AndreaDomeni i 1
andFlaviaDonno 2
1
DIIEIT,UniversityofPisa,v.Diotisalvi2,I-56122Pisa,Italy
2
CERN,EuropeanOrganizationforNu learResear h,CH-1211 Geneva23,
Switzerland
Abstra t. TheStorageResour eManagerhasbeenproposedasastan-
dardinterfa e for high-endstorage systemsdeployedonGrid ar hite -
tures.In this paperwe propose a on eptual model for the SRMthat
should supplement itsAPI spe i ation with a lear and on ise de -
nitionofitsunderlyingstru tural andbehavioral on epts.Thismodel
would make it easierto de ne itssemanti s, it wouldhelp servi e and
appli ation developers, and provide for a more rigorous validation of
implementations. Di erent notations are used as appropriate to de ne
di erentaspe tsofthemodel.
1 Introdu tion
The WorldwideLHC Computing Grid (WLCG)is the infrastru turethat will
providethe omputationalandstoragefa ilitiesneededtopro essthedata ol-
le ted bythefour experimentsattheLarge HadronCollider(LHC)at CERN,
amountingto severalPetabytesea hyear.
Oneofthe riti alissues thatWLCG hasto fa eis theprovisionofaGrid
storageservi ethat allows fordynami spa eallo ation,the negotiationof le
a essproto ols,supportforqualityofstorage,authenti ationandauthorization
me hanisms,storage and le management, s heduling of spa e and le opera-
tions,supportfortemporary les,andotherstoragemanagementfun tions.
TheStorage Resour e Manager (SRM)hasbeenproposed[1℄asastandard
interfa eforhigh-endstoragesystemsdeployedonGridinfrastru tures.Inpar-
ti ular,asigni ante ortbyaninternational ollaboration oordinatedbythe
WLCG hasledto thede nition ofthe SRM2.2proto oland to itsimplemen-
tationonallthestoragesolutionsdeployedin WLCG.
The SRM is spe i ed primarily as an appli ation programming interfa e
(API). Inthis paperwe propose a on eptual model for theSRM that should
supplementtheAPIandotherspe i ationswith anexpli it, learand on ise
de nitionofitsunderlyingstru turalandbehavioral on epts.Thismodelwould
makeiteasiertode netheservi esemanti s,itwouldhelpservi eandappli a-
tiondevelopersandprovideforamorerigorousvalidationofimplementations.
Theproposed model is meant to strikea satisfa tory ompromisebetween
larity and formality. Di erent notations (e.g., basi set-theoreti and logi al
formalism, UML [2℄ diagrams, and plain English) are used as appropriate to
AStorageElement(SE)isaGridServi eimplementedonamassstoragesystem
(MSS) that maybebased ona pool of diskservers,onmore spe ialized high-
performingdisk-basedhardware,oronadisk a hefront-endba kedbyatape
system,orsomeotherreliable,long-termstoragemedium.Remotedataa essis
providedbyaGridFTPservi e[3℄andpossiblybyotherdatatransferservi es,
whilelo ala ess isprovidedbyPOSIX-likeinput/output alls.
ASE providesspa es where users reate and a ess les. A le isalogi al
set ofdatathat isembodied inoneormorephysi al opies.
Storagespa esmaybeofdi erentqualities,relatedtoreliabilityand a es-
sibility, andsupport di erentdatatransferproto ols.Di erent usersmayhave
di erent requirementson spa e quality and a ess proto ol, therefore, besides
the data transferand le a ess fun tions, aSE must support more advan ed
resour emanagementservi es,in ludingdynami spa eallo ation.
TheStorageResour eManager (SRM)isamiddleware omponentthatpro-
videstheresour emanagementservi esthroughastandardinterfa e,indepen-
dent of the underlying MSS. The interfa e is de ned by the SRM Interfa e
Spe i ation (IS)[4℄thatliststheservi erequeststhata lientappli ationmay
issue,alongwiththedatatypesfortheirargumentsandresults.
Requestsignaturesaregroupedbyfun tionality,su hasspa e management
requeststhat allow lientsto reserve,release,and managespa es,spe ifyingor
negotiating their qualityand lifetime, and data transfer requests that get les
intoSRMspa eseitherfrom a lient'sspa eorfromotherstoragesystems,and
retrievethem. Othergroupsaredire tory,permission,anddis overy fun tions.
3 A Model for the Storage Resour e Manager
ThemainSRMspe i ationaretheabovementionedISandtheStorageElement
ModelforSRM2.2andGLUEs hemades ription[5℄.Otherrelevantdo uments
are [1,6,7℄.Weproposedamodelto extendthespe i ations withasyntheti
des riptionofthebasi entities,theirrelationships,andtheirbehaviors.
