architecture patterns

7/27/2019 Architecture patterns

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A-36

Examples to Accompany:

Pattern-Oriented SoftwareArchitecture

O ff ice C en sus

Judy,Can I walk you home?

Johnny


Johnny

Pattern-Oriented Software Architecture (also known as PoSA) was written by

Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad and

Michael Stal.

It was published by John Wiley and Sons in 1996.

The examples here are the result of an OOPSLA ‘98 workshop on Non-Software Examples of PoSA patterns, conducted by Michael Duell, Linda

Rising, Peter Sommerlad and Michael Stal.

In addition to the workshop organizers, contributors to this body of work

include Russ Frame, Kandi Frasier, Rik Smoody and Jun’ichi Suzuki.

© AG Communication Systems - 1999



A-37

Layers

John Doe

C/O 12344Town, State 98352

Speedy Delivery

John DoeC/O 12344

Town, State 98352

Bike

Disassemble/Assemble

Pack/Unpack

Ship/Receive

Transport

The Layers pattern helpsstructure applications that can

be decomposed into groupsof subtasks at different layers

of abstraction. Consider shipping a bike. The person atthe shipping or receiving end

of the process, or the driver may not know how to

assemble or disassemble a bike. Then again, they don’t

need to know how.

Architecture

The purpose of Layers is to structure applications that can be decomposed into

different subtasks, with each subtask at an appropriate level of abstraction.

Participant Correspondence:

When considering disassembling a bike for transport, and reassembling it at its

destination, each step corresponds to a layer.The disassembly/assembly, packing/unpacking, shipping/receiving are all tasks

at different levels of abstraction.

Consequences:

The layers are exchangeable. The disassembler, packer, or shipping clerk can

be changed without altering the basic structure of the example.

Layers can be used in a variety of context. The packer and pack items other

than bicycles, just as the shipping clerk can ship items other than bicycles.

Dependencies are kept local. If a bike requires non-metric tools, the

packing/unpacking and shipping/receiving layers are not impacted.



A-38

Pump

Filter Sediment

Chlorinate

Tank

Heater

Tee

Pipes and FiltersPublic water systems demonstrate

Pipes and Filters. The water flowing through pipes is the

input to a filter. Pipes are also usedas the output from a filter. Thisexample shows that name

correspondence between theexample and pattern can produce a

viable example.

Architecture

Pipes and Filters provides a structure for systems that process a stream of data.


In the public water system example, the water pipes correspond to data pipes.

The various stages that add something to or take something from the water

correspond to the filters.

Consequences:

Changing filters adds flexibility. For example, fluoride could be added to the

water supply without upsetting the overall system.

Recombining filters adds flexibility. Many homes have water treatment

systems that soften water used for cleaning and filter chemicals from the

drinking water. All of these home systems take advantage of water supplied via

the upstream pipes and filters.



A-39

Blackboard

Forensics:

Evidence:

Ballistics:

Motives:

Watch just about any police

show on television, andnotice that the crimes cannot

be solved immediately.There is often a blackboard

with forensic evidence, ballistic reports, crime scenedata, etc. posted. When all

of the experts add their pieces of the puzzle, the

police are able to solve thecrime!

Architecture

The Blackboard pattern is used to assemble knowledge from a variety of

sources when no deterministic solution strategy is known.


Obviously the blackboard corresponds to the blackboard class (where data is

centrally managed).Each person having subject matter expertise corresponds to a knowledge

source.

The lead detective is the controller, who brings together the subject matter

experts, and manages access to the blackboard.

Consequences:

It is easy to experiment with different approaches when data from different

sources is brought together.

It is easy to change and maintain the blackboard, because the independence

between the participants. If a crime involved a knife, there would be no needto involve a ballistics subject matter expert.

A blackboard is robust. Only conclusions supported by data and other

hypothesis survive. All others are weeded out.



A-40

Broker A travel agent is a broker for travel related services. Acustomer dealing with atravel agent can book

passage on ships, planes andtrains, reserve a hotel room

and rental car, and book tours. The customer deals

only with the travel agent,although several companiesare involved. Note that

there are often complexcodes on the itinerary. The

broker and travel servicescompanies understand thesecodes. The customer often

does not. Architecture

The Broker pattern is used to structure distributed systems with decoupled

components that interact by remote service invocations. The Broker

component is responsible for coordinating communication between clients and

servers.


