers of information internal interfaces between various design components.

Sequence diagrams

used to model interactions between system components, although external agents may also be included.

external perspective

where you model the context or environment of the system.

behavioral perspective

where you model the dynamic behavior of the system and how it responds to events.

interaction perspective

where you model the interactions between a system and its environment, or between the components of a system

structural perspective

where you model the organization of a system or the structure of the data that is processed by the system

Class

which show the object classes in the system and the associations between these classes

Generic Task Set for Design

• 1. Examine the information domain model, and design appropriate data structures for data objects and their attributes. • 2. Using the analysis model, select an architectural style that is appropriate for the software. 3. Partition the analysis model into design subsystems and allocate these subsystems within the architecture 4. Create a set of design classes or components 5. Design any interface required with external systems or devices 6. Design the user interface 7. Conduct component-level design 8. Develop a deployment model

Fundamental Concepts

• Abstraction—data, procedure, control • Architecture—the overall structure of the software • Patterns—"conveys the essence" of a proven design solution • Separation of concerns—any complex problem can be more easily handled if it is subdivided into pieces • Modularity—compartmentalization of data and function • Hiding—controlled interfaces • Functional independence—single-minded function and low coupling • Refinement—elaboration of detail for all abstractions • Aspects—a mechanism for understanding how global requirements affect design • Refactoring—a reorganization technique that simplifies the design • OO design concepts—Appendix II • Design Classes—provide design detail that will enable analysis classes to be implemented

Statecharts

• Allow the decomposition of a model into sub- models (see following slide). • A brief description of the actions is included following the 'do' in each state. • Can be complemented by tables describing the states and the stimuli.

Why is Design so Difficult?

• Analysis: Focuses on the application domain • Design: Focuses on the solution domain - Design knowledge is a moving target - The reasons for design decisions are changing very rapidly • Halftime knowledge in software engineering: About 3-5 years • Cost of hardware rapidly sinking • "Design window": - Time in which design decisions have to be made • Technique - Time-boxed prototyping

Separation of Concerns

• 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.

Common elements

• Architecture defines component relationships (structures) and interactions • Architecture omits content information about components that does not pertain to their interactions • Behavior of components is a part of architecture insofar as it can be discerned from the point of view of another component

Architecture defines..

• Architecture defines the rationale behind the components and the structure • Architecture definitions do not define what a component is • Architecture is not a single structure -- no single structure is the architecture

Behavioral models

• Behavioral models are models of the dynamic behavior of a system as it is executing. They show what happens or what is supposed to happen when a system responds to a stimulus from its environment.

The Purpose of System Design

• Bridging the gap between desired and existing system in a manageable way • Use Divide and Conquer - We model the new system to be developed as a set of subsystems

Design Class Characteristics

• Complete - includes all necessary attributes and methods) and sufficient (contains only those methods needed to achieve class intent) • Primitiveness - each class method focuses on providing one service • High cohesion - small, focused, single-minded classes • Low coupling - class collaboration kept to minimum

Characteristics of Good Design

• Component independence - High cohesion - Low coupling • Exception identification and handling • Fault prevention and fault tolerance

Context models

• Context models are used to illustrate the operational context of a system - they show what lies outside the system boundaries. • Social and organisational concerns may affect the decision on where to position system boundaries. • Architectural models show the system and its relationship with other systems.

Process perspective

• Context models simply show the other systems in the environment, not how the system being developed is used in that environment. • Process perspective show the overall process and the processes that are supported by the system. • Data flow models may be used to show the processes and the flow of information from one process to another.

Data flow diagrams

• DFDs model the system from a functional perspective. • Tracking and documenting how the data associated with a process is helpful to develop an overall understanding of the system. • Data flow diagrams may also be used in showing the data exchange between a system and other systems in its environment. The development of models such as data-flow models should be a 'top-down' process. Notation - Functional processing (rounded rectangles) - Data stores (rectangles) - Data movements between functions (labeled arrows)

Data-processing models

• Data flow diagrams (DFDs) may be used to model the system's data processing. • These show the processing steps as data flows through a system. • DFDs are an intrinsic part of many analysis methods. • Simple and intuitive notation that customers can understand. • Show end-to-end processing of data.

Component Elements

• Describes the internal detail of each software component • Defines - Data structures for all local data objects - Algorithmic detail for all component processing functions - Interface that allows access to all component operations

OO Design Concepts

• Design classes - Entity classes - Boundary classes - Controller classes • Inheritance—all responsibilities of a superclass is immediately inherited by all subclasses • Messages—stimulate some behavior to occur in the receiving object • Polymorphism—a characteristic that greatly reduces the effort required to extend the design

Software Design

• Encompasses the set of principles, concepts, and practices that lead to the development of a high quality system or product - Design principles establish and overriding philosophy that guides the designer as the work is performed - Design concepts must be understood before the mechanics of design practice are applied - Software design practices change continuously as new methods, better analysis, and broader understanding evolve

Modularization Goals

• Good modularity eases - Simplicity: Makes it easy to understand, implement and test - Extension: Facilitates the addition of features - Change: Facilitates the modification of requirements • A module can be replaced with minimal effects on the rest of the software system

Design goals, System decomposition

• Identify design goals (choose aspects of the system to be optimized) Design goals are often derived from non- functional requirements. - Guide designers in assessing trade offs • Develop and refine a subsystem decomposition that satisfies the maximum number of design goals and or the most critical design goals • Refine the decomposition to better satisfy the design goals

Deployment Elements

• Indicates how software functionality and subsystems will be allocated within the physical computing environment • Modeled using UML deployment diagrams • Descriptor form • Instance form

Interface Elements

• Interface is a set of operations that describes the externally observable behavior of a class and provides access to its public operations • Important elements - User interface (UI) - External interfaces to other systems - Internal interfaces between various design components • Modeled using UML communication diagrams

Data-driven modeling

• Many business systems are data-processing systems that are primarily driven by data. They are controlled by the data input to the system, with relatively little external event processing. • Data-driven models show the sequence of actions involved in processing input data and generating an associated output. • They are particularly useful during the analysis of requirements as they can be used to show end-to-end processing in a system.

