dr izzat m alsmadi edited from ©ian sommerville & others software engineering, chapter 5 slide...

Dr Izzat M Alsmadi Edited from ©Ian Sommerville & others Software Engineering, chapter 5 Slide 1

Architectural Design(Chapter 11 from the textbook)


Objectives

To introduce architectural design and to discuss its importance

To explain the architectural design decisions that have to be made

To introduce three complementary architectural styles covering organisation, decomposition and control

To discuss reference architectures are used to communicate and compare architectures


Topics covered

Architectural design decisions System organisation Decomposition styles Control styles Reference architectures


Analysis Model -> Design Model

Analysis Model

use-cases - text use-case diagrams activity diagrams swim lane diagrams

data flow diagrams control-flow diagrams processing narratives

f low-oriented elements

behavioralelements

class-basedelements

scenario-basedelements

class diagrams analysis packages CRC models collaboration diagrams

state diagrams sequence diagrams

Data/ Class Design

Architec tural Design

Interface Design

Component- Level Design

Design Model


From Analysis to Design, The what and the How

We should imagine two domains: problem and solution domains. The analysis model analyses the problem in the problem domain, while the design model analyses the solution in the solution domain.

I explain in the analysis model the problem that the software will try to solve while in design, I will explain how I am going to solve that problem.


Design and Quality

The design must implement all of the explicit requirements contained in the analysis model, and it must accommodate all of the implicit requirements desired by the customer.

The design must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software.

The design should provide a complete picture of the software, addressing the data, functional, and behavioral domains from an implementation perspective.


Software architecture

The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is called architectural design.

The output of this design process is a description of the software architecture.

““The overall structure of the software and The overall structure of the software and the ways in which that structure provides the ways in which that structure provides conceptual integrity for a system.” [SHA95a]conceptual integrity for a system.” [SHA95a]


Architectural design

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.


Advantages of explicit architecture

Stakeholder communication• Architecture may be used as a focus of

discussion by system stakeholders. System analysis

• Means that analysis of whether the system can meet its non-functional requirements is possible.

Large-scale reuse• The architecture may be reusable across a

range of systems.


Architecture and system characteristics Performance

• Localise critical operations and minimise communications. Use large rather than fine-grain components. High level components.

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.


Fundamental Concepts abstraction—data, procedure, control – Ignore some details.

architecture—the overall structure of the software patterns—”conveys the essence” of a proven design solution

(i.e. templates whether for design or for anything else). modularity—compartmentalization of data and function. i.e.

into modules. Hiding- controlled interfaces- Only expose what should be

exposed. Functional independence—single-minded function and low

coupling or relations between the components. refinement—elaboration of detail for all abstractions Refactoring—a reorganization technique that simplifies the

design. i.e. refining the design into more understandable format.


Data Abstraction

doordoorimplemented as a data structure

manufacturermanufacturermodel numbermodel numbertypetypeswing directionswing directioninsertsinsertslightslights typetype numbernumberweightweightopening mechanismopening mechanism

Those are the main aspects or the things we need to know about the door, although the door has many more details (that maybe we don’t need to know at this time).


Procedural Abstraction

openopen

implemented with a "knowledge" of the object that is associated with enter

details of enter details of enter algorithmalgorithm


PatternsTo help improve reusability and extensibility. By following a pattern To help improve reusability and extensibility. By following a pattern in writing things.in writing things.Design Pattern TemplateDesign Pattern TemplatePattern namePattern name—describes the essence of the pattern in a short but —describes the essence of the pattern in a short but expressive name expressive name IntentIntent—describes the pattern and what it does—describes the pattern and what it doesAlso-known-asAlso-known-as—lists any synonyms for the pattern—lists any synonyms for the patternMotivationMotivation—provides an example of the problem —provides an example of the problem ApplicabilityApplicability—notes specific design situations in which the pattern is —notes specific design situations in which the pattern is applicableapplicableStructureStructure—describes the classes that are required to implement the —describes the classes that are required to implement the patternpatternParticipantsParticipants—describes the responsibilities of the classes that are —describes the responsibilities of the classes that are required to implement the patternrequired to implement the patternCollaborationsCollaborations—describes how the participants collaborate to carry —describes how the participants collaborate to carry out their responsibilitiesout their responsibilitiesConsequencesConsequences—describes the “design forces” that affect the pattern —describes the “design forces” that affect the pattern and the potential trade-offs that must be considered when the and the potential trade-offs that must be considered when the pattern is implementedpattern is implementedRelated patternsRelated patterns—cross-references related design patterns—cross-references related design patterns


Modular Design

easier to build, easier to change, easier to fix ...

