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Software ArchitecturesSoftware ArchitecturesChapter 2: Architectural StylesChapter 2: Architectural Styles

Prof. Steven A. Demurjian, Sr.Computer Science & Engineering Department

The University of Connecticut371 Fairfield Road, Box U-255

Storrs, CT 06269-2155

steve@engr.uconn.eduhttp://www.engr.uconn.edu/

~steve(860) 486 - 4818

Copyright © 2000 by S. Demurjian, Storrs, CT.

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Taxonomy of Architectural StylesTaxonomy of Architectural Styles

Data Flow SystemsData Flow Systems Batch Sequential Pipes and Filters

Call & Return SystemsCall & Return Systems Main/Subroutines

(C, Pascal) Object Oriented Implicit Invocation Hierarchical Systems

Virtual MachinesVirtual Machines Interpreters Rule Based Systems

Data Centered SystemsData Centered Systems DBS Hypertext Blackboards

Independent Independent ComponentsComponents Communicating

Processes/Event Systems

Client/ServerClient/Server Two-Tier Multi-Tier

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Taxonomy of Architectural StylesTaxonomy of Architectural Styles

Establish Framework of … Establish Framework of … Components

Building Blocks for Constructing Systems A Major Unit of Functionality Examples Include: Client, Server, Filter, Layer, DB

Connectors Defining the Ways that Components Interact What are the Protocols that Mandate the Allowable

Interactions Among Components? How are Protocols Enforced at Run/Design Time? Examples Include: Procedure Call, Event

Broadcast, DB Protocol, Pipe

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Overall FrameworkOverall FrameworkConsider a Question-Based ApproachConsider a Question-Based Approach

What Is the Design Vocabulary?What Is the Design Vocabulary? Connectors and Components

What Are Allowable Structural Patterns?What Are Allowable Structural Patterns? Constraints on Combining Components &

Connectors What Is the Underlying Conceptual Model?What Is the Underlying Conceptual Model?

Von Newman, Parallel, Agent, Etc. What Are Essential Invariants of a Style?What Are Essential Invariants of a Style?

Limits on Allowable Components & Connectors

Common Examples of UsageCommon Examples of Usage Advantages and Disadvantages of a StyleAdvantages and Disadvantages of a Style Common Specializations of a StyleCommon Specializations of a Style

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Pipes and FiltersPipes and Filters

Filters:Filters: Invariant: Unaware of up and Down Stream

Behavior Streamed Behavior: Output Could Go From

One Filter to the Next One Allowing Multiple Filters to Run in Parallel.

SortSort

SortSort

MergeMerge

Connectors for Flow Streams of I/O

Components with Input and Output

Components are IndependentEntities. No Shared State!

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Pipes and FiltersPipes and Filters

Possible Specializations:Possible Specializations: Pipelines - Linear Sequence Bounded - Limits on Data Amounts Typed Pipes - Known Data Format

What is a Classic Example?What is a Classic Example? Other Examples:Other Examples:

Compilers Sequential Processes Parallel Processes

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Pipes and FiltersPipes and FiltersAnother ExampleAnother Example

Text Information Retrieval Systems Text Information Retrieval Systems Scanning Newspapers for Key Words, Etc. Also, Boolean Search Expressions

User

SearchSearchControllerController

SearchSearchDBDB

QueryQueryResolverResolver

Term Term ComparatorComparator

DiskDiskControllerController

CommandsCommands

ProgrammingProgrammingControlControl

ResultResult

DataData

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ADTs and OO ArchitecturesADTs and OO Architectures

Widespread Usage in the 1990’sWidespread Usage in the 1990’s Advantages Are Well Known Advantages Are Well Known

Disadvantages:Disadvantages: Interaction Required Object Identity If Identity Changes, It Is Difficult to Track All

Affected Objects.

obj

obj obj

obj obj

objobj

obj

op

opopop

op

op op

op

opop op

opop

Connectors

Components

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Implicit InvocationImplicit Invocation

Similar to OO in the Sense that Components Can Similar to OO in the Sense that Components Can Call Services on Other ComponentsCall Services on Other Components

How Does this Work?How Does this Work? Components Have List of Events they can

Raise and List of Procedures to Handle Events When Event is Raised, it is Broadcast All Components that Have Procedure to

Handle Broadcast Event will Act Upon it The Component That Raised the Event has no

Knowledge of Which Component(s) will Handle Event

What are Some Examples?What are Some Examples?