The Travel Agent’s client corresponds to the Client.

The Travel Agent corresponds the Broker.

The Travel Services, such as airlines, rental cars, hotels, etc. correspond to the

Servers.

The reservation systems for the Travel Services correspond to the Server-side

Proxies.

Consequences:

The Travel Agent, like the Broker offers location transparency. The client does

not need to know where particular travel services are located.If a server changes, the interface is the same to the client. The interface is the

same to the client, regardless of the airline booked.

Details of internal systems are hidden. A travel agent switching from SABRE

to APOLLO reservation system is transparent to the client.



A-41

Model-View-Controller

Some types of dancers areknown to pay special attention

to those who tip the highest.In this situation, the dancer is

the model, and whoever istipping is controlling the viewof the model for himself, as

well as all others.

Architecture

The Model-View-Controller pattern divides an interactive application into a

model (or core data), view (display) and controller.


The dancer corresponds to the model.

The view of the dancer from different vantage points corresponds to the viewof the model.

The big tipper, corresponds to the controller, since the view of the model

changes while he is getting special attention.

Consequences:

With a dancer, there are multiple views of the same model. These views are

synchronized, since the view of a model is changing simultaneously as she

walks away from one observer, and towards another.



A-42

Presentation-Abstraction-Control

Polls will often gather information from all over

the country, and displayresults as national totals,

regional totals, state totalsand city totals. Theinformation is gathered at

each site, and then reportedto a central site.

Architecture

0

10

20

30

40

50

60

70

80

90

San Francisco

St. Louis

New York

Washinton

The Presentation-Abstraction-Controller pattern defines a structure for

interactive systems in the form of a hierarchy of cooperating agents. Each

agent is responsible for a specific aspect of the application’s functionality.


The national data repository corresponds to the Top-level Agent. It controlsthe PAC hierarchy.

The regional and state data repositories correspond to the Intermediate-level

Agents.

The city data repositories correspond to the Bottom-level Agents.

Consequences:

Detailed information can be obtained from each level. A synopsis of voter

behavior for a city can be obtained at the city level.

The model is easily extendible. A county agent can be added between the city

and state level, without impacting the regional and national levels.Multi-tasking is supported. Voter behavior for multiple cities can be examined

simultaneously.



A-43

Microkernel

POSA Patterns Game

Computer Programmersof

DeathGameboy Ca m

Nintendo makes a GameboyTM

hand-held video game. TheGameboy unit provides thecore functionality, such as

displays and input/output for numerous games and the

Gameboy camera. A gamecartridge or the Gameboy

camera can be inserted toallow the user to play.

Architecture

The Microkernel pattern separates the minimal functional core from extended

functionality.


The player corresponds to the client. The GameboyTM unit corresponds to the

microkernel, since it contains the core functionality.The game cartridges correspond to the internal servers, which provide

additional functionality.

The pin connections on the unit and cartridges correspond to the adapter.

The buttons on the unit provide an interface to clients, and therefore

correspond to the external server.

Consequences:

The client applications can be easily shared between hardware without porting

to a new environment. Kids can trade game cartridges without violating

software copyrights.The Gameboy is very flexible. Originally it was only for games. Eventually, it

could be used as a camera.

The Gameboy has separated policy and mechanism, so that new external

servers could be added that implement their own views. For example, on the

Gameboy camera cartridge, the lens is a new external server.



A-44

Reflection

Reflection provides a

mechanism for changingstructure and behavior dynamically. Consider a

constitution. Changing the behavior of a legislative body is

done by changing theconstitution. The constitution

can be considered to be a meta-object, while the legislative

body is the base object.

Architecture

The Reflection pattern provides a mechanism for changing structure and

behavior of a system dynamically. A system is structured as a base level, and

meta level. The meta level contains information about system properties. The

base level exhibits the application structure and behavior. Changes to meta

level information affect subsequent base level structure and behavior.


The Constitution corresponds to the meta level. It contains information on

how the congress is to conduct itself.

The congress is the base level. It conducts itself according to the Constitution.

Changes to the Constitution affect how the congress behaves.

Consequences:

There is no explicit modification of the base. A change is accomplished by

calling a function in the metaobject protocol. For example, the makeup of

congress changes every two years due to elections as outlined in the

Constitution.