Divide the product into reusable, and replaceable modules (as much independent as it is possible).


Information Hiding

modulemodulecontrolledcontrolledinterfaceinterface

"secret""secret"

• • algorithmalgorithm

• • data structuredata structure

• • details of external interfacedetails of external interface

• • resource allocation policyresource allocation policy

clientsclients

a specific design decisiona specific design decision

A driver doesn’t need to know how the car works. He needs to know how to drive it only.


Why Information Hiding?

reduces the likelihood of “side effects”. Know only what you need to know.

limits the global impact of local design decisions. Variables affect only their own locations.

emphasizes communication through controlled interfaces only. You should go out from a house through a door only.

discourages the use of global data. Locate variable on the place they will be used only.

leads to encapsulation—an attribute of high quality design. Hiding unnecessary information.

results in higher quality software


Functional Independence

COHESION - the degree to which a module performs one and only one function. COUPLING - the degree to which a module is "connected" to other modules in the system.

Use a pen only for writing on papers.

If modules are independent from each other, they can be easily reused, replaced, upgraded, ..etc.


Content coupling example


Refactoring Fowler [FOW99] defines refactoring in the following

manner: • "Refactoring is the process of changing a software system in

such a way that it does not alter the external behavior of the code [design] yet improves its internal structure.”

When software is refactored, the existing design is examined for • redundancy• unused design elements• inefficient or unnecessary algorithms• poorly constructed or inappropriate data structures• or any other design failure that can be corrected to yield a

better design.


Architectural conflicts

Using large-grain components (too many details) improves performance but reduces maintainability. We need to compromise between performance and maintainability.

Introducing redundant data improves availability but makes security more difficult. Another compromise.

Localising safety-related features usually means more communication so degraded performance. Another one.


System structuring – Detail Design

Concerned with decomposing the system into interacting sub-systems.

The architectural design is normally expressed as a block diagram presenting an overview of the system structure.

More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed.


Packing robot control system

Visionsystem

Objectidentification

system

Armcontroller

Grippercontroller

Packagingselectionsystem

Packingsystem

Conveyorcontroller

Component Diagram


Box and line diagrams (i.e. Component Diagrams).

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.


Architectural design decisions

Architectural design is a creative process so the process differs depending on the type of system being developed.

However, a number of common decisions span all design processes.


Architectural design decisions

Is there a generic application architecture that can be used? Reuse an existing design.

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? Design

verification. How should the architecture be documented? Diagrams?

Documents? Etc.


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.

Application architectures are covered in Chapter 13 and product lines in Chapter 18.


Architectural styles

The architectural model of a system may conform to a generic architectural model or style.

An awareness of these styles can simplify the problem of defining system architectures.

However, most large systems are heterogeneous (a mix of more than one style) and do not follow a single architectural style.


Examples of Architectural Styles / Patterns

There are many common ways of designing computer software modules and their communications, among them:

Blackboard Client-server Database-centric architecture Distributed computing Event Driven Architecture Implicit invocation Monolithic application Peer-to-peer Pipes and filters


Plugin Representational State Transfer Structured (module-based but usually monolithic within

modules) Software componentry (strictly module-based, usually object-

oriented programming within modules, slightly less monolithic) Service-oriented architecture Search-oriented architecture Space based architecture Shared nothing architecture Three-tier model


Architectural models

Used to document an architectural design. Static structural model that shows the major system

components. Such as a class diagram. Dynamic process model that shows the process

structure of the system. Such activity or sequence diagrams.

Interface model that defines sub-system interfaces. Relationships model such as a data-flow model that

shows sub-system relationships. Distribution model that shows how sub-systems are

distributed across computers.


Architectural Styles According to Sommerville


Levels of decomposition


System organisation

Reflects the basic strategy that is used to structure a system.

Three organizational styles are widely used:• A shared data repository style; similar to a

database.• A shared services and servers style;• An abstract machine or layered style.


The repository model 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. Suited in cases where one part of the system produces

data, others consumes this data, Example University data structure, command and control systems, Management information systems, CAD systems, CASE tools


Example, CASE toolset architecture

Projectrepository

Designtranslator

Programeditor

Designeditor

Codegenerator

Designanalyser

Reportgenerator

All can provide the same – level – of service. Others utilizes the service provided by each They just need a central location to save all their information.


Repository model characteristics Advantages

• Efficient way to share large amounts of data; no explicit data transfer between subsystems.