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Implicit InvocationImplicit Invocation

AdvantagesAdvantages No Need to Know the Targeted Components Single Event can Impact Multiple Components New Event Handlers can Easily be Added New Events Can then be Raised

DisadvantagesDisadvantages No Control Over the Order of Processing

When an Event is Raised No Control Over “Who” and “How Many”

Process Events Very Non-Deterministic System Behavior

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Layered SystemsLayered Systems

Components - Virtual Machine at Each LayerComponents - Virtual Machine at Each Layer Connectors - Protocols That Specify How Layers InteractConnectors - Protocols That Specify How Layers Interact Interaction Is Restricted to Adjacent LayersInteraction Is Restricted to Adjacent Layers

Users

Corelevel

Base UtilityUseful Systems

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Layered SystemsLayered Systems

Advantages:Advantages: Increasing Levels of Abstraction Support Enhancement - New Layers Support for Reuse

Drawbacks:Drawbacks: Not Feasible for All Systems Performance Issues With Multiple Layers Defining Abstractions Is Difficult.

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ISO as Layered ArchitectureISO as Layered Architecture

ISO Open Systems Interconnect (OSI) ModelISO Open Systems Interconnect (OSI) Model Now Widely Used as a Reference Architecture 7-layer Model Provides Framework for Specific Protocols

(Such as IP, TCP, FTP, RPC, UDP, RSVP, …)

ApplicationPresentation

SessionTransportNetwork

Data LinkPhysical

ApplicationPresentation

SessionTransportNetwork

Data LinkPhysical

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Application ApplicationPresentation Presentation

Session SessionTransport TransportNetwork Network

Data LinkPhysical

Data LinkPhysical

ISO OSI ModelISO OSI Model

Physical (Hardware)/Data Link Layer Networks: Ethernet, Token Ring, Physical (Hardware)/Data Link Layer Networks: Ethernet, Token Ring, ATMATM

Network Layer Net: The InternetNetwork Layer Net: The Internet

Transport Layer Net: Tcp-based NetworkTransport Layer Net: Tcp-based Network Presentation/Session Layer Net: Http/html, RPC, PVM, MPIPresentation/Session Layer Net: Http/html, RPC, PVM, MPI

Applications, E.g., WWW, Window System, AlgorithmApplications, E.g., WWW, Window System, Algorithm

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RepositoriesRepositories

Knowledge Sources Interact With the Blackboard.Knowledge Sources Interact With the Blackboard. Blackboard Contains the Problem Solving State Data.Blackboard Contains the Problem Solving State Data. Control Is Driven by the State of the Blackboard.Control Is Driven by the State of the Blackboard. DB Systems Are a Form of Repository With a Layer DB Systems Are a Form of Repository With a Layer

Between the BB and the KSs - Supports Between the BB and the KSs - Supports Concurrent Access, Security, Integrity, Recovery

ks8

ks6

ks7

ks1

ks2

ks3

ks4 ks5

Blackboard(shared data)

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Database System as a RepositoryDatabase System as a Repository

Clients Interact With the DBMSClients Interact With the DBMS Database Contains the Problem Solving State DataDatabase Contains the Problem Solving State Data Control is Driven by the State of the DatabaseControl is Driven by the State of the Database

Concurrent Access, Security, Integrity, Recovery Single Layer System: Clients have Direct Access Control of Access to Information must be

Carefully Defined within DB Security/Integrity

c8

c6

c7

c1

c2

c3

c4 c5

Database(shared data)

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InterpretersInterpreters

What Are Components and Connectors?What Are Components and Connectors? Where Have Interpreters Been Used in CS&E?Where Have Interpreters Been Used in CS&E?

LISP, ML, Java, Other Languages, OS Command Line

Data(program state)

Program beinginterpreted

Simulatedinterpretation

engine

Internalinterpreter

state

Inputs

OutputsSelectedinstructionSelected

data

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Java as Interpreter Java as Interpreter

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Process Control ParadigmsProcess Control Paradigms

Also:Also: Open vs. Close Loop Systems Well Defined Control and Computational

Characters Heavily Used in Engineering Fields.

Process

ProcessController

Set point

Set point

Input variables

Input variables

Controlledvariable

Controlledvariable

s tomanipulated

variables

s tomanipulated

variables

Controller

With FeedbackWith Feedback

Without FeedbackWithout Feedback

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Process Architecture Process Architecture UML Statechart Diagram?UML Statechart Diagram?