Change to the system is easy. The Constitution provides safe and uniform

mechanism for changing the congress.

Many types of change are supported. Looking at the types of change that have

can affect the structure of congress, elections, representation based on census,

and adding a new state can cause changes in structure.

Meta level changes can damage the system, just as changes to the Constitution

can cause damage.



A-45

Whole-Part

Drive

SparkPlugDistributor FuelInjector Piston

Combines

InjectFuelInCylinder ProvideSpark IgniteFuel TurnCam

A car encapsulates many partsthat a driver cannot access

directly. While driving, thefuel injector will supply fuel to

the cylinder, the distributor will provide spark to thesparkplug in that cylinder, the

sparkplug will fire, and the piston will be forced down,

thereby turning the cam. Thedriver has no direct access to

any of these parts.

Design

The Whole-Part pattern helps aggregate components that form a semantic unit.


The driver corresponds to the client. The driver will ultimately use all of thecomponents through the interface of the whole.

The car corresponds to the Whole. It is made up of smaller systems.

The individual systems, such as the fuel system, ignition system, cooling

system, etc. correspond to the Parts.

Consequences:

Parts are changeable without impacting the client. If a starter fails, it can be

replaced, and the interface to the care remains the same for the driver.

Each concern is implemented by a separate part, making it easier to implement

complex strategies. Many of the parts on a car are fairly simple, yet the

composite is quite complex.

Reusability is supported since parts of a whole can be used in other aggregates.The auto salvage industry is build on this concept. A part can be taken from

one car for use on another.



A-46

Master-SlaveWhen tasked with taking

the census, the CensusBureau utilizes census

takers. The census takersare semantically the same,

(a census taker for one areacould just a easily work adifferent area), and the

results are tabulated by theCensus Bureau. Any

“clients” of the CensusBureau deal directly withthe Bureau, and not withthe individual censustakers.

U.S. Census Bureau

Design

A master component distributes work to identical slave components and

computes a final result from the results when the slaves return


The congress corresponds to the client. Congress authorizes the census, and

uses its results.The Census Bureau corresponds to the Master. It is responsible for taking the

census, and does so by dividing the work into smaller tasks.

The Census takers correspond to the Slaves. They report the work to the

Census Bureau. They are interchangeable.

Consequences:

Makes slaves exchangeable and extensible. A census taker can cover any

designated area. If one finishes early, he can be reassigned to another area.

Separates slave and client code from the work partitioning code. Neither

congress or the individual census takers are concerned with how work is partitioned.

Allows for parallel computing. With multiple census takers, they can all be

working simultaneously. There is no need to finish one area before starting

another.



A-47

Proxy

To:jim @acm.org

[email protected]

jim@monkeysoft .com

Many organizations offer a

proxy e-mail address. Whensomeone sends mail to a

proxy address, he or shemay have no idea that it is a

proxy address. The proxythen forwards the messageto the “real” e-mail address.

Design

The Proxy provides a surrogate or place holder to provide access to an object.


The organization e-mail address corresponds to the Proxy.The “real” e-mail address corresponds to the Original.

The individual having the proxy e-mail and original e-mail address correspondsto the AbstractOriginal.

The person sending e-mail to the AbstractOriginal corresponds to the Client.

Consequences:

The proxy introduces a level of indirection when accessing an object. The

indirection can be used for security, or disguising the fact that an object is

located in a different address space.

In the case of the e-mail example, the AbstractOriginal can change real e-mail

addresses (like when changing companies) and still receive e-mail through the

proxy.



A-48

Command Processor In catalog sales, orders can

be received by Mail, Phone,or FAX. When a request is

received, an order isgenerated. The order will

cause items to be removedfrom stock in order to fill

the order. If an order isreturned, the item will bereturned to stock, and the

account will be credited. If a web site based order form

is implemented, the basicorder processing willremain intact.

Design

Order

(Command)

Fill

Return

CustomerInterface

(Controller)

OrderEvent

Order

(Command)

Fill

Return

Inventory

(Supplier)

RemoveFromStock

ReturnToStock

Abstract Order

(Abstract Command)

Fill

Return

Order

(Command)

Fill

Return

Creates

Uses

Order Fulfillment

(Command Processor)

FillOrder

ReturnOrder

Transfers Commands

Performs& Stores

Order

Thank You!