• Sub-systems need not be concerned with how data is produced.• Centralised management e.g. backup, security, etc.• Sharing model is published as the repository schema.

Disadvantages• Sub-systems must agree on a repository data model. Inevitably a

compromise;• Data evolution is difficult and expensive; large volume of data to

transfer to a new model.• No scope for specific management policies; Centralized system

enforces same policies across all data from all subsystems.• Difficult to distribute efficiently. Data redundancy, inconsistency.


Repository Model Variants

Passive data store: • Sub systems must interact with the data store.

Active data store (e.g. BlackBoard):• Data store can contact and trigger actions in

subsystems when data becomes available.


Client-server model Distributed system model which shows how data

and processing is distributed across a range of components.

Client server elements: Servers, Clients and Network.

Servers: Set of stand-alone servers which provide specific services such as printing, data management, etc.

Clients: Set of clients which call on these services. Network which allows clients to access servers.


Example, Film and picture library

CatalogueserverLibrary

catalogue

Videoserver

Film clipfiles

Pictureserver

Digitisedphotographs

Web server

Film andphoto info.

Client 1 Client 2 Client 3 Client 4

Internet

Servers provide services and clients use those services.


Client-server characteristics Advantages

• Distribution of data is straightforward;• Makes effective use of networked systems. May require

cheaper hardware;• Easy to add new servers or upgrade existing servers.

Disadvantages• No shared data model so sub-systems use different data

organisation. Data interchange may be inefficient;• Redundant management in each server;• No central register of names and services – Client must know

in advance name and interface of services. It may be hard to find out what servers and services are available.


Abstract machine (or layered) model

Used to model the interfacing of sub-systems. Organises the system hierarchy into a set of layers (or

abstract machines) each of which provide a set of services.

Each layer provides a set of services to layer above. 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. Each layer depends on the services provides by the layer

below.


Version management system

Configuration management system layer

Database system layer

Operating system layer

Object management system layer


Layer model example, OSI model


Layer model advantages


Layered Model disadvantages


Modular decomposition styles

Styles of decomposing sub-systems into modules.

No rigid distinction between system organisation and modular decomposition.


Sub-systems and modules A sub-system is a system in its own right whose

operation is independent of the services provided by other sub-systems. Sub-systems are composed of modules and have defined interfaces which are used for communication with other sub-systems.

A module is a system component that provides services to other components but would not normally be considered as a separate system. It is not normally considered to be an independent system. Modules are usually composed from other, simpler system components.


Modular decomposition

Another structural level where sub-systems are decomposed into modules.

Two modular decomposition models covered• An object model where the system is decomposed into

interacting object;• A pipeline or data-flow model where the system is

decomposed into functional modules which transform inputs to outputs.

If possible, decisions about concurrency should be delayed until modules are implemented.


Object models Structure the system into a set of loosely coupled objects

with well-defined interfaces. Object-oriented decomposition is concerned with

identifying object classes, their attributes and operations. When implemented, objects are created from these

classes and some control model used to coordinate object operations.

In many programming projects, developers prefer to use OO model as it is easy to learn and apply. Same classes can be defined in requirements, design and used in coding.


Example, Invoice processing system

issue ()sendReminder ()acceptPayment ()sendReceipt ()

invoice#dateamountcustomer

invoice#dateamountcustomer#

invoice#dateamountcustomer#

customer#nameaddresscredit period

Customer

Payment

Invoice

Receipt


Company Class diagram


OBJECT-ORIENTED DECOMPOSITIONObject classes have names and a set of associated attributes.Operations if any are defined in the lower part of the rectangle.Dashed arrows indicate that an object uses the attributes or services provided by another object.


Object-oriented decomposition Describes object classes, attributes and methods Loosely coupled objects with well defined

interfaces Advantages:

• Easy to understand, maintain, reuse• Can be implemented straightforwardly from

architectural components Disadvantages: To use services, objects must explicitly reference

the name and interface of other objects. An interface change affect all users of that

interfaces.


Function-oriented pipelining Functional transformations process their inputs to

produce outputs. May be referred to as a pipe and filter model (as in

UNIX shell). The term filter is used because a transformation

filters out the data it can process from its input data stream.

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 such as billing systems.

Not really suitable for interactive systems.


Invoice processing system

Read issuedinvoices

Identifypayments

Issuereceipts

Findpayments

due

Receipts

Issuepaymentreminder

Reminders

Invoices Payments

The same OO model is more abstract as it does not include information about the sequence of operations.