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Breath

Waiting forResp. Signal

Resp Signal

timeout

TriggerLocalAlarm

TriggerRemoteAlarm

Heartbeat

Waiting forHeart Signal

Heart Signal

irregular beat

Alarm Reset

Process Architecture Process Architecture UML Activity Diagram?UML Activity Diagram?

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Client/ServerClient/ServerSingle and Multi-Tier ArchitecturesSingle and Multi-Tier Architectures

Widespread use in Practice for All Types of Widespread use in Practice for All Types of Distributed Systems and ApplicationsDistributed Systems and Applications

Two Kinds of ComponentsTwo Kinds of Components Servers: Provide Services - May be Unaware of

Clients Web Servers (unaware?) Database Servers and Functional Servers (aware?)

Clients: Request Services from Servers Must Identify Servers May Need to Identify Self A Server Can be Client of Another Server

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Client/ServerClient/ServerSingle and Multi-Tier ArchitecturesSingle and Multi-Tier Architectures

Normally, Clients and Servers are Independent Normally, Clients and Servers are Independent Processes Running in ParallelProcesses Running in Parallel

Connectors Provide Means for Service Requests Connectors Provide Means for Service Requests and Answers to be Passes Among Clients/Serversand Answers to be Passes Among Clients/Servers

Connectors May be RPC, RMI, etc.Connectors May be RPC, RMI, etc. AdvantagesAdvantages

Parallelism, Independence Separation of Concerns, Abstraction Others?

DisadvantagesDisadvantages Complex Implementation Mechanisms Scalability, Correctness, Real-Time Limits Others?

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Example:Example:Software Architectural StructureSoftware Architectural Structure

Initial Data Entry Operator(Scanning & Posting)

10-100MB Network

Advanced Data Entry

Operators

DocumentServerStored

Images/CD

DatabaseServerRunningOracle

RMI Registry

Functional Server

RMI Act.Obj/Server

RMI Act.Obj/Server

Analyst Manager

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Business Process Model: Business Process Model: Scanning and Initial Data EntryScanning and Initial Data Entry

Scanner

Licensing

LicensingDivisionScanningOperator

StoredImages

BasicInformationEntered

DB

CompletedApplications

HistoricalRecords

Printer

New LicensesNew AppointmentsFOI

Letters (RequestInformation, etc.)

Licensing DivisionData Entry Operator

SupervisorReview

DB

DB DB

DB

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Two-Tier ArchitectureTwo-Tier Architecture

Small Manufacturer Previously on C++ Small Manufacturer Previously on C++ New Order Entry, Inventory, and Invoicing New Order Entry, Inventory, and Invoicing

Applications in Java Programming Language Applications in Java Programming Language Existing Customer and Order Database Existing Customer and Order Database Most of Business Logic in Stored Procedures Most of Business Logic in Stored Procedures Tool-generated GUI Forms for Java ObjectsTool-generated GUI Forms for Java Objects

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Three-Tier ArchitectureThree-Tier Architecture

Passenger Check-in for Regional Airline Passenger Check-in for Regional Airline Local Database for Seating on Today's Flights Local Database for Seating on Today's Flights Clients Invoke EJBs at Local Site Through RMI Clients Invoke EJBs at Local Site Through RMI EJBs Update Database and Queue Updates EJBs Update Database and Queue Updates JMS Queues Updates to Legacy System JMS Queues Updates to Legacy System DBC API Used to Access Local Database DBC API Used to Access Local Database

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Four-Tier ArchitectureFour-Tier Architecture

Web Access to Brokerage Accounts Web Access to Brokerage Accounts Only HTML Browser Required on Front End Only HTML Browser Required on Front End "Brokerbean" EJB Provides Business Logic "Brokerbean" EJB Provides Business Logic Login, Query, Trade Servlets Call Brokerbean Login, Query, Trade Servlets Call Brokerbean Use JNDI to Find EJBs, RMI to Invoke ThemUse JNDI to Find EJBs, RMI to Invoke Them

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Architecture ComparisonsArchitecture Comparisons

Two-tier Through JDBC API is Simplest Two-tier Through JDBC API is Simplest Multi-tier: Separate Business Logic, Protect Multi-tier: Separate Business Logic, Protect

Database Integrity, More Scaleable Database Integrity, More Scaleable JMS Queues vs. Synchronous (RMI or IDL): JMS Queues vs. Synchronous (RMI or IDL):

Availability, Response Time, Decoupling JMS Publish & Subscribe: Off-line Notification JMS Publish & Subscribe: Off-line Notification

