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Its important

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Its importantthat the

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release

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http://www.infoq.com/articles/scalability-principles

22/09/200922:26

InfoQ:Scalability Princip

les

Page1of 4

http://www.infoq.com/a

rticles/scalability-princi

ples

Article

ScalabilityPrinciples

Posted bySimonBrown

onMay21,2008

CommunityArchitecture

TopicsPerformance&Scal

ability TagsConcurrenc

y

Atthesimplestlevel,sca

labilityisaboutdoingm

oreof something.Thisco

uld beresponding tomo

reuser requests,execut

ingmore

workorhandling moreda

ta. Whiledesigningsoftw

are hasitscomplexities,

makingthatsoftware ca

pableofdoinglotsofwo

rkpresents itsownset

ofproblems.Thisarticle

presentssomeprinciples

and guidelinesfor buildin

gscalablesoftware syste

ms.

RelatedVendorContent

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tion Performance

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otectionforREST, SOA,a

nd Web2.0Applications

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r

TheAgileChecklist

AdobeFlashPlatform

OverviewPDF

1.Decreaseprocessingtim

e

Onewaytoincreasethe

amountofwork that an a

pplication does isto dec

rease the timetakenforin

dividualworkunits to co

mplete. For example,

decreasingthe amounto

ftimerequiredto proces

sauser requestmeans t

hatyouareableto hand

lemore userrequestsin

thesameamountoftime

.

Herearesomeexamples

of where thisprincipleis

appropriateand somepo

ssiblerealisationstrateg

ies.

Collocation:reduce any

overheads associatedwi

thfetchingdata required

forapieceofwork, by c

ollocating thedataand th

ecode.

Caching: ifthe dataand

the codecan'tbecolloc

ated,cache thedatatore

duce the overheadof fetc

hingitoverandoveraga

in.

Pooling:reduce the ove

rheadassociated with us

ingexpensiveresources

bypooling them.

Parallelization :decrea

sethetimetakento com

pleteaunit of workbyde

composingtheprobleman

dparallelizing the individ

ual steps.

Partitioning: concentra

terelatedprocessingas

closetogether as possibl

e, bypartitioning thecod

e andcollocatingrelated

partitions.

Remoting :reduce thea

mount oftime spentacce

ssing remote services by

, for example, makingth

einterfacesmorecoarse

-grained. It'salso

worthrememberingthatr

emotevslocalisan expl

icitdesign decisionnota

switchandto considert

hefirstlawofdistributed

computing-do

notdistributeyourobjec

ts.

Assoftwaredevelopers, w

etend tointroduceabstr

actions andlayerswhere

theyareoftennotrequire

d.Yes,theseconcepts are

great tools for

decouplingsoftwarecomponents,butthey

haveatendencytoincr

easecomplexityandimp

actperformance, particul

arly ifyou'reconverting b

etween

datarepresentationsate

achlayer.Therefore,theother wayinwhich p

rocessing timecanbemin

imisedistoensurethat t

heabstractionsaren'ttoo

abstractandthatthere's

nottoomuch layering.I

naddition,it's worthund

erstandingthecostofruntimeservice

sthatwe take forgrante

dbecause,

unlesstheyhaveaspeci

ficservicelevel agreemen

t,it'spossiblethatthese

couldendupbeing the b

ottlenecksinourapplications.

2.Partition

Decreasingtheprocessin

gtime associatedwitha

particularwork unit willg

etyousofar, butultimat

elyyou'll needtoscale ou

tyoursystemwhen

youreachthelimitsofa

singleprocess deployme

nt. Inatypical web applic

ation, scalingoutcould

beaseasyasstartingup

additional webservers to

handle theuserrequests

andload balancingbetw

een them.Whatyou migh

t find,however,isthatpa

rtsofyour overallarchit

ecturewillstartto

becomepointsofconten

tion becauseeverything w

illgetbusy atthesame t

ime. Agoodexample is

asingledatabaseserver

sittingbehindallthose

webservers.Whenthats

tartstobecomethebottl

eneck, youhave tochang

e yourapproachandone

waytodothisistoadop

tapartitioningstrategy.