Pipeline model advantages Supports transformation reuse. [Input-operation- output]. Intuitive organization for stakeholder communication. As

many think of their work as inputs and outputs. Easy to add new transformations [ Evolution]. Relatively simple to implement as either a concurrent or

sequential system. However, requires a common format for data transfer along the

pipeline to be recognized by all transformers and difficult to support event-based interaction. For example,

GUI events have complex events to be simplified by through such model.


Control styles

Are concerned with the control flow between sub-systems. Distinct from the system decomposition model. Two generic formats:

Centralised control• One sub-system has overall responsibility for

control and starts and stops other sub-systems. Event-based control

• Each sub-system can respond to externally generated events from other sub-systems or the system’s environment.


Control Examples


Centralised control A control sub-system takes responsibility for managing

the execution of other sub-systems. Call-return model

• Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems only. (Embedded programming like C)

Manager model (event-loop model) • Applicable to concurrent systems. One system component

controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement. Control decisions are usually determined by the value of some system state variables (internal event). (Usually implemented as case statements)


Call-return model

Routine 1.2Routine 1.1 Routine 3.2Routine 3.1

Routine 2 Routine 3Routine 1

Mainprogram


Real-time system control – manager model

Systemcontroller

Userinterface

Faulthandler

Computationprocesses

Actuatorprocesses

Sensorprocesses


Event-driven systems

Driven by externally generated events where the timing of the event is outwith the control of the sub-systems which process the event.

Two principal event-driven models• Broadcast models. An event is broadcast to all sub-

systems. Any sub-system which can handle the event may do so;

• Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing.

Other event driven models include spreadsheets and production systems.


Broadcast model Effective in integrating sub-systems on different

computers in a network. Sub-systems register an interest in specific

events. When these occur, control is transferred to the sub-system which can handle the event.

Control policy is not embedded in the event and message handler. Sub-systems decide on events of interest to them.

However, sub-systems don’t know if or when an event will be handled. UDP protocol in networking uses this model or algorithm.


Broad cast model – Selective broadcasting

Sub-system1

Event and message handler

Sub-system2

Sub-system3

Sub-system4


Interrupt-driven systems

Used in real-time systems where fast response to an event is essential.

There are known interrupt types with a handler defined for each type.

Each type is associated with a memory location and a hardware switch causes transfer to its handler.

Allows fast response but complex to program and difficult to validate. CPUs uses such model.


Interrupt-driven control

Handler1

Handler2

Handler3

Handler4

Process1

Process2

Process3

Process4

Interrupts

Interruptvector


Reference Architectures

Reference models are derived from a study of the application domain rather than from existing systems.

May be used as a basis for system implementation or to compare different systems.

It acts as a standard against which systems can be evaluated.

OSI Open System Interconnection, model is a layered model for communication systems.


Reference architectures

Architectural models may be specific to some application domain.

Two types of domain-specific model• Generic models which are abstractions from a number of

real systems and which encapsulate the principal characteristics of these systems. Covered in Chapter 13.

• Reference models which are more abstract, idealised model. Provide a means of information about that class of system and of comparing different architectures.

Generic models are usually bottom-up models; Reference models are top-down models.


OSI reference model

Presentation

Session

Transport

Network

Data link

Physical

7

6

5

4

3

2

1

Communications medium

Network

Data link

Physical

Application

Presentation

Session

Transport

Network

Data link

Physical

Application


Case reference model examples

Data repository services• Storage and management of data items.

Data integration services• Managing groups of entities.

Task management services• Definition and enaction of process models.

Messaging services• Tool-tool and tool-environment communication.

User interface services• User interface development.


The European Computer Manufacturer's Association ECMA reference model

Toolslots

Messageservices

Task management servicesUser interface services

Data repository servicesData integration services


Key points

The software architecture is the fundamental framework for structuring the system.

Architectural design decisions include decisions on the application architecture, the distribution and the architectural styles to be used.

Different architectural models such as a structural model, a control model and a decomposition model may be developed.

System organisational models include repository models, client-server models and abstract machine models.


Key points

Modular decomposition models include object models and pipelining models.

Control models include centralised control and event-driven models.

Reference architectures may be used to communicate domain-specific architectures and to assess and compare architectural designs.


Key points

As businesses have much in common, their application systems also tend to have a common architecture that reflects the application requirements.

A generic architecture is configured and adapted to create a system that meets specific requirements.


Key points

software architecture: Systems and subsystems and their interactions.

Architectural styles: Data repository, client server, and layered.

Steps in modeling: • Define software architecture• Subsystems• Architectural style(s) used• Define subsystems• Component/packages