Model driven architecture

• Model-driven architecture (MDA) was the precursor of more general model-driven engineering • MDA is a model-focused approach to software design and implementation that uses a subset of UML models to describe a system. • Models at different levels of abstraction are created. From a high-level, platform independent model, it is possible, in principle, to generate a working program without manual intervention.

Model-driven engineering

• Model-driven engineering (MDE) is an approach to software development where models rather than programs are the principal outputs of the development process. • The programs that execute on a hardware/software platform are then generated automatically from the models. • Proponents of MDE argue that this raises the level of abstraction in software engineering so that engineers no longer have to be concerned with programming language details or the specifics of execution platforms.

Usage of model-driven engineering

• Model-driven engineering is still at an early stage of development, and it is unclear whether or not it will have a significant effect on software engineering practice.

Interaction models

• Modeling user interaction is important as it helps to identify user requirements. • Modeling system-to-system interaction highlights the communication problems that may arise. •Modelling component interaction helps us understand if a proposed system structure is likely to deliver the required system performance and dependability. • Use case diagrams and sequence diagrams may be used for interaction modeling.

Event-driven modeling

• Real-time systems are often event-driven, with minimal data processing. For example, a landline phone switching system responds to events such as 'receiver off hook' by generating a dial tone. • Event-driven modeling shows how a system responds to external and internal events. • It is based on the assumption that a system has a finite number of states and that events (stimuli) may cause a transition from one state to another.

High Cohesion within a module

• Responsibilities of module or subsystem are cohesive when - Elements related to one concept or resource are grouped to hide the detailed function of that concept or device from other modules, no side effects (informational) - Elements contribute to a single well defined task (functional) - Elements operate on the same input or produce the same output (communicational, not usually OO except perhaps within objects)

Sequence diagrams

• Sequence diagrams are part of the UML and are used to model the interactions between the actors and the objects within a system. • A sequence diagram shows the sequence of interactions that take place during a particular use case or use case instance. • The objects and actors involved are listed along the top of the diagram, with a dotted line drawn vertically from these. • Interactions between objects are indicated by annotated arrows.

Structural models

• Structural models of software display the organization of a system in terms of the components that make up that system and their relationships. • Structural models may be static models, which show the structure of the system design, or dynamic models, which show the organization of the system when it is executing. • You create structural models of a system when you are discussing and designing the system architecture.

Structured methods

• Structured methods incorporate system modelling as an inherent part of the method. • Methods define a set of models, a process for deriving these models and rules and guidelines that should apply to the models. • CASE tools support system modelling as part of a structured method.

High-Level Architecture

• Such "lines and boxes" diagrams show the basic system context and relations • Not usually directly useful to programming, but good as a system overview and as an introduction to what you are doing

System modeling

• System modeling has now come to mean representing a system using some kind of graphical notation, which is now almost always based on notations in the Unified Modeling Language (UML). • System modeling helps the analyst to understand the functionality of the system and models are used to communicate with customers.

Objectives of Design

• The design phase takes the results of the requirements analysis phase and evolves these results further • The results of the design phase feeds directly into the implementation phase • Requirements analysis → WHAT the system must do • Next Goal: determine HOW the software system is to accomplish what it must do

Basis of System Design

• The design phase uses the analysis model - Non-functional requirements / constraints - Use case model: (from users point of view) • Use cases and use case diagrams • state diagrams - Object model: • Context diagram, class diagrams

State machine models

• These model the behaviour of the system in response to external and internal events. • They show the system's responses to stimuli so are often used for modelling real-time systems. • State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another.

Method weaknesses

• They do not model non-functional system requirements. • They do not usually include information about whether a method is appropriate for a given problem. • The may produce too much documentation. • The system models are sometimes too detailed and difficult for users to understand.

Objectives of system design

• Transforms analysis model (from requirements analysis) into a system design model - Identify, model system architecture - Develop an efficient system decomposition - Identify boundary use cases describing configuration, startup, shutdown, exceptional conditions.

Use case modeling

• Use cases were developed originally to support requirements elicitation and now incorporated into the UML. • Each use case represents a discrete task that involves external interaction with a system. • Actors in a use case may be people or other systems. • Represented diagramatically to provide an overview of the use case and in a more detailed textual form.

Design goals

• When assessing design goals consider - Selection of existing components (off the shelf modules or components) - Hardware / software mapping, • Are there multiple nodes or systems • What is each node responsible for - selecting solutions for managing persistent data - Access control policies - Control flow on a solution wide basis - Boundary conditions (startup, error, shutdown)

Why Information Hiding?

• reduces the likelihood of "side effects" • limits the global impact of local design decisions • emphasizes communication through controlled interfaces • discourages the use of global data • leads to encapsulation—an attribute of high quality design • results in higher quality software