Architectures (2)

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2003 Jaelson Castro and John Mylopoulos

CSC340Information Systems Analysis and Design

OO Architectures -- 1

XVIII. Software ArchitecturesXVIII. Software Architectures

Software Architectures

UML Packages

Client-Server vs Peer-to-Peer

3-Tier and 4-Tier Architectures

Horizontal Layers and Vertical Partitions

The Model-View-Controller Architecture

Broker Architectures for Distributed Systems




Software ArchitecturesSoftware Architectures

A software architecture defines the components of a software systemand how they use each others functionality and data.

For example, theclient-serverclient-serverarchitecture consists ofserversservers, whichsupport some kind of service, and clientsclients which request and useserver services. With a client-server architecture, an informationsystem need not be seen as a monolithic program.

Instead, input/output functions are placed on clients, running on PCsand workstations; data storage is assigned to a server, implementedin terms of a DBMS (e.g., DB2, Ingres, Sybase or Oracle) and placedon a mainframe or mini. Consistency checking is located with theserver, applications are located with clients.

Thick serversThick serversoffer a lot of functionality,thinthinones little.

Thick clientsThick clients have their own services, thinthin ones get almosteverything from servers.

In these lecture notes, we emphasizeobject-oriented architectures.


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SubsystemsSubsystems

A subsystem is a component of a system or of anothersubsystem.

ModulesModules are atomic subsystems (which are not furtherdecomposed into subsystems.)

Its useful to subdivide a software system into subsystems For better-managed software development; For improved reuse potential (through components); For improved portability (platform-specific code isolated to

particular subsystems.) For easier maintenance.

Each subsystem has a well-defined interface with respect to the

rest of the system.




System

Subsystem Subsystem Subsystem

Subsystem

Subsystem

Subsystem

Subsystem

Subsystem

...

Subsystem

Subsystem

Subsystem

Subsystem

Module

Module

Module

Module

Module

Module

... ... ...

componentOf


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Components and ConnectorsComponents and Connectors

The architecture shown in the previous slide is one exampleof a software architecture where the nodes representsubsystems or modules and the connectors between themdescribe componentOf relationships.

There are many others kinds of connectors that can be used,such as:

Uses -- one component uses data defined in anothercomponent;

Calls-- one component calls methods defined in anothercomponent;

I/O -- the output of one component is fed as input to

another;




The SoftwareThe Software

BookshelfBookshelf


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Architectural Styles

It is useful to classify software architectures into classes ofarchitectural styles.

For example, the client-server architecture discussed earlier isan architectural style.

The styles well discuss below are as follows:

Pipes and filters;

Object-Orientation;

Event-Based

Layered;

Repository-Based;

Client-Server; Three-Tier;

more...




PackagesPackages

A package in UML is a grouping of elements; these elements

May be packages (representing subsystems or modules);

May be classes;

Each element of a software architecture (subsystem, moduleor class) is owned by a single package;

Packages may reference other packages.

There are many criteria to use in decomposing a software systeminto packages:

Ownership -- who is responsible from which diagrams;

Application -- each application has its own obvious partitions;e.g., a university dept model may be partitioned into staff,courses, degree programmes,

Clusters of classes used together, e.g., course, coursedescription, instructor, student,


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A Package DiagramA Package Diagram

A dependency means thatif you change a class in onepackage (Meetings), you

maymay have to changesomething in the other(Constraints).

The concept is similar tocompilation dependencies.

Its desirable to minimizedependency cycles, if at allpossible.

Persons

Meetings

Constraints

dependency




Decomposition into SubsystemsDecomposition into Subsystems

A software system may be decomposed into horizontal layershorizontal layers,and/orvertical partitionsvertical partitions.

For a horizontal layer decomposition, each layer corresponds toone or more subsystems, and each layer uses services providedby the layers below it.

Layered architectures have two forms:

closed architecture- each layer only uses services of thelayer immediatebelow;

open architecture- a layer can use services from any lower

layer.


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ClosedClosedvsvsOpenOpen

Layered ArchitectureLayered Architecture




Closed layered architectures

Minimize dependencies between layers and reduce the impactof a change to the interface of any one layer.

Open layered architectures

Lead to more compact code, since the services of all lowerlayers can be accessed directly without the need for extraprogram code to pass messages through each interveninglayer;

Break the encapsulation of layers, increase dependenciesbetween layers and increase the complexity of changes to thesystem.

ClosedClosedvsvsOpenOpenLayered ArchitecturesLayered Architectures


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Client Server Architectures

A client server architecture consists of service consumers(clients) andservice providers (servers). Clients and serversmay or may not be running on dedicated machines.

Information exchange between clients and servers is donethrough messages.

Service requests and responses are accomplished through oneof the following protocols:

Remote Procedure Call (RPC) -- client invokes a remotelylocated procedure, which is executed and the results sent tothe client; RPC is widely supported;

Remote Data Access (RDA)-- here the invoked procedure isa database query (say, in SQL) and the response is an oftenlarge set of data; supported by DBMS vendors;

Queued Message Processing -- here requests are queuedand processed whenever possible.




Three-Tier Client Server Architectures

Used widely in industry

E.g., architecture for a meeting scheduling system (MSS)

User Processing Dataclient server serverclient

request request

response response

MSS Client MSS Server

DatabaseServer

SchedulingLogic

Facility MgtLogic

User MgtLogic

InitializationLogic

DB InterfaceLogic

Conflict ResLogic

PaymentLogic

PanelInitialization

Main MenuPanel

SchedulingPanels

FacilityMgt Panels

ConfigurationPanels

User MgtPanels

User Processing(Business Logic)

Data


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Many Possible Variations

User Processing Data




Web-Based Software Architectures

These are client server too, but they are based on WWWtechnologies.

Such architectures are becoming very popular because of staticHTML-based applications, but also dynamic ones, such as thosethat involve Ecommerce.

HTTP --HyperText Transfer Protocol, used to transfer hypertextdocuments over the internet;

HTML --HyperText Markup Language, used to define hypertextdocuments;

CGI -- Common Gateway Interface is a program (e.g., a unixshell script, or a perl script)

CGI scripts are programs that reside on a web server and areexecuted with a click to retrieve data, generate graphics etc.


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Dynamic HTML-Based Architecture

The CGI gateway serves as demon which dispatches a request, dealtwith by an application or a database server.

HTTP

Server

HTMLDocuments

Stream

Manager

Path

Resolver

CGI

Applications

Files +

Databases




Document Interchange ExampleDocument Interchange Example

ANALYST, the General Motors Dealer Review Advisor.ANALYST, the General Motors Dealer Review Advisor.

Assists credit analysts in 230 GM Acceptance Corporationbranch offices analyzing dealership operations in order todecide on credit applications.

Offers many benefits, including faster reviews, reduced trainingof personnel and consistency in decision-making.

Uses an expert system, integrated into vast, conventional dataprocessing architecture


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ANALYST Local ArchitectureANALYST Local Architecture

REMOTE HOST

INTERFACE

DEALERSHIP DATA

MANAGEMENT

EXPERT SYSTEM

REVIEW ADVISOR

DEALERSHIP

DATABASE

CLERICAL

DATA

ENTRY

CLERK

CREDIT

ANALYST




ANALYST Global ArchitectureANALYST Global Architecture

GMAC

WHOLESALE

SYSTEM

GMAC

RETAIL

SYSTEM

DEALER

TIME SALES

SYSTEM

GMAC

400 CARD

SYSTEM

ANALYST

SUPPORT

SYSTEM

GM DEALERSHIP

FINANCIAL

SYSTEM

SNA

NETWORK

BRANCH BRANCH BRANCH

nightly feeds

onlinequeries,updates




nightlyuploads,

downloads


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Four-Layer ArchitecturesFour-Layer Architectures

for Information Systemsfor Information Systems

This is a variation of the 3-tier architectureThis is a variation of the 3-tier architecture

we discussed earlierwe discussed earlier




Vertical PartitioningVertical Partitioning Now the idea is to partition each layer into subsystems.

Partitioning identifies weakly coupled subsystems within alayer.

Each partition provides a self-contained service for the rest ofthe system.


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Architecture for the A-7E AircraftFunction Drivers

Shared Services

S/W

UtilitiesData

Banker Physical

ModelsFilter

Behaviours

Device Interfaces

Application Data Types

Extended Computer




Notes on the A-7E Architecture

This is a uses architecture, I.e., shows which component usesresources in another component.Modules in the different components of the architecture:

Extended computer: virtual memory module, parallelismmodule, timer module;

Device interfaces: air data module, audible signal devicemodule, Doppler radar set module,;

Function driving module: flight information display module,,panel module, ground test module,;

Application data types: numeric, state transition data types;

Data banker module: singular values module, complex eventmodule,;

Physical model: aircraft motion module, earth characteristicsmodule, human factors module;

Software utilities: powerup module.


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Styles of Communication:Styles of Communication:

Client-Server vs Peer-to-PeerClient-Server vs Peer-to-Peer




The Model View Controller (MVC)The Model View Controller (MVC)ArchitectureArchitecture

First used with Smalltalk but has since become widely used asan architecture for object-oriented software systems.

Capable of supporting user requirements that are presentedthrough differing interface styles

Aids maintainability and portability

This architecture is best suited for software systems where userinterfaces play an important role.


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Responsibilities of MVCResponsibilities of MVC

ComponentsComponents




MVC Component InteractionMVC Component Interaction


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Broker ArchitecturesBroker Architectures

for Distributed Systemsfor Distributed Systems A broker increases the flexibility of the system by decoupling the

client and server components

Each client sends its requests to the broker rather thancommunicating directly with the server component

The broker then forwards the service request to anappropriate server

The client need not know where the server is locate (it may be inlocal or remote computer)

Only the broker needs to know the location of the servers that ithandles




Simplified Broker ArchitectureSimplified Broker Architecture


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BrokerBroker

ArchitectureArchitecture

for Localfor Local

ServerServer




Threading and ConcurrencyThreading and Concurrency Each independent flow of control can be modelled as an active

object that represents a process or thread that can initiate controlactivity.

A process is a heavyweight flow (known to the operatingsystems itself) that can execute concurrently with otherprocesses

A thread is a lightweight flow that can execute concurrentlywith other threads within the same process.

Dynamic model of the design identifies concurrent parts of thesystem:

Sequence diagrams imply sequential threads of execution -sequences of messages that invoke each other procedurally;

State and activity diagrams can model concurrent executionwhere different event sequences can lead to concurrentexecution.


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SummarySummary

Architectural system (I.e., hardware and software) designaddresses issues of hardware and software location,interconnectivity, distribution of I/O processes, data storesand application processes.

Architectural system design is the most important step ofsystem design and can, literally, make or break aninformation system development project.




Additional ReadingsAdditional Readings

[Booch99] Booch, G. Rumbaugh, J., Jacobson, I., The UnifiedModeling Language User Guide. Chapter 22. Addison-Wesley.

[Rumbaugh91] Rumbaugh, J et al. Object-Oriented Modelingand Design. Chapter 9, Prentice-Hall.