Querying Business Processes

12 Pages·2006·0.33 MB·English

Querying Business Processes
∗
Catriel Beeri Anat Eyal Simon Kamenkovich
The Hebrew University Tel Aviv University Tel Aviv University
[email protected] [email protected] [email protected]
Tova Milo
Tel Aviv University
[email protected]
ABSTRACT view of the process, as a graph of data and activity nodes, con-
nectedbycontrolanddataflowedges. Designsareautomatically
WepresentinthispaperBP-QL,anovelquerylanguageforquery-
convertedtoBPELspecifications.Thesecanbeautomaticallycom-
ingbusinessprocesses. TheBP-QLlanguageisbasedonanintu-
piledintoexecutablecodethatimplementsthedescribedBP[30].
itivemodelofbusinessprocesses, anabstractionoftheemerging
DeclarativeBPELspecificationsgreatlysimplifythetaskofsoft-
BPEL(BusinessProcessExecutionLanguage)standard. Itallows
waredevelopmentforBPs.Moreinterestinglyfromaninformation
userstoquerybusinessprocessesvisually,inamannerveryanal-
managementperspective,theyalsoprovideanimportantnewmine
ogous to how such processes are typically specified, and can be
ofinformation.Considerforinstanceauserwhotriestounderstand
employedinadistributedsetting,whereprocesscomponentsmay
howaparticulartravelagencyoperates. Shemaywanttofindan-
beprovidedbydistinctproviders(peers).
swerstoquestionssuchas: CanIgetapricequotewithoutgiving
We describe here the query language as well as its underlying
firstmycreditcarddetails? Whatshouldonedotoconfirmapur-
formalmodel. Weconsiderthepropertiesofthevariouslanguage
chase?Whatkindofcreditservicesareusedbytheagency,directly
componentsandexplainhowtheyinfluencedthelanguagedesign.
orindirectly,(i.e. bytheotherprocessesitinteractswith)? Ob-
In particular we distinguish features that can be efficiently sup-
viously,suchqueriesareofgreatinteresttobothindividualusers
ported,andthosethatincuraprohibitivelyhighcost,orcannotbe
andtoorganizationsinterestedinusingoranalyzingBPs.Answer-
computedatall. Wealsopresentourimplementationwhichcom-
ingthemisextremelyhard(ifnotimpossible)whentheBPlogic
plies with real life standards for business process specifications,
iscodedinacomplexprogram. Itispotentiallymucheasiergiven
XML,andWebservices,andisusedintheBP-QLsystem.
adeclarativespecificationlikeBPEL.Foranorganizationthathas
accesstoitsownBPELspecifications,aswelltothoseofcooperat-
1. INTRODUCTION
ingorganizations,theabilitytoanswersuchqueries,inapossibly
A business process (BP for short) consists of a group of busi- distributedenvironment,isofgreatpracticalpotential.
nessactivitiesundertakenbyoneormoreorganizationsinpursuit Tosupportsuchqueries,oneneedsanadequatequerylanguage,
ofsomeparticulargoal. Itusuallydependsuponvariousbusiness andanefficientexecutionengineforit. Toaddressthisneed, we
functions for support, e.g. personnel, accounting, inventory, and presentinthispaperBP-QL,anewquerylanguagewhichallows
interactswithotherBPs/activitiescarriedbythesameorotheror- for an intuitive formulation of queries on BP specifications, and
ganizations. Consequently, the software implementing such BPs queryexecutioninadistributedcross-organizationenvironment.
typicallyoperatesinacross-organization,distributedenvironment. Before presenting our results, let us highlight briefly some of
It is common practice to use XML for data exchange between thechallengesinqueryingBPspecificationsingeneral,andBPEL
BPs,andWebservicesforinteractionwithremoteprocesses[34]. onesinparticular.
TherecentBPELstandard(BusinessProcessExecutionLanguage
FlexiblegranularityBPspecificationsmaybeabstractlyviewed
[7], alsoidentifiedasBPELWSorBPEL4WS),developedjointly
as a set of nested graphs, possibly with recursion: The graphs
byBEASystems,IBM,andMicrosoft,combinesandreplacesIBM’s
structure captures the execution flow of the process components;
WebServicesFlowLanguage(WSFL)[27]andMicrosoft’sXLANG
Thenestingcomesfromthefactthattheoperations/servicesused
[35]. ItprovidesanXML-basedlanguagetodescribenotonlythe
in a process are not necessarily atomic and may have a complex
interfacebetweentheparticipantsinaprocess,butalsothefullop-
internal structure (which may itself be represented by a graph);
erationallogicoftheprocessanditsexecutionflow.
The recursion is due to the fact that a process may call itself in-
Commercial vendors offer systems that allow to design BPEL
directly,throughcallsitmakestootherprocesses.Usersmaywish
specification via a visual interface, using a conceptual, intuitive
to ask coarse-grain queries that consider certain process compo-
∗TheresearchhasbeensupportedbytheIsraelScienceFoundation. nentsasblackboxesandallowforhighlevelabstraction,aswellas
fine-grainedqueriesthat“zoom-in”onalltheprocesscomponents,
possiblyrecursively. Anadequatequerylanguagemustthusallow
Permissiontocopywithoutfeeallorpartofthismaterialisgrantedprovided userstoquerytheprocessesatdifferent,flexible,granularitylevels.
thatthecopiesarenotmadeordistributedfordirectcommercialadvantage,
DistributionAsmentionedabove,BPstypicallyoperatesinacross-
theVLDBcopyrightnoticeandthetitleofthepublicationanditsdateappear,
andnoticeisgiventhatcopyingisbypermissionoftheVeryLargeData organization,distributedenvironmentwhereeachpeerholdsaset
BaseEndowment. Tocopyotherwise,ortorepublish,topostonservers ofBPsandmayprovide(resp.use)servicesto(of)remotepeers.If
ortoredistributetolists,requiresafeeand/orspecialpermissionfromthe aservice’sinternalflowhasbeendefinedinBPEL,andtheservice
publisher,ACM. providersmakethisspecificationavailabletotheircooperatingor-
VLDB’06,September12-15,2006,Seoul,Korea.
ganizations(sayviaawebservice),usersmaywishtozoom-inon
Copyright2006VLDBEndowment,ACM1-59593-385-9/06/09
theseremotecomponentsaswelltoquerytheservicespecification. Observe that, in general, several modes of querying business
processes are possible. One can query the specifications as data
PathsextractionWhenqueryingBPs,usersmaybeinterestedin
(e.g. “doesthespecificationincludeapathfromactivityAtoac-
retrieving,asananswer,thequalifyingflowpaths(asforinstance
tivityB”). Onecanalsoaskaboutpatternsthatmayoccurwhen
inthequery“WhatshouldIdotoconfirmmypurchase?”). Asthe
theprocessesareexecuted(e.g. “cantherebearunofthesystem
numberofrelevantpathsmaybelarge(oreveninfiniteinrecursive
whereactivityAisfollowedbyactivityB”).Onecanalsomonitor
processes)amajorchallengeistoprovidestheuserswithacom-
runsasprocessesexecute,orposequeriesonlogsofpastruns.
pactfiniterepresentationofthe(possibleinfinite)answer.
BP-QLisaquerylanguageforprocessspecifications,1notabout
EaseofqueryingAsmentionedabove,theBPELstandardoffers
theirpossibleruns. Thisisfortwomainreasons. First,querying
anXML-basedlanguagefordescribingtheoperationallogicofa
thepossiblerunsofasystemisaverificationproblem[22]andis
BP.SinceaBPELspecificationisessentiallyanXMLdocument,a
typically of very high complexity (from NP-hard for very simple
naturalquestioniswhynotqueryitdirectly,usingXQuery?Akey
specificationstoundecidableinthegeneralcase[28]).Second,the
observationisthattheBPELXMLformatis(1)verycomplexand
analysisofrunsrequiresaspecificationtohaveawelldefinedse-
(2)wasdesignedwitheaseofautomaticcodegenerationinmind;
mantics.Unfortunately,BPELisnotbasedonaformalmodel[28].
however,itisextremelyinconvenientforquerying.Toexpresseven
To avoid these obstacles and guaranty complexity that is polyno-
averysimpleinquiryaboutaprocessexecutionflow,oneneedsto
mialinthesizeofthedata,BP-QLignorestherun-timesemantics
write a fairly complex XQuery query that performs an excessive
ofcertainBPELconstructssuchasconditionalexecutionandvari-
numberofjoins.Furthermore,evenifamorequery-friendlyXML
ablevaluesandfocusesonthegivenspecificationflow.Webelieve
representationforithadbeenchosen(asindeedisdoneinternallyin
this approach offers a reasonable balance between expressibility
ourimplementation),XQuery,asis,wouldstillnotbeadequatefor
and complexity. Note that querying of specifications in fact “ap-
thetask: XQueryonlyreturnsdocument elements, butnotpaths,
proximates”thequeryingofruns(e.g.onlyspecificationsthatcon-
itdoesnotsupportqueryingatdifferentlevelsofgranularity,and
taintwogivenactivitiesmaypotentiallyhaverunswherebothoc-
it does not offer tools for controlling distributed querying. Last
cur).Hence,evenwhenfullrunverificationisdesired,BP-QLcan
butnotleast,queryinganXMLrepresentationismuchmorediffi-
be used as an efficient means to narrow the search space for the
cultthanqueryingdirectlyaconceptualmodel.Essentially,easeof
morecostly, interpretationdependent, verification. Itcan alsobe
queryingrequiresananintuitive,conceptual,datamodel,coupled
usedtoselecttheprocesspartstobemonitoredatruntime[32].
withamatching,equallyintuitive,querylanguage.
ContributionsWenowstatethecontributionsofthispaper.
The BP-QL query language presented in this paper addresses 1. WepresentBP-QL,anewgraphicalquerylanguagethatal-
theseissues. ItisbasedonanintuitivemodelofBPs,anabstrac- lowsforintuitivequeryingofprocessspecifications, byof-
tionoftheBPELspecification,alongwithagraphicaluserinterface feringadatamodelandaninterfacesimilartothoseusedfor
thatallowsforsimpleformulationofqueriesoverthismodel. Ina BPsspecification. Itallowstoretrievepaths,andoffersfa-
sense,itfollowsthesamedesignprinciplesthatguidedcommercial cilitiesforqueryingatdifferentlevelsofgranularity,andfor
vendorsinthedevelopmentofgraphicaleditorsforthespecifica- controllingdistributedquerying.
tionofBPELprocesses: ithidesfromtheusersthetediousBPEL 2. Wepresentaformalmodelforsystemsofprocesses,andfor
XML details and allows for more natural query formulation. In- ourquerylanguageonsuchsystems,basedongraphgram-
deed,wewillseethatthetightanalogybetweenhowBPsarespec- mars[21]. Thismodelallowstodistinguishbetweenquery
ifiedinsucheditorsandhowtheyaregraphicallyqueriedinBP- featuresthatcanbeefficientlysupported,andthosethatin-
QL,facilitatesintuitivequerying. BP-QLalsooffersfacilitiesfor curaprohibitivelyhighcost, orcannotbecomputedatall.
controlling granularity and distribution in query formulation and Using this model, we explain how to construct a finite and
allowspathsinqueryresults. intuitiverepresentationofthe(possiblyinfinite)answersof
At the core of the BP-QL language are BP patterns that allow queriesintimepolynomialinthesizeofthespecifications.
userstodescribethepatternofactivities/dataflowthatareofinter-
3. Finally,wedescribethesystem’simplementation,highlight-
est. BPpatternsaresimilartothetree-andgraph-patternsoffered
ingthemainchallengesfacedandthesolutionstaken.
byexistingquerylanguagesforXML[36]andgraph-shapeddata
AfirstprototypeofBP-QLwasdemonstratedin[5],whereonlya
[15,13,31],butincludetwonovelfeaturesdesignedtoaddressthe
veryhighlevelviewofthelanguagewaspresented. Thepresent
issues mentioned above. First, BP-QL supports navigation along
paperprovidesacomprehensivedescriptionofthelanguage,ofits
twoaxis: (1)thestandardpath-basedaxis,thatallowstonavigate
underlyingformalformalandofitsimplementation. Thepaperis
through,andquery,pathsinprocessgraphs,and(2)anovelzoom-
organizedasfollows. Section2introducesBP-QL informallyvia
inaxis,thatallowstonavigate(transitively)insideprocesscompo-
arunningexample. Theunderlyingformalmodelispresentedin
nents(localaswellasremoteones)andquerythematanydepth
Section 3. The system implementation is described in Section 4.
ofnesting. Second,pathsareconsideredfirstclassobjectsinBP-
WeconcludeinSection5,consideringrelatedandfuturework.
QL and can be retrieved, and represented compactly, even when
involvingactivitiesperformedondistinctpeers.
2. SYSTEMOVERVIEW
Together,thesefeaturesallowforsimpleformulationofqueries
onBPs. However,theymaketheevaluationofqueriesmuchmore WepresenthereaninformaloverviewofBP-QLviaarunning
intricatethanthatoftraditionalXML/graphpatterns.Indeed,some example. To illustrate the features of BP-QL , we will consider
queries that can easily be evaluated on flat graphs/trees may be- a set of business processes (BPs) used by a consortium offering
comecomputationallyexpensive(orevenundecidable)whennested travel-relatedservices. Theseincludeflightandhotelreservation,
graphsareconcerned. Tokeeptheevaluationofqueriestractable, carrental,creditandaccountingservices. Theprocesses,andtheir
we had identified these problematic scenarios and carefully de- BPEL specifications, reside and operate on distinct peers. The
signedthelanguagesothattheyareavoided,andpolynomial-time specificationsincludetheinteractionsbetweenthevariousprocesses.
queryevaluationisguaranteed. Ouranalysisisbasedonmodeling
1A variant for monitoring and querying of logs is planned future
systemsofprocessesandqueriesasgraphgrammars[21].
research.
Wefirstshowhowprocessesarespecified,viathesystem’sgraph-
icaluserinterface,andthenillustratehowtheycanbeinterrogated
andqueriedwithBP-QL.ThegraphicalspecificationofBPsthat
weuseisfairlystandard,andissimilartothoseofferedbycommer-
cialvendors(e.g.[30]).ThenoveltyhereisintheBP-QLgraphical
query language, designed especially for querying such specifica-
tions. The ease of query formulation is illustrated by comparing
thequerygraphicalinterfacetothatusedfortheprocessesspecifi-
cation;thereisatightanalogybetweenhowprocessesarespecified
andhowtheyarequeried.
RunningexampleOurrunningexampleisalongthelinesofW3C’s
travelagentscenario[1].Alpha-Tours,afictionaltravelagency,of-
fers to its potential clients the ability to book complete vacation
packages: plane tickets, hotels, car rentals, and so on. The main
stepsofthereservationprocessareasfollows: Theuserprovides
adestination,somedates,possiblysomeconstraints,tothetravel
agencyservice. Next,theserviceobtainsinformationaboutpossi-
bledealsfromairlines,hotelsandcarrentalagenciesandpresents
themtotheuser,whichselectstheonessheisinterestedin. Those
are reserved by the agency. Finally, the user may cancel or con- Figure1:TravelAgency.
firmthereservation,passinghercreditcarddetails.Theairline,car
rentalandhotelservicescontactacreditcardserviceforpayment
authorizationbeforetheyacknowledgesthereservation.
Wenowdemonstratehowtheservicesarespecifiedandqueried.
All screenshots were taken with our BP-QL visual designer and
querytool.
2.1 BusinessProcesses
AsystemconsistsofasetofBPs, possiblyresidingondistinct
peers.ABPspecificationincludes:
1. Somegeneraldescriptionoftheprocessproperties,including
itsname,capabilities,theserviceprovider,andsoon.
2. The data used in the process, namely the process variables
andtheinputandoutputparametersfortheparticipatingac-
tivities/services.
3. Theactivitiesofwhichtheprocessiscomposed.
4. Adescriptionoftheprocessoperationalanddataflow.
Visually, the specification of a BP is represented as a directed
labeled graph, with three types of nodes: property nodes (for 1),
drawnashexagons; datanodes(for2),drawnasellipses; andac-
tivitynodes(for3),drawnasrectangles. Edgesthatconnectdata
Figure2:ZoomingintosearchTrip.
andactivitynodes,calleddataflowedges,describewhichdatais
read or output by which activity. Edges between activity nodes, taxonomiesandontologiesthatprovidestandarddefinitionsofthe
calledactivityflowedges,describetheoperationalflow.Tocapture servicedomain.2
certain particular aspects of the operational flow of BPs, activity Theprocessflow(ontheleft)anditsdataelements(ontheright),
nodesmaybeidentifiedasprovidedoperationsorrequestedoper- are displayed in separate boxes. The small rectangles at the top
ations. These describe the services offered by a process to other andbottomoftheactivityflowareitsentryandexitpoints. The
processes,andtheexternalservicesthatitrequests,resp. Activity BPcontainsfourcompoundactivitynodes,namelysearchTrip,
nodesmayalsobedistinguishedasatomicorcompound.Thelatter reserveTrip,confirmTripandcancelTrip.Ashortthick
representinvocationsofcomposite(possiblyremote)processesand incomingarrowindicatesaprovidedoperation. Aclientmayin-
aredenotedbytwolittleboxesatthetopleftcorneroftheactivity vokeeachofthefourprovidedoperations(attheappropriatepoint
icon. Theinterpretationofcompoundnodesisbasedontheideas intheflow). Edgesbetweendatanodesandactivitynodesdepict
ofstatechart[23]:azoom-inallowstoreplaceacompoundactivity the data flow. For example, the client’s trip request is imported
byadetaileddescriptionoftheprocessthatitinvokes. whenthesearchTripactivityisentered. Theresultsarestored
Forillustration,considertheBPdepictedinFigure1. Itrepre- inatripResultvariable.
Onecanzoomintoacompoundactivitynodetoseewhatisin-
sentsthetravelagencyfromourrunningexample. Thelabelun-
side. Figure 2 shows the details of searchTrip. We can see
dereachnodeisitsname. Eachnodecarriessomeinformationon
thatthetravelagencyinteractswithotherservicestofulfillclient
the process property/data/activity that it represents, which can be
requests. The short thick arrows outgoing of the searchCars,
viewed by clicking on it. For instance, the property nodes at the
top of the figure describe the process, its provider, and its capa- 2Implementation-wisetheyarestoredinaUDDIrepository.
bilities. Most attributes of these nodes are references to external
Figure4:Creditservicesinvokedwhensearchingfortrips.
Figure3:Findprovidedoperations.
searchFlights,andsearchRoomsiconsindicatethatthose
arerequestedoperations. Here,thenodeattributes(notdisplayed
in the figure) provide the parameters (URL, operation name, ...)
that allow one to invoke the relevant Web service. If the service
providersmaketheirBPELspecificationavailable,onecanzoom
inalsointothesenodesaswelltoseetheservicespecification.
Thefigurealsoshowsdataflowedges(forclaritysomeofthese
edges are omitted). For example, the set of airlines that the
agency works with is imported when searchFlights is en-
tered. Theresultsofsearchingexternalairlineservicesarestored
inpossibleFlights.
Beforemovingontoquerying,wehighlighttwotypesofcycles Figure5:(a)Negation (b)Pathconstraints.
thataspecificationmaycontain.First,thegraphofagivenBPmay
containcycles,indicatingthatcertainactivitiesmayberepeatedan inselectionconditionsontheirattributesanddataandforjoins.We
unboundednumberoftimes.Second,inasystemconsistingofsev- alsosupportnegation(denotedbydashednodesandedges).
eralprocessesthatcalleachother,aBPmaycallitselfindirectly, We demonstrate the use of BP-QL via some example queries.
throughcallsitmakestootherprocesses. Thisisanotherkindof Each query describes a process pattern that a user is looking for.
Thecheckboxesnexttonodesandedgesmarkselectednodesand
cyclicstructure: hereonecouldzoomintothecorrespondingcom-
paths,resp.,thattheuserwantstotoretrieveasthequeryresult.
poundoperationanunboundednumberoftimes. Notethatwhen
querying BPs, users are often interested to retrieve flow paths as
EXAMPLE 2.1. ThequeryinFigure3searchesforoperations
answers(asforinstanceinthequery“Whatarethepossibleways providedbytheAlpha-ToursBPandtheservicesituses. The
topurchaseaplaneticket?”).Inthepresenceofcycles,thenumber doubleheadededgesinsidethebehaviorboxindicatethatactivi-
ofqualifyingpathsmaybeinfinite.Oneofthecontributionsofour tiesatanydistancefromthestart/endnodesmayqualify;theshape
workistoprovideanintuitive,finite(andcompact)representation ofthenoderestrictsthesearchtoprovidedoperations. Thedou-
forsuchpossiblyinfiniteanswers. bleboundingofthebehaviorboxdenotesunboundedzoom-in;we
lookforoperationsprovidedbytheBPand(transitively)thecom-
2.2 TheBP-QLQueryLanguage
poundactivities/servicesthatitinvokes. Thezoom-inisrestricted
GiventhatBPsaredefineddeclaratively,wecanquerythespec- toactivities/serviceswhosespecificationsresideonthesamepeer,
ificationstolearnabouttheprocesses. Inourrunningexample,a since the deepSearch attribute is set to local. Setting it to global
user may want to ask questions like: ’Which operations are pro- willextendthesearchtoremoteservicesaswell. (cid:163)
vided by the travel agency service?’, ’Which services are called
directly or indirectly by the service?’, ’Does the service allow to EXAMPLE 2.2. Figure4illustratesajoinoperation.Thequery
makeareservationwithoutfirstgivingcreditcarddetails? andif checkswhichVISAcreditcardservicesarecalled(directlyorindi-
rectly)bytheAlphaTour’sconfirmTripactivity. Weusevari-
so,whatdoesoneneedtodoformakingareservation?’. Wepro-
ablestodefinethejoinconditions. Thejoinisvaluebased,i.e. the
ceedtoexplainhowBP-QLcanbeusedtoexpresssuchqueries.
nodes’attributesarecheckedtohavethesamevalues. (cid:163)
BP-QLquerieslookmuchlikethespecifications. Forquerying
BPs,BP-QLoffersBPpatternswhich,intuitively,playforBPsa
roleanalogoustothatplayedforXMLtreesbytreepatternqueries. EXAMPLE 2.3. ThequeryinFigure5(a)illustratestheuseof
Theydescribethepatternofactivity/dataflowthatisofinterestto negation. IttestswhethertheusersofAlpha Toursarenever
theuserandallownavigationalongtwoaxis:path-basedandzoom- requiredtologinwhensearchingforflights. Formally, thisisex-
inbased. Followingtheuseof/and//inXPath[36]fordenoting pressed by stating that a path to the searchFlights activity
singleandmultiplestepnavigation,ourPBpatternsuseedgeswith thatpassesthroughaloginactivitydoesnotexists(dashededges
singleanddoubleheadstodenotesingleandmultipleedgepaths, and nodes denote negation). The existing flow paths leading to
resp.Similarly,toallowausertoqueryaboutflowsthatarenested searchFlights are then retrieved (as indicated by the small
at any depth in the zoom-in hierarchy, compound activity nodes checkboxnexttothedoubleheadededge).
mayhavedoublyboundedboxes,todenoteanunboundedzoomin A more lenient query, that retrieves, the paths without a login
leading to searchFlights, can be expressed by attaching a
intotheactivities’internalspecifications. Thenodesandedgesof
variable, say x, to the edge, along with the selection condition
BPpatternscanbeassociatedwithvariables,andthesecanbeused
Figure6:Dataflow.
x ∈ (Σ\“login”)∗. SeeFig.5(b). Regularpathexpressionsas
constraintsonpathsarediscussedinSection3.4. (cid:163) Figure7:(a)searchFlights. (b)Infinitesetofresults.
EXAMPLE 2.4. Finally, Figure 6 illustrates querying the data
flow. Thequerysearchesfordataelementsthatare(transitively)
affectedbythesearchRequest,andserveasinputforsending
thesuggestedtripsbacktotheclient. Bydefault,adoubleheaded
edgebetweentwodata(resp.activity)nodesdenotespathsconsist-
ingonlyofdata(activity)flowedges. Tooverridethedefault,(e.g.
considerpathswithallsortsofedges)onecanattach,asabove,a
variabletotheedgewithanappropriateselectioncondition. (cid:163)
2.3 Query Semantics (informally)
Whenaqueryisevaluated,itspatternsarematchedagainstthe
systemBPs.Itsnodesandedgesareassignedactivity/data/property
nodesandexecution/dataflowpaths,resp. Thesearethenusedto
constructthequeryresult.
More precisely, the semantics of a query q on system S is de-
finedasfollows. Anembeddingisafunctionfromthenodesand Figure8:Finiteresultrepresentation.
edges of q to nodes, edges and paths of S, that satisfies the ob-
system’sdesign. Inparticularwedistinguishfeaturesthatcanbe
vious constraints: Nodes are mapped to nodes of the same type,
efficientlysupported,andthosethatincuraprohibitivelyhighcost,
single/double-head edges are mapped to edges/paths between the
orcannotbecomputedatall. Tosimplifythepresentationwefirst
correspondingendpoints.Whenacompoundquerynodeisdoubly-
considerabasicdatamodelandquerylanguage,thenenrichthem
bounded,nodesandedgesinitmaybemappedtonodesandpaths
toobtainthefullfledgedmodel.
inaprocessobtainedbyanynumberofzoom-insintotheactivity’s
specification. For nodes and edges are associated with variables, 3.1 SimpleBusinessProcessesandSystems
thequeryconstraintsonthesevariablesmustbesatisfiedaswell.
WeassumetheexistenceofoftwodomainsN ofnodesandL
Eachembeddingdefinesoneresultforthequery.Thenumberof
ofnodelabels.Listhedisjointunionofseveraldomainsincluding
qualifyingresultsmaybelarge(possiblyinfiniteinthepresenceof
datavalues,attributenames,dataelementnames,processproperty
cycles). However,BP-QLprovidesaconcise,intuitive(andfinite)
names,andatomicandcompoundactivitynames.Weassumesome
representationfortheset.Weillustratethisbelowwithanexample
distinguishedpropertynames. Theseareintroducedbelow,inthe
andprovidemoredetailsontheconstructioninSection3.
appropriatecontexts.
EXAMPLE 2.5. AssumethatthesearchFlightsservice(in-
vokedbysearchTripinFigure2)hasthestructuredepictedin Business graphs and processes. We model a (simple) BP
Figure7(a). Theusercaneitherloginandcheckfortheavailabil- asadirectedlabeledgraphwithnodesoftwotypes: concreteand
ity of various flights, or call, again, Alpha Tours’ searchTrip compound.Concretenodesrepresentprocessproperties,attributes,
servicetostartanewsearch. Now,reconsiderthequeryinFigure data elements, and atomic activities. Compound nodes represent
5(b),thatretrievesthepathsleadingtosearchFlightsthatdo compoundactivities,namelycallsof(possiblyremote)operations.
notrequirealogin. Becauseofthepotentialcyclicserviceinvoca- TwodistinguishednodesoftheBPgraphrepresentitsstartandend
tion, searchTripcaninfactbereachedbyaninfinitenumber activities.Formally,
ofpaths, asdepictedinFigure7(b). Ratherthanlistingallthese
DEFINITION 3.1. A(simple)businessgraphisapairg=(G,λ),
paths,theuserisprovidedwithacompactrepresentation(seeFig-
whereG = (N,E)isadirectedgraphinwhichN ⊂ N isafi-
ure8)thathighlightstherecursivestructureoftheresults. (cid:163)
nitesetofnodes,andEisasetofedgeswithendpointsinN;and
λ : N → L isalabelingfunctionforthenodes. Dependingon
3. THE FORMAL MODEL theirlabeltype,werefertothenodesingasactivitynodes,value
nodes, property nodes, etc. Nodes labeled by compound activity
In this section we briefly present the formal model underlying
namesarecalledcompoundnodes;allothernodesingarecalled
theBP-QLquerylanguage. Wediscussthepropertiesofthevar-
concrete.
ious language components and explain how these influenced our
A(simple)businessprocess(BP)isatriplep=(g,start,end),
where: g isabusinessgraph;start,endaretwodistinguished
activity nodes in g; and each activity node in g resides on some
pathfromstarttoend. (cid:163)
Notethatthestartandendnodesneednotbedistinct.Forexample,
aprocessmayconsistsofjustoneactivitynode,whichisbothits
startanditsend.Alsonotethatonlyactivitynodesarerestrictedto
bebetweenthestartandendnodes.Recallfromsection2thatactiv-
itiescanbeclassifiedasrequestedorprovided.Thisismodeledby
assumingtwoparticularpropertynamesprovidedandrequested,
andattachingtoactivitynodesappropriatepropertynodes.
Forexample,Figure9showsseveralBPs(ignorethe“bubbles”
fornow).Asbefore,weusesquaresforactivitynodesandhexagons
Figure9:AsystemofBPs.
forpropertynodes. TheleftmostBPhasasinglecompoundactiv-
itynode,whichisbothitsstartandend. Theoneinthecenterhas
twodistinctstartandendnodes,andfourprovidedoperations.
As mentioned above, compound nodes represent calls to com-
positeoperations. Theinternalstructureoftheseoperationsisnot
part of the business process graph and is given separately, as we
explainnext.
Simple systems. Asystemisacollectionofbusinessprocesses
(orgraphs), alongwithamappingbetweencompoundnodesand
theirimplementations–theprocessestheyinvoke. Inthegeneral
case, a system may be distributed. This is ignored for now, for
simplicity,andisdiscussedinSection3.4.
DEFINITION 3.2. AsystemSofbusinessprocesses(resp.graphs)
isapair(P,τ),wherePisafinitesetofbusinessprocesses(graphs),
and τ is a (possibly partial) function, called the implementation
function,fromthecompoundactivitynodesinPtobusinessprocesses
(graphs)inP.3 (cid:163)
Figure10:Arefinedsystem(afteronestep).
Thisdefinitioncaneasilybeextendedtodistinguishbetweenroot
processes,thataredirectlyaccessible,andimplementationprocesses, initsplace,withtheincoming/outgoingedgesofnnowbeingcon-
thatareaccessibleonlyasimplementationