Software Architecture and Design

Chapter 1 – Architectural Design
Mutah University
Faculty of IT
Department of Software Engineering
Dr. Ra’Fat A. AL-Msie’Deen
rafatalmsiedeen@mutah.edu.jo
https://rafat66.github.io/Al-Msie-Deen/
Software Architecture and Design

Reference - Text Book:
 Chapter 6 of Software Engineering book - by Ian
Sommerville. l0th Edition. Addison Wesley, 2015, ISBN-
10: 0137035152.
2

Objectives
 The objective of this chapter is to introduce the concepts
of software architecture and architectural design. When
you have read the chapter, you will:
 understand why the architectural design of software is important;
 understand the decisions that have to be made about the
software architecture during the architectural design process;
 have been introduced to the idea of Architectural patterns, well-
tried ways of organizing software architectures that can be
reused in system designs;
 understand how Application-Specific Architectural patterns may
be used in transaction processing and language processing
systems.
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Topics covered
 Architectural design decisions
 Architectural views
 Architectural patterns
 Application architectures
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Architectural design
 Architectural design is concerned with understanding
how a software system should be organized and
designing the overall structure of that system.
 Architectural design is the critical link between design
and requirements engineering, as it identifies the main
structural components in a system and the relationships
between them.
 The output of the architectural design process is an
architectural model that describes how the system is
organized as a set of communicating components.
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Architectural design … cont.
 In the model of the
software development
process, architectural
design is the first stage in
the software design
process.
Figure 0.
A general model of the design process.

Agility and architecture
 In agile processes, it is generally accepted that an early
stage of an agile development process should focus on
designing an overall system architecture.
 Incremental development of architectures is not usually
successful. Refactoring components response to
changes is usually relatively easy. However, refactoring
the system architecture is expensive because you may
need to modify most system components to adapt them
to the architectural changes.
7

Agility and architecture … cont.
 In practice, there is a significant overlap between the
processes of requirements engineering and architectural
design.
 Ideally, a system specification should not include any design
information. This ideal is unrealistic, however, except for very
small systems.
 You need to identify the main architectural components as
these reflect the high-level features of the system.
 Therefore, as part of the requirements engineering process,
you might propose an abstract system architecture where you
associate groups of system functions or features with large-
scale components or sub-systems.
 You then use this decomposition to discuss the requirements
and more detailed features of the system with stakeholders. 8

The architecture of a packing robot control
system
 To help you understand what I mean by system
architecture, look at Figure 1. This diagram shows an
abstract model of the architecture for a packing robot
system.
 This robotic system can pack different kinds of objects.
 It uses a vision component to pick out objects on a conveyor,
identify the type of object, and select the right kind of
packaging.
 The system then moves objects from the delivery conveyor to
be packaged. It places packaged objects on another
conveyor.
 The architectural model shows these components and the
links between them. 9

The architecture of a packing robot control
system … cont.
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Architectural abstraction
 You can design software architectures at two levels of
abstraction, which I call architecture in the small and
architecture in the large:
1. Architecture in the small 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.
 This chapter is mostly concerned with program architectures.
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Architectural abstraction … cont.
 You can design software architectures at two levels of
abstraction, which I call architecture in the small and
architecture in the large:
2. Architecture in the large is concerned with the
architecture of complex enterprise systems that include
other systems, programs, and program components.
 These enterprise systems may be distributed over different
computers, which may be owned and managed by different
companies.
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Architectural abstraction … cont.
 Software architecture is important because it affects the
performance, robustness, distributability, and
maintainability of a system.
 Individual components implement the functional system
requirements, but the dominant influence on the non-
functional system characteristics is the system’s
architecture.
 Chen et al. confirmed this in a study of “architecturally
significant requirements” where they found that non-
functional requirements had the most significant effect
on the system’s architecture.
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Advantages of explicit architecture
 Bass et al. suggest that explicitly designing and
documenting software architecture has three
advantages:
 Stakeholder communication
 The architecture is a high-level presentation of the system
that may be used as a focus for discussion by a range of
different stakeholders.
14

Advantages of explicit architecture … cont.
 System analysis
 Making the system architecture explicit at an early stage
in the system development requires some analysis.
 Architectural design decisions have a profound effect on
whether or not the system can meet critical
requirements such as performance, reliability, and
maintainability.
15

Advantages of explicit architecture … cont.
 Large-scale reuse
 An architectural model is a compact, manageable
description of how a system is organized and how the
components interoperate.
 The architecture may be reusable across a range of
systems.
• The system architecture is often the same for systems with
similar requirements and so can support large-scale
software reuse.
 Product-line architectures may be developed.
• Product-line architectures are an approach to reuse where
the same architecture is reused across a range of related
systems.
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Architectural representations
 Simple, informal block diagrams showing entities and
relationships are the most frequently used method for
documenting software architectures.
 But these have been criticised because they lack
semantics, do not show the types of relationships
between entities nor the visible properties of entities in
the architecture.
 Depends on the use of architectural models. The
requirements for model semantics depends on how the
models are used.
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Block diagram
 System architectures are often modeled informally using
simple block diagrams, as in Figure 1.
 Each box in the diagram represents a component.
 Boxes within boxes indicate that the component has
been decomposed to subcomponents.
 Arrows mean that data and or control signals are passed
from component to component in the direction of the
arrows.
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Block diagram … cont.
 Block diagrams present a high-level picture of the
system structure, which people from different disciplines,
who are involved in the system development process,
can readily understand.
 In spite of their widespread use, Bass et al. dislike
informal block diagrams for describing an architecture.
 They claim that these informal diagrams are poor
architectural representations, as they show neither the
type of the relationships among system components nor
the components’ externally visible properties.
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 Very abstract - they do not show the nature of
component relationships nor the externally visible
properties of the sub-systems.
 However, useful for communication with stakeholders
and for project planning.
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 Block diagrams are a good way of supporting
communications between the people involved in the
software design process. They are intuitive, and domain
experts and software engineers can relate to them and
participate in discussions about the system.
 Managers find them helpful in planning the project. For
many projects, block diagrams are the only architectural
description.
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Use of architectural models
 The apparent contradictions between architectural theory
and industrial practice arise because there are two ways
in which an architectural model of a program is used:
 As a way of facilitating discussion about the system
design.
 As a way of documenting an architecture that has
been designed.
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Use of architectural models … cont.
 As a way of facilitating discussion about the system
design
 A high-level architectural view of a system is useful for
communication with system stakeholders and project planning
because it is not cluttered with detail.
 Stakeholders can relate to it and understand an abstract view of
the system. They can then discuss the system as a whole
without being confused by detail.
 The architectural model identifies the key components that are to
be developed so that managers can start assigning people to
plan the development of these systems.
23

Use of architectural models … cont.
 As a way of documenting an architecture that has been
designed
 The aim here is to produce a complete system model that
shows the different components in a system, their
interfaces and their connections.
 The argument for such a model is that such a detailed
architectural description makes it easier to understand and
evolve the system.
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Key points
 A software architecture is a description of how a software
system is organized.
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Software Architecture and Design

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