RMI IIOP vs. JRMP vs. Java IDL: RMI IIOP vs. JRMP vs. Java IDL: Standard Cross-language Calls or Full Java

Functionality JTS: Distributed Integrity, Lockstep ActionsJTS: Distributed Integrity, Lockstep Actions

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Java Client to Legacy App via RDBSJava Client to Legacy App via RDBS

Java ClientJava Client

LegacyLegacyApplicationApplication

Relational Relational DatabaseDatabase

System(RDS)System(RDS)

Transformed Transformed Legacy DataLegacy Data

Updated DataUpdated Data

Extract and Extract and Generate DataGenerate Data

Transform andTransform andStore DataStore Data

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Java Client with Wrapper Java Client with Wrapper to Legacy Applicationto Legacy Application

LegacyLegacyApplicationApplication

NetworkNetwork

Java ClientJava Client

Java Application CodeJava Application Code

WRAPPERWRAPPER

Mapping ClassesMapping Classes

JAVA LAYERJAVA LAYER

NATIVE LAYERNATIVE LAYER

Native Functions (C++)Native Functions (C++)RPC Client Stubs (C)RPC Client Stubs (C)

Interactions Between Java ClientInteractions Between Java Clientand Legacy Appl. via C and RPCand Legacy Appl. via C and RPC

C is the Medium of Info. ExchangeC is the Medium of Info. Exchange

Java Client with C++/C WrapperJava Client with C++/C Wrapper

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One COTS and One Legacy Application One COTS and One Legacy Application to Java Clientsto Java Clients

NetworkNetwork

Java Application CodeJava Application Code

JAVA NETWORK WRAPPERJAVA NETWORK WRAPPER

Mapping ClassesMapping Classes

NATIVE LAYERNATIVE LAYER

JAVA LAYERJAVA LAYER

Native Functions that Native Functions that Map to COTS ApplMap to COTS Appl

Java ClientJava Client Java ClientJava Client

Java Application CodeJava Application Code

JAVA NETWORK WRAPPERJAVA NETWORK WRAPPER

Mapping ClassesMapping Classes

NATIVE LAYERNATIVE LAYER

JAVA LAYERJAVA LAYER

Native Functions that Native Functions that Map to Legacy ApplMap to Legacy Appl

COTS ApplicationCOTS Application Legacy ApplicationLegacy Application

Java is Medium of Info. Exchange - C/C++ Appls with Java Wrappers

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Design Patterns Design Patterns Finer Grained Software ArchitecturesFiner Grained Software Architectures

Emerged as the Recognition that in Object-Emerged as the Recognition that in Object-Oriented Systems Repetitions in Design OccurredOriented Systems Repetitions in Design Occurred

Gained Prominence in 1995 with Publication of Gained Prominence in 1995 with Publication of “Design Patterns: Elements of Reusable Object-“Design Patterns: Elements of Reusable Object-Oriented Software”, Addison-WesleyOriented Software”, Addison-Wesley “… descriptions of communicating objects and

classes that are customized to solve a general design problem in a particular context…”

Akin to Complicated Generic Usage of Patterns RequiresUsage of Patterns Requires

Consistent Format and Abstraction Common Vocabulary and Descriptions

Simple to Complex Patterns – Wide RangeSimple to Complex Patterns – Wide Range

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The Observer PatternThe Observer Pattern

Utilized to Define a One-to-Many Relationship Utilized to Define a One-to-Many Relationship Between ObjectsBetween Objects

When Object Changes State – all Dependents are When Object Changes State – all Dependents are Notified and Automatically UpdatedNotified and Automatically Updated

Loosely Coupled Objects Loosely Coupled Objects When one Object (Subject – an Active Object)

Changes State than Multiple Objects (Observers – Passive Objects) Notified

Observer Object Implements Interface to Specify the Way that Changes are to Occur

Two Interfaces and Two Concrete Classes

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The Observer PatternThe Observer Pattern

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Concluding RemarksConcluding Remarks

Architectural Styles Provide Patterns Architectural Styles Provide Patterns Suppose Designing a New System During Requirements Discovery, Behavior and

Structure of System Will Emerge Attempt to Match to Architectural Style Modify, Extend Style as Needed

By Choosing Existing Architectural StyleBy Choosing Existing Architectural Style Know Advantages and Disadvantages Ability to Focus in on Problem Areas and

Bottlenecks Can Adjust Architecture Accordingly

Architectures Range from Large Scale to Small Architectures Range from Large Scale to Small Scale in their ApplicabilityScale in their Applicability