Putsimply,this involvesb

reaking upthatsinglepie

ceofthearchitectureint

osmallermoremanageab

le chunks. Partitioningtha

tsingleelementinto

smallerchunksallows yo

utoscale themoutand t

hisis exactly thetechniq

uethat largesitessuch a

s eBayusetoensurethat

theirarchitectures

scale. Partitioningisago

od solution,althoughyou

mayfind thatyoutrade-o

ffconsistency.

Astohowyoupartitiony

our system,wellthatdep

ends.Trulystatelesscom

ponents cansimplybe sc

aledoutandthe work loa

d balancedbetween

them,ideallywithallinst

ancesofthecomponent

runninginanactiveman

ner.If,on the otherhand

,there isstate that needs

tobe maintained, you

needto findaworkload p

artitioning strategy thatw

ill allow youtohave mult

iple instancesofthosest

atefulcomponents,where

each instanceis

responsible foradistinct

subset oftheworkand/

ordata.

3. Scalabilityisabout concu

rrency

Scalabilityisinherentlya

bout concurrency; aftera

ll,it'saboutdoingmore

workatthesametime.T

echnologiessuchasthe

earlyversions of

Enterprise JavaBeans(EJB

) attemptedto providea

simplifiedprogramming

model andencouragedu

stowritecomponentsth

atwere single-threaded.

Unfortunately,thesecom

ponents typicallyhad de

pendencies onothercomp

onents andthisledto co

ncurrencyproblems.Ifco

ncurrency isn't thought

about, youhave system

s where datacaneasily b

ecome corrupted.Onthe

otherhand,toomanygu

ardsaroundconcurrency

lead tosystems thatare

essentially serialin natureand limitedi

nthedegreetowhich the

y canscale. Concurrentpr

ogrammingisn'tthathar

dtodo,buttherearesom

e

simple principles that can

helpwhenbuildingscalablesystems.

Ifyou doneedto holdloc

ks(e.g. localobjects, dat

abase objects, etc),tryto

holdthem foraslittle timeaspossi

ble.

Trytominimizecontentio

nofshared resources an

dtry totakeanycontenti

on off ofthecriticalproc

essingpath(e.g. by scheduling work

asynchronously).

Any designforconcurren

cyneedstobedoneup-

front,sothat it's well un

derstoodwhich resources

can besharedsafely and

wherepotential

scalabilitybottlenecks wi

llbe.

4.Requirementsmustbek

nown

Inorder to buildasucces

sfulsoftwaresystem,you

need toknow whatyour

goalsareand whatyou'r

eaimingfor. Whilethef

unctionalrequirements

are often well-known, it'

sthenon-functionalreq

uirements(orsystemqua

lities)thatareusuallyabs

ent. If youdo genuinely

needtobuildapieceof

softwarethatis highly sc

alable,thenyouneedto

understandthefollowing

typesof thingsup-front

for thecritical compone

nts/workflows.

Targetaverage andpeak

performance(i.e. respon

setime,latency, etc).

Targetaverage andpeak

load(i.e.concurrentuse

rs, messagevolumes, etc

).

Acceptable limitsfor per

formance andscalability.


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(andwhy?)


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Will these software systems

perform and scale acceptably?


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Stepone


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Steptwo


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Stepthree


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Stepfour


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Stepfive


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Stepsix


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Stepseven


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timeStamp,elapsed,

label,responseCode

,threadName,succes

s,URL

1242921099104,77,0

1.Logonpage,200,

Corporateuser1-1

,true,http://ib4cu

rrencydev/user/Log

on.aspx

1242921099318,684,

02.Enterusername

andpassword,200,

Corporateuser1-1

,true,http://ib4cu

rrencydev/user/Log

on.aspx

1242921100101,1057

,03.Entermemorab

leinformation,302

,Corporateuser1-

1,true,http://ib4c

urrencydev/user/Lo

gon.aspx

1242921101170,10,0

4.Post-logonland

ingpage,200,Corpo

rateuser1-1,true

,http://ib4currenc

ydev/user/landing-

page.aspx

1242921101191,42,05.Viewclie

nts,200,Corporate

user1-1,true,http

://ib4currencydev/

view/relationship.

aspx

1242921104394,13,0

1.Logonpage,200,Corporateuser

1-2,true,http://ib

4currencydev/user/

Logon.aspx

1242921104420,788,

02.Enterusername

andpassword,200,

Corporateuser1-2,true,htt

p://ib4currencydev

/user/Logon.aspx

1242921105232,872,

03.Entermemorabl

einformation,302,

Corporateuser1-2

,true,http://ib4currencydev/us

er/Logon.aspx

1242921106120,9,04

.Post-logonlandi

ngpage,200,Corpor

ateuser1-2,true,

http://ib4currency

dev/user/landing-page.aspx

1242921106141,715,

05.Viewclients,2

00,Corporateuser

1-2,true,http://ib

4currencydev/view/

relationship.aspx

1242921109600,166,

01.Logonpage,200

,Corporateuser1-

3,true,http://ib4c

urrencydev/user/Lo

gon.aspx

1242921109775,582,

02.Enterusername

andpassword,200,

Corporateuser1-3

,true,http://ib4cu

rrencydev/user/Log

on.aspx

1242921110377,835,

03.Entermemorabl

einformation,302,

Corporateuser1-3

,true,http://ib4cu

rrencydev/user/Log

on.aspx

1242921111221,9,04

.Post-logonlandi

ngpage,200,Corpor

ateuser1-3,true,

http://ib4currency

dev/user/landing-p

age.aspx

1242921101254,1031

2,06.Viewclient

details,200,Corpor

ateuser1-1,true,

http://ib4currency

dev/view/account-l

ist.aspx

1242921111239,925,

05.Viewclients,2

00,Corporateuser

1-3,true,http://ib

4currencydev/view/

relationship.aspx

1242921111584,1612

,07.Recipientsla

ndingpage,200,Cor

porateuser1-1,tr

ue,http://ib4curre

ncydev/recipients/

Default.aspx

1242921113208,51,0

8.Createrecipien

t,200,Corporateus

er1-1,true,http:/

/ib4currencydev/re

cipients/Create.as

px

1242921113274,268,

09.Createrecipie

nt-choosepaymen

ttype,200,Corpora

teuser1-1,true,h

ttp://ib4currencyd

ev/recipients/Crea

te.aspx

1242921113558,1613,10.Create

recipient-enter

recipientdetails

,200,Corporateuse

r1-1,true,http://

ib4currencydev/rec

ipients/Create.asp

x

1242921114709,480,

01.Logonpage,200,Corporateuse

r1-4,true,http://

ib4currencydev/use

r/Logon.aspx

1242921115197,626,

02.Enterusername

andpassword,200,Corporateuser1-

4,true,http://ib4c

urrencydev/user/Lo

gon.aspx

1242921115857,957,

03.Entermemorabl

einformation,302,

Corporateuser1-4

,true,http://ib4currencydev/u

ser/Logon.aspx

1242921116823,11,0

4.Post-logonland

ingpage,200,Corpo

rateuser1-4,true

,http://ib4currenc

ydev/user/landing-page.aspx

1242921116842,37,0

5.Viewclients,20

0,Corporateuser1

-4,true,http://ib4

currencydev/view/r

elationship.aspx


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This showspoorscalability because

response times havestarted to increase

exponentiallyNumber of users

Respon

setime

This showsgoodscalability because

response times increasein a predictable way Number of users

Responsetime


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Areyousure?


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Softwarearchitecturefor

developers

Documents

Bcs2010 a Developers Guide to Load Testing