Software Architecture and patterns by Adelphi unicersity

Chapter 6 – Architectural Design
Lecture 1
1
Chapter 6 Architectural design

Topics covered
 Architectural design decisions
 Architectural views
 Architectural patterns
 Application architectures
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Software architecture Definition
 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.
3

The architecture of a packing robot control
system – Box and Line Diagram
4

Architectural design decisions – common
questions (though a creative process)
 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?
5

Architecture reuse
 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.
 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.
 Discussed later in this lecture.
6

Architecture and system characteristics
 Performance
 Localise critical operations and minimise communications. Use large
rather than fine-grain components or replicate.
 Security
 Use a layered architecture with critical assets in the inner layers.
 Safety
 Localise 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.
7

4 + 1 view model of software architecture
 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)
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Architectural patterns
 Patterns are a means of representing, sharing and
reusing knowledge.
 An architectural pattern is a stylized description of good
design practice, which has been tried and tested in
different environments.
 Patterns should include information about when they are
and when the are not useful.
 Patterns may be represented using tabular and graphical
descriptions.
9

The Model-View-Controller (MVC) pattern
Name MVC (Model-View-Controller)
Description 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. See Figure 6.3.
Example Figure 6.4 shows the architecture of a web-based application system
organized using the MVC pattern.
When used 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.
Advantages 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.
Disadvantages Can involve additional code and code complexity when the data model and
interactions are simple.
10

The organization of the Model-View-Controller
11

Web application architecture using the MVC
pattern
12

Layered architecture
 Used to model the interfacing of sub-systems.
 Organises the system into a set of layers (or abstract
machines) each of which provide a set of services.
 Supports the incremental development of sub-systems in
different layers. When a layer interface changes, only the
adjacent layer is affected.
 However, often artificial to structure systems in this way.
13

The Layered architecture pattern
Name Layered architecture
Description 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. See Figure 6.6.
Example A layered model of a system for sharing copyright documents
held in different libraries, as shown in Figure 6.7.
When used 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.
Advantages 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.
Disadvantages 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.
14

A generic layered architecture
15

The architecture of the LIBSYS system
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Layered information system architecture
17

The architecture of the MHC-PMS
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Generic Architecture Models
 Repository
 Client Server
 Pipe and Filter
Chapter 6 Architectural design 19

Repository architecture
 Sub-systems must exchange data. This may be done in
two ways:
 Shared data is held in a central database or repository and may
be accessed by all sub-systems;
 Each sub-system maintains its own database and passes data
explicitly to other sub-systems.
 When large amounts of data are to be shared, the
repository model of sharing is most commonly used a
this is an efficient data sharing mechanism.
20

The Repository Architecture pattern
Name Repository
Description All data in a system is managed in a central repository that is
accessible to all system components. Components do not
interact directly, only through the repository.
Example Figure 6.9 is an example of an IDE where the components
use a repository of system design information. Each software
tool generates information which is then available for use by
other tools.
When used You should use this pattern when you have a system in which
large volumes of information are generated that has to be
stored for a long time. You may also use it in data-driven
systems where the inclusion of data in the repository triggers
an action or tool.
Advantages Components can be independent—they do not need to know
of the existence of other components. Changes made by one
component can be propagated to all components. All data can
be managed consistently (e.g., backups done at the same
time) as it is all in one place.
Disadvantages The repository is a single point of failure so problems in the
repository affect the whole system. May be inefficiencies in
organizing all communication through the repository.
Distributing the repository across several computers may be
difficult.
21

A repository architecture for an IDE
22

Client-server architecture
 Distributed system model which shows how data and
processing is distributed across a range of components.
 Can be implemented on a single computer.
 Set of stand-alone servers which provide specific
services such as printing, data management, etc.
 Set of clients which call on these services.
 Network which allows clients to access servers.
23

The Client–server pattern
Name Client-server
Description In a client–server architecture, the functionality of the system is
organized into services, with each service delivered from a
separate server. Clients are users of these services and access
servers to make use of them.
Example Figure 6.11 is an example of a film and video/DVD library
organized as a client–server system.
When used Used when data in a shared database has to be accessed from a
range of locations. Because servers can be replicated, may also
be used when the load on a system is variable.
Advantages The principal advantage of this model is that servers can be
distributed across a network. General functionality (e.g., a printing
service) can be available to all clients and does not need to be
implemented by all services.
Disadvantages Each service is a single point of failure so susceptible to denial of
service attacks or server failure. Performance may be
unpredictable because it depends on the network as well as the
system. May be management problems if servers are owned by
different organizations.
24

A client–server architecture for a film library
25

Pipe and filter architecture
 Functional transformations process their inputs to
produce outputs.
 May be referred to as a pipe and filter model (as in UNIX
shell).
 Variants of this approach are very common. When
transformations are sequential, this is a batch sequential
model which is extensively used in data processing
systems.
 Not really suitable for interactive systems.
26

The pipe and filter pattern
Name Pipe and filter
Description The processing of the data in a system is organized so that each
processing component (filter) is discrete and carries out one type of
data transformation. The data flows (as in a pipe) from one component
to another for processing.
Example Figure 6.13 is an example of a pipe and filter system used for
processing invoices.
When used Commonly used in data processing applications (both batch- and
transaction-based) where inputs are processed in separate stages to
generate related outputs.
Advantages Easy to understand and supports transformation reuse. Workflow style
matches the structure of many business processes. Evolution by
adding transformations is straightforward. Can be implemented as
either a sequential or concurrent system.
Disadvantages The format for data transfer has to be agreed upon between
communicating transformations. Each transformation must parse its
input and unparse its output to the agreed form. This increases
system overhead and may mean that it is impossible to reuse
functional transformations that use incompatible data structures.
27

An example of the pipe and filter architecture
28

Application architectures
 A generic application architecture is an architecture for a
type of software system that may be configured and
adapted to create a system that meets specific
requirements.
 As a starting point for architectural design.
 As a design checklist.
 As a way of organising the work of the development team.
 As a means of assessing components for reuse.
 As a vocabulary for talking about application types.
29

Examples of application types
 Data processing applications
 Data driven applications that process data in batches without explicit
user intervention during the processing.
 Transaction processing applications
 Data-centred applications that process user requests and update
information in a system database.
 Event processing systems
 Applications where system actions depend on interpreting events
from the system’s environment.
 Language processing systems
 Applications where the users’ intentions are specified in a formal
language that is processed and interpreted by the system.

Summary
Architectural design decisions
Pattern or Application; Distribution of responsibilities;
Control strategy; Modules; How to model
Architectural views
Logical, process, development, physical + use cases
Architectural patterns
MVC, Layered,
Application architectures
Repository, client-server, pipe and filter

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