Lecture-_-5-_SDA_software design and architecture.doc

Software Design & Architecture
Lecture-5
Lecturer: Junaid Arshad
Outlines
 What Software Architecture Is and What It Isn’t
 Other Points of View

 Architectural Patterns, Reference Models, and Reference
Architectures
 Why Is Software Architecture Important?
What Software Architecture Is and
What It Isn't
The figure is taken from a system description for an underwater
acoustic simulation

What It Isn’t cont’d
what can we tell from the figure?
1) The system consists of four elements.

2) Three of the elements— Prop Loss Model (MODP), Reverb Model (MODR),
and Noise Model (MODN)—might have more in common with each other
than with the fourth—Control Process (CP)—because they are positioned
next to each other.
3) All of the elements apparently have some sort of relationship with each
other, since the diagram is fully connected.
Is this an architecture?
what can we not tell from the diagram?

1) What is the nature of the elements?
2) What are the responsibilities of the elements?
3) Do the connections mean that the elements communicate with each other,
control each other, send data to each other, use each other, invoke each
other, synchronize with each other, share some information-hiding secret with
each other, or some combination of these or other relations?
The diagram does not show a software architecture
what does form a software architecture?

 The software architecture of a program or computing system is the
structure or structures of the system, which comprise software
elements, the externally visible properties of those elements, and the
relationships among them
 “Externally visible” properties are those assumptions other elements
can make of an element, such as its provided services, performance
characteristics, fault handling, shared resource usage
Architecture defines software elements

 The architecture is foremost an abstraction of a system that hold back
details of elements that do not affect how they use or interact with
other elements.
 In nearly all modern systems, elements interact with each other by
means of interfaces that partition details about an element into
public and private parts.
 Architecture is concerned with the public side of this division; private
details are not architectural.
Architecture defines software elements cont’d

 The architecture can comprise more than one kind of structure
 every computing system with software has a software architecture because
every system can be shown to comprise elements and the relations among
them
 The behavior of each element is part of the architecture
Architectural Patterns, Reference
Models, and Reference Architectures
There are three important architectural structures
1. architectural pattern

2. reference model
3. reference architecture
Architectural pattern
 An architectural pattern is a description of element and relation types
together with a set of constraints on how they may be used.
 A pattern can be thought of as a set of constraints on an architecture
(on the element types and their patterns of interaction)
 these constraints define a set or family of architectures that satisfy
them.

 For example, client-server is a common architectural pattern.
 Client and server are two element types, and their coordination is described in
terms of the protocol that the server uses to communicate with each of its
clients.
 Use of the term client-server implies only that multiple clients exist; the clients
themselves are not identified, and there is no discussion of what functionality,
other than implementation of the protocols, has been assigned to any of the
clients.
 Countless architectures are of the client-server pattern under this (informal)
definition.

 An architectural pattern is not an architecture, but it still conveys a useful
image of the system (it imposes useful constraints on the architecture and, in
turn, on the system).
 patterns exhibit known quality attributes. This is why the architect
chooses a particular pattern and not one at random.
 Some patterns represent known solutions to performance problems,
others lend themselves well to high-security systems, still others have
been used successfully in high-availability systems.
 Choosing an architectural pattern is often the architect's first major
design choice.
 The term architectural style has also been widely used to describe the
same concept of patterns

Reference model
 A reference model is a division of functionality together
with data flow between the pieces.
 A reference model is a standard decomposition of a known
problem into parts that cooperatively solve the problem.
 For example: Can you name the standard parts of a
compiler ? Can you explain in broad terms how the parts
work together to accomplish their purpose? If so, it is
because you have been taught a reference model of these
applications.

Lecture-_-5-_SDA_software design and architecture.doc

Reference architecture
 A reference architecture is a reference model mapped onto
software elements and the data flows between them.
 The mapping may be one to one. A software element may
implement part of a function or several functions.
 Reference models, architectural patterns, and reference
architectures are not architectures; they are useful concepts
that capture elements of an architecture.

Why Is Software Architecture
Important?
There are fundamentally three reasons for software
architecture's importance:
1) Communication among stakeholders.
2) Early design decisions.
1) Transferable abstraction of a system

1. Architecture is the vehicle for
stakeholders communication
Each stakeholder of a software system is concerned with different
system characteristics that are affected by the architecture.
For example:
 the user is concerned that the system is reliable and available when
needed;
 the customer is concerned that the architecture can be implemented
on schedule and to budget;
 the manager is worried (as well as about cost and schedule) that the
architecture will allow teams to work largely independently,
interacting in disciplined and controlled ways.

 The architect is worried about strategies to achieve all of those goals.
 Architecture provides a common language in which different concerns
can be expressed, negotiated, and resolved at a level that is
intellectually manageable even for large, complex systems.
 Without such a language, it is difficult to understand large systems
sufficiently to make the early decisions that influence both quality
and usefulness.
 Architectural analysis, depends on this level of communication and
enhances it.

2. Architecture represents the earliest
set of design decisions
These early decisions are the most difficult to get correct and the
hardest to change later in the development process, and they have the
most far-reaching effects.
1. The Architecture Defines Constraints on Implementation
2. The Architecture Dictates Organizational Structure
3. The Architecture inhibits or Enables a System's Quality Attributes
4. Predicting System Qualities by Studying the Architecture
5. The Architecture Makes It Easier to Reason about and Manage Change
6. The Architecture Helps in Evolutionary Prototyping

7. The Architecture Enables More Accurate Cost and Schedule Estimates
3. Architecture as a transferable
reusable model
 The earlier in the life cycle re-use is applied, the greater the benefit
that can be achieved.
 Not only code can be re-used but so can the requirements that led to
the architecture in the first place, as well as the experience of
building the re-used architecture.

 When architectural decisions can be re-used across multiple systems,
all of the early decision consequences we just described are also
transferred.
Class Assignment
Self Study
Other Points of View in Software Architecture

Lecture-_-5-_SDA_software design and architecture.doc

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