Order

Thank You!

M icrophone $19 .95

Headsets $24.95

The Command Processor pattern separates the request for a service from its

execution.


The Customer Interface corresponds to the Controller.

The Order Fulfillment department corresponds to the Command Processor.This department is responsible for processing the order, making sure that an

item is taken from stock, and sent to the customer. If the item is returned, the

order fulfillment department will return the item to stock.

The order corresponds to the Capitalize command. Once a phone call, order

form, or fax is received, it becomes a record is created containing the customer name, address, desired items, and method of payment.

The inventory corresponds to the supplier.

Consequences:

There is flexibility in the way that requests are activated. A customer can order

via mail, phone, or FAX. If a web site based order form is implemented, the

basic order processing will remain intact.

Commands can be implemented concurrently. Several customers will request

items daily. The orders are placed in bins, and shipped when complete.



A-49

View Handler

Although window managersare common to software, the

visual nature of windowsallows one to examine

behavior without looking at

source code. The windowmanager opens, manipulates,

and disposes of views. It alsocoordinates dependencies

between views, andcoordinates their updates.

Design

The View Handler pattern helps manage all views provided by a system.


The windowing system corresponds to the view handler.

The displayed windows correspond to the views.

The underlying files or documents correspond to the supplier(s)

Consequences:

The View Handler provides uniform handling of views. New views can beadded without affecting existing views.



A-50

Forwarder-Receiver


Johnny


Johnny School children often use the

Forwarder-Receiver pattern.Johnny will write a message to

Judy, fold it, pass it to Mary, andask her to pass it to Judy.

Mary will pass it to Billy, and ask him to pass it to Judy.Billy will pass it to Judy, who

will unfold the message and readit.

Design

The Forwarder-Receiver pattern introduces a peer-to-peer communication

model that decouples the forwarders and receivers from the underlying

communication mechanisms.


Each student corresponds to a peer. Students will act as Forwarders when they pass a note, and Receivers when they receive the note.

Consequences:

Note passing is efficient inter-process communication. A forwarder knows the

location of potential receivers, but does not need to know the location of the

ultimate destination.

A change in communication mechanism does not impact forwarders and

receivers. For instance, if the note were written in French, it would still be

passed in the same manner.



A-51

Client-Dispatcher-Server

Design

May I speak with thesales manager?

One moment while Iconnect you.

When a client calls a companyto talk with an individual, a

receptionist often answers thecall, and then routes it to the

appropriate person. The clientcannot tell if the person is inthe same building, or at a

remote site. Furthermore, theclient does not know if the

phone rings on the salesmanager’s desk, or if the

receptionist puts him on hold,and yells, “Hey Larry, pick upthe phone!”

The Client-Dispatcher-Sever introduces an intermediate layer between clients

and servers to provide location transparency and hide connection details.


The customer corresponds to the Client.

The receptionist corresponds to the Dispatcher.

The sales manager (or anyone else being called) corresponds to the server.

Consequences:

The Client-Dispatcher-Server introduces an intermediate layer between clients

and servers.

It provides location transparency, by means of a name service.

It hides the details of the communication connection between clients and

servers.

If a new sales manager is hired, the customer is not required to have this

knowledge in order to reach him.



A-52

Publisher-Subscriber To communicate to others,

one can post or publish anotice on a bulletin board.

The subscribers (readers) of the bulletin board can vary

dynamically. Stateinformation, such as havingextra tickets can be

communicated to unknownindividuals when it is not

feasible to poll peopleexplicitly to determine if they require the tickets.

Design

Readers/Subscribers

N e e d 2 t i c k e t s , C a l l M i k e

F r e e K i t t e

n s

t o a g o o d

h o m e.

N e e d R i d e

t o

D e n v e r

Posters/Publishers

The Publisher-Subscriber pattern helps to keep the state of cooperating agents

synchronized. A publisher notifies any number of subscribers of changes in

state.


Someone posting a message on a bulletin board corresponds to the publisher.Someone reading the bulletin board corresponds to a subscriber.

Consequences:

Changes in state must be communicated to unknown parties. The number of

subscribers can vary dynamically.

When an individual needs tickets to an event, a message is posted indicating

that their state is “NeedTickets”. When tickets are obtained, the message is

removed.

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