Wehave hosentousetwosubmodels,withdi erentlevelsofformality.The
semi-formalmodelusesplainEnglishandUMLdiagrams,anditismeanttogive
anoverallviewofthesystem,identifyingitsmain omponents,theirrelationships
andbehavior,andtode neand larifythetermsusedintheIS.Amoreformal
modelusesset-theoreti andlogi alnotationstoexpress onstraints.Thismodel
ismeanttoresolveambiguitiesthatmightremaininthesemi-formalmodel,and
to supportthedesignandtestingofSRMimplementations.
3.1 Des ribing Con epts and Properties
In the stati model, the SRM on eptsare represented asobje t lasses, their
propertiesandre ipro alrelationshispsbeingmodeledbyattributesandasso ia-
tionssubje ttovarious onstraints.Figure1showsapartialUML lassdiagram
surl: anyURI
fileRetentionPolicy: TRetentionPolicy fileAccessLatency: TAccessLatency fileStorageType: TFileStorageType locality: TFileStorageType fileLifetimeAssigned: int fileLifetimeLeft: int
File
spaceRetentionPolicy: TRetentionPolicy spaceAccessLatency: TAccessLatency
copyRetentionPolicy: TRetentionPolicy copyAccessLatency: TAccessLatency copyStorageType: TFileStorageType requestToken: RequestToken
Copy
copyPinLifetime: int turl: anyURI handlePinLifetime: int
Handle
totalReservedSpace: long int guaranteedReservedSpace: long int
Space spaceToken: string
spaceLifetime: int
file 1 .. *
file 1
space 1
0 .. *
master {subsets copies}
copies
0 .. *
1 copy space
1
0 .. *
Fig.1.Stati modeloftheSRM.
Someoftheattributesrepresentimportantproperties:retentionpoli y,a ess
laten y,and ( le)storagetype. Retentionpoli y des ribesthelikelihood thata
le opy may be lost in a given storage spa e. This likelihood may be high
(REPLICAretentionpoli y),intermediate(OUTPUT),orlow(CUSTODIAL).
The OUTPUT poli y is urrently not implemented. A ess laten y des ribes
data a essibility: A spa e where data areimmediately a essible is ONLINE,
otherwise it is NEARLINE. A NEARLINE spa e is supported by me hani al
media,su hastape,thatrequiredatatobestagedtotemporarydiskstoragefor
a ess. A third laten y, OFFLINE, is urrently not implemented. Thestorage
type refers to le lifetime. A VOLATILE le is deleted automati ally after a
giventime.ADURABLE lealsohasalimitedlifetime,butitmustberemoved
expli itly by its owner. A PERMANENT le has an unlimited lifetime, until
removedbyitsowner.Durable lesare urrentlynotimplemented.ASiteURL
(SURL) identi esthe lewithin theSE,andtheSEitself.
Theasso iationsin thediagram showthat aspa ehosts zeroormore les,
thatea h lehasoneormore opies,oneofwhi histhemaster opy,thatea h
opyresidesonaspa e,andithasoneormorehandles.
3.2 Des ribing Behavior
Thedynami model of theSRMisdes ribedbyUML statediagrams.Figure2
showsapartofthemodel,relatedto les.
A le is reated with aprepareToPut or a opy request. A request an be
served asyn hronously, so a le may remain for sometime in a waiting state
(SURLUnassigned)beforeit isassigned aSURL.Inthis state,the le anbe
destroyed by an abortFiles orabortRequest operation. Otherwise, eventuallya
option,whenitslifetimeexpiresandthe leisvolatile,orwhenthepinlifetime
ofitslast opyexpiresandthe leisvolatile.
Some requests are a epted in the SURLAssigned state, but they do not
alterthebehavior.Su hrequestsarelistedasinternaltransitions (showninside
the statei on in the diagram) and leave the le in SURLAssigned and in its
urrentsubstate,whi heveritbe.
SURL_Unassigned
extendFileLifetime extendFileLifetimeInSpace setPermissions prepareToPut [busy]
copy [busy]
SURL_Assigned
abortFiles
abortRequest rm
releaseSpace [force]
when (fileLifetimeLeft = 0) [type = VOLATILE]
when (expiredCopy(c) and lastCopy(c)) [type = VOLATILE]
Fig.2.Statema hineforFile(1).
4 A More Formal Stati Model
Whilethesemi-formal modelexposed aboveishelpful forusersanddevelopers
oftheSRM,a nerlevelofdetailandagreaterdegreeofformalityareneededto
ensureinteroperabilityandfull omplian ewiththespe i ation.Thereforewe
proposeaninitial,stillin omplete,formalmodelexpressedinbasi mathemati al
notation. Sin e the SRM is still evolvingand several issuesare still open, the
model is limited to fundamental features, upon whi h further extensions and
re nements anbebuilt.
Anelementarymathemati alnotationwas hoseninsteadofsomemorespe-
ializedlanguage,su hastheUMLObje tConstraintLanguage[8℄ortheZSpe -
i ation Language [9℄, but itshould beeasy to translatethenotation adopted
heretothoseformalisms,ifneeded.
First,weintrodu esomebasi sets whosemembersareunstru turedvalues,
su hasatomi symbols(meantto representnamesofobje tsordis retevalues
for their attributes) ornumbers. Then we de ne the onstru ted sets of stor-
age elements, spa es, opies, handles, and les asCartesian produ ts of some
previoulyde nedsets.
Fun tions are used to represent various properties and relationships. Con-
identi altothesetofnaturalnumbers(Sz =IN ),andthesetofretentionpoli ies,
de ned asaset ofthreevalues(Rp=fREPLICA;OUTPUT;CUSTODIALg).
Theset ofspa esisde nedasS =TLPropSzR
s
,whereT isthe
set ofspa e tokens(i.e., identi ers),Ltheset of lifetimes, Prop asetof tuples
ofspa eproperties, andR
s
isthesetofrequestissued forea hspa e.
Asanexampleof a onstraint,thestatementthat \a le annot outliveits
spa e"isexpressedas
8
f2F;t>stime(f)
0<lleft(f;t)<lleft(mspa e(f);t)
whereF isthesetof les,tisatimevalue,stime(f)isthetimeof le reation,
lleft(;t) isthe le orspa elifetime remainingat time t,and mspa e(f)is the
spa eholdingthemaster opyof lef.
5 Validation of Existing SRM Implementations
TheSRMhasbeenimplementedfor vedi erentMSSs,namelyCASTOR[10℄,
developedatCERNandbasedontapelibrariesanddiskservers,dCa he[7℄,de-
velopedatDESY,usedwithmultipleMSSba kends,DPM[11℄,adisk-onlyMSS
developedatCERN,DRM/BeStMan,adisk-basedsystemdevelopedatLBNL,
the rstpromoter ofSRM, and StoRM [12℄,adisk-basedsystem developed at
CNAF,basedonparallel lesystemssu hasGPFSorPVFS.
All these systems are being tested for omplian e with the SRM IS. Us-
ing various te hniques ofbla k-box testing[13℄, vefamilies oftest aseshave
beendesigned:Availability to he ktheavailabilityoftheSRMend-points;Ba-
si to verify basi fun tionality of the implemented SRM APIs;Use Cases to
he k boundary onditions, fun tion intera tions, and ex eptions; Exhaustion
to exhaust allpossiblevaluesofinputand outputargumentssu h aslengthof
lenames,SURLformat,andoptionalarguments;Stresstests tostress thesys-
tems, identify ra e onditions,study the behaviorof the system when riti al
on urrentoperationsareperformed,andinother exa ting onditions.
TheSRMmodelproposedin thispaperhasbeenused toderiveseveraltest
asesintheBasi and UseCases testsuites.
6 Con lusions
A omprehensive model of the SRM is being developed to support the devel-
opmentandveri ationofSRMimplementations,usingdi erentnotationsand
levelsof formality in order to satisfy theneeds ofdi erentstakeholders in the
SRMdevelopment.
The rstdraftofthemodelisavailable,andfeedba kfromitsusersisawaited.
Infa t,themodelhasalready ontributedtothevalidationofexistingimplemen-
tationsbyassistinginthedesignofafewfamiliesoftests,anditsdevelopment
hashelpedin identifyingunanti ipated behaviorsandintera tions. Thetesting
ampaign itself has helped the developers to nd solutions that better satisfy
The work of the manypeople in the SRM ollaborationis gratefully a knowl-
edged.Inparti ularwewouldliketothankArieShoshani,AlexSimandJunmin
Gu from LBNL, Jean-Philippe Baud, Paolo Badino, Maarten Litmaath from
CERN, TimurPerelmutovfrom FNAL,Patri kFuhrmann fromDESY, Shaun
DeWittfrom RAL,EzioCorsofromICTP,Lu aMagnoniandRi ardoZappi
from CNAF/INFN, for theirvaluable inputin thespe i ationde nition pro-
ess. Finally, wewould liketo thankthe WLCG proje t for givingus the op-
portunity to olle t the requirements and test the proposed proto ol for real
use- ases.TheauthorshavebeensupportedbyCERNandINFN,respe tively.
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