Design Concepts and System Modelling

Question 1

What is software architecture?

Answer 1

Software architecture is the fundamentalorganizationof a system, embodied in its components, their relationshipsto each other and the environment, and the principlesgoverning its design and evolution

Question 2

What should good software design exhibit?

Answer 2

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 a pleasurable one.

Question 3

What is software design?

Answer 3

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

Question 4

What are some components of software design?

Answer 4

Data/Class design
–transforms analysis classes into implementation classes and data structures
Architectural design
–defines relationships among the major software structural elements
Interface design
– defines how software elements, hardware
elements, and end-users communicate
Component-level design
– transforms structural elements into procedural
descriptions of software components

Question 5

What are some common elements of all software architecture?

Answer 5

• Architecture defines major components
• Architecture defines component relationships (structures) and interactions
• Architecture omits content information about components that do 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
• Every system has an architecture (even a system composed of one component)
• 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

Question 6

Why is design so difficult?

Answer 6

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

Question 7

What is the purpose of the system design?

Answer 7

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

Question 8

What is the objective of the design phase?

Answer 8

•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
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.

Question 9

What is the basis of system design?

Answer 9

•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

Question 10

What does the design phase produce?

Answer 10

•The design phase produces a system model
–That is based on specific design goals for the designers
–That 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

Question 11

What are the steps in the design process?

Answer 11

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

Question 12

What to consider when assessing the design goals?

Answer 12

–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)

Question 13

What is a systems modelling?

Answer 13

• System modelling is the process of developing abstract models of a system, with each model presenting a different view or perspective of that system.
• System modelling 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 modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers.

Question 14

What are models of existing and new systems made for?

Answer 14

• Models of the existing systemare used during requirements engineering. 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.
• Models of the new systemare used during requirements engineering to help explain the proposed requirements to other system stakeholders. Engineers use these models to discuss design proposals and to document the system for implementation.

Question 15

What are some different system perspectives?

Answer 15

• An external perspective, where you model the context or environment of the system.
• An interaction perspective, where you model the interactions between a system and its environment, or between the components of a system.
• A structural perspective, where you model the organization of a system or the structure of the data that is processed by the system.
• A behavioural perspective, where you model the dynamic behaviour of the system and how it responds to events.

Question 16

What are the types of UML diagrams?

Answer 16

• Activity diagrams, which show the activities involved in a process or in data processing .
• Use case diagrams, which show the interactions between a system and its environment.
• Sequence diagrams, which show interactions between actors and the system and between system components.
• Class diagrams, which show the object classes in the system and the associations between these classes.
• State diagrams, which show how the system reacts to internal and external events.

Question 17

What are some uses of graphical models?

Answer 17

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.

Question 18

What are context models?

Answer 18

• Context models are used to illustrate the operational context of a system -they show what lies outsidethe 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.

Question 19

What are system boundaries?

Answer 19

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.

Question 20

What is the process perspective?

Answer 20

• 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.

Question 21

What are interaction models?

Answer 21

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

Question 22

What are two approaches to interaction modelling?

Answer 22

• 1. Use case modeling, which is mostly used to model interactions between a system and external actors (users or other systems).
• 2. Sequence diagrams, which are used to model interactions between system components, although external agents may also be included.

Question 23

What is use-case modelling?

Answer 23

• 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.

Question 24

What are sequence diagrams?

Answer 24

• 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.

Question 25

What are structural models?

Answer 25

• 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.

Question 26

What is a class diagram?

Answer 26

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

Question 27

What are the weaknesses of the structured method?

Answer 27

• 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.

Question 28

What are generalizations?

Answer 28

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.
In object-oriented languages, such as Java, generalization is implemented using the class inheritance mechanisms built into the language.
In a generalization, the attributes and operations associated with higher-level classes are also associated with the lower-level classes.
The lower-level classes are subclasses inherit the attributes and operations from their superclasses. These lower-level classes then add more specific attributes and operations.

Question 29

What are behavioural models?

Answer 29

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.
•You can think of these stimuli as being of two 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.

Question 30

What are the types of behavioural models?

Answer 30

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

Question 31

What is data-driven modelling?

Answer 31

• 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.

Question 32

What are data-processing models?

Answer 32

• 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.

Question 33

What is a data flow diagram?

Answer 33

• 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.

Question 34

What is an event-driven modelling?

Answer 34

• 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.

Question 35

What are state machine models?

Answer 35

• These model the behaviour of the system in response to externaland internalevents.
• 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.

Question 36

What is model-driven engineering?

Answer 36

• 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 MDEargue 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.

Question 37

What are pros and cons of model-driven engineering?

Answer 37

•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.
•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.

Question 38

What is model-driven architecture?

Answer 38

• 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 UMLmodels 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.

Question 39

What are the different types of models?

Answer 39

A computation independent model (CIM)
These model the important domain abstractions used in a system. CIMsare 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 UMLmodels that show the static system structure and how it responds to external and internal events.
Platform specific models (PSM)
These are transformations of the platform-independent model with a separate PSMfor each application platform. In principle, there may be layers of PSM, with each layer adding some platform-specific detail.

Question 40

What is the relationship between agile methods and model-driven architecture?

Answer 40

• 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 modelling 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.

Question 41

What are the characteristics of a good desgn?

Answer 41

the design must implement all of the explicit requirements contained in the analysis model
the design must be a readable, understandable guidefor those who generate code and for those who test and subsequently support the software.
the design should provide a complete picture of the software
A design should exhibit an architecturethat (1) has been created using recognizable architectural styles or patterns, (2) is composed of components that exhibit good design characteristics and (3) can be implemented in an evolutionary fashion
A design should be modular
A design should contain distinct representationsof data, architecture, interfaces, and components.
A design should lead to data structures that are appropriate for the classes tobe implemented and are drawn from recognizable data patterns.
•A design should lead to components that exhibit independent functional characteristics.
•A design should lead to interfaces that reduce the complexityof connections between components and with the external environment.
•A design should be derived using a repeatable methodthat is driven by information obtained during software requirements analysis.
•A design should be represented using a notation that effectively communicates its meaning.

Question 42

What are software quality attributes in design?

Answer 42

• Functionalityis 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.
• Usabilityis assessed by considering human factors, overall aesthetics, consistency, and documentation
• Reliability is evaluated by measuring the frequency and severity of the failure, the accuracy of output results, the mean-time-to-failure (MTTF), the ability to recover from failure, and the predictability of the program.
• Performanceis measured by considering processing speed, response time, resource consumption, throughput, and efficiency.
• Supportabilitycombines 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.

Question 43

What is the generic task set for design?

Answer 43

•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