ARCHITECTURAL DESIGN I

Question 1

What is architectural design and what is its output?

Answer 1

 The design process for identifying the sub-systems
making up a system and the framework for sub-system
control and communication is architectural design.
 The output of this design process is a description of the software architecture.

Question 2

Describe Architectural design

Answer 2

 An early stage of the system design process.
 Represents the link between specification and design
processes. Often carried out in parallel with some specification activities.
 It involves identifying major system components and
their communications.

Question 3

What is the importance of architectural design?

Answer 3

 The most important part of design because it unifies both types of design

Question 4

Describe architectural abstraction in small systems

Answer 4

 Architecture in the small systems is concerned with the architecture of individual programs.
 At this level, we are concerned with the way that an
individual program is decomposed into components.

Question 5

Describe architectural abstraction in large systems

Answer 5

 Architecture in the large is concerned with the architecture of complex enterprise systems that include other systems, programs, and program components.
 These enterprise systems are distributed over different networks and computers, which may be owned and managed by different companies.

Question 6

What are the main issues involved with using simple informal block diagrams for architectural representation

Answer 6

 Simple, informal block diagrams showing entities and
relationships are the most frequently used method for
documenting software architectures.
 But these have been criticized because:
-they lack semantics
-do not show the types of relationships between
entities
-do not show the visible properties of entities in the
architecture.

Question 7

What are the uses of architectural models?

Answer 7

 As a way of facilitating discussion about the system design. Stakeholders are are able to understand an abstract view of the system and can discuss it without confusion
 As a way of documenting an architecture that has been designed. A complete system model is produced that shows different components in the system, their interfaces and their connections.

Question 8

Give examples of architectural design decisions that span across all design processes

Answer 8

 Is there a generic application architecture that can be used?
 How will the system be distributed?
 What architectural styles are appropriate?
 What approach will be used to structure the system?
 How will the system be decomposed into modules?
 What control strategy should be used?
 How will the architectural design be evaluated?
 How should the architecture be documented?

Question 9

Describe some situations in which architecture can be reused

Answer 9

 Systems in the same domain often have similar
architectures that reflect domain concepts.
 Application product lines are built around a core
architecture with variants that satisfy particular customer
requirements.

Question 10

Describe the use of architecture patterns or styles

Answer 10

 The architecture of a system may be designed around
one of more architectural patterns or ‘styles’.
 These capture the essence of an architecture and can be instantiated in different ways.

Question 11

Describe architecture and system characteristics

Answer 11

 Performance- Localize critical operations and minimize communications.
 Security- Use a layered architecture with critical assets in the inner layers.
 Safety- Localize safety-critical features in a small number of sub-systems.
 Availability- Include redundant components and mechanisms for fault tolerance.
 Maintainability- Use fine-grain, replaceable components.

Question 12

Describe 4+1 View Model of Software Architecture

Answer 12

 A logical view, which shows the key abstractions in the system as objects or object classes.
 A process view, which shows how, at run-time, the system is composed of interacting processes.
 A development view, which shows how the software is decomposed for development.
 A physical view, which shows the system hardware and how software components are distributed across the processors in the system.
 Related using use cases or scenarios (+1

Question 13

Define an architectural pattern

Answer 13

An architectural pattern is a stylized description of good design practice, which has been tried and tested in different environments.

Question 14

Describe architectural patterns in general

Answer 14

 Patterns are a means of representing, sharing and
reusing knowledge
 Patterns should include information about when they are and when the are not useful.
 Patterns may be represented using tabular and graphical descriptions.

Question 15

Describe the MVC model and its components

Answer 15

Separates presentation and interaction from the system data. The system is structured into three logical components that interact with each other.
* The Model component manages the system data and associated operations on that data.
* The View component defines and manages how the data is presented to the user.
* The Controller component manages user interaction (e.g., key presses, mouse clicks, etc.) and passes these interactions to the View and the Model.

Question 16

Illustrate the MVC model

Answer 16

*See notes for the illustration

Question 17

When is the MVC model used?

Answer 17

• Used when there are multiple ways to view and interact with data.
• Also used when the future requirements for interaction and presentation of data are unknown.

Question 18

What are the advantages of the MVC model

Answer 18

• Allows the data to change independently of its representation and vice versa.
• Supports presentation of the same data in different ways with changes made in one representation shown in all of them.

Question 19

What is a possible disadvantage of the MVC model

Answer 19

• Can involve additional code and code complexity when the data model and interactions are simple.

Question 20

Describe layered architecture

Answer 20

• Organizes the system into layers with related functionality associated with each layer.
• A layer provides services to the layer above it so the lowest-level layers represent core services that are likely to be used throughout the system.
• Organizes the system into a set of layers (or abstract
  machines) each of which provide a set of services.

Question 21

Illustrate both a generic and specific layered architecture

Answer 21

*See notes for illustration

Question 22

When is the layered architecture used?

Answer 22

• Used when building new facilities on top of existing systems;
• When the development is spread across several teams with each team responsibility for a layer of functionality;
• When there is a requirement for multi-level security.

Question 23

What are the advantages of layered architecture

Answer 23

• Allows replacement of entire layers so long as the interface is maintained.
• Redundant facilities (e.g., authentication) can be provided in each layer to increase the dependability of the system.
• Supports the incremental development of sub-systems
  in different layers.
• When a layer interface changes, only the adjacent layer is affected.

Question 24

What are the disadvantages of layered archtitecture

Answer 24

• In practice, providing a clean separation between layers is often difficult and a high-level layer may have to interact directly with lower-level layers rather than through the layer immediately below it.
• Performance can be a problem because of multiple levels of interpretation of a service request as it is processed at each layer.

Question 25

What does the architectural model reveal in terms of views of the system?

Answer 25

 Each architectural model only shows one view or
perspective of the system.
 It might show how a system is decomposed into
modules, how the run-time processes interact or the
different ways in which system components are
distributed across a network.
 For both design and documentation, you usually need
to present multiple views of the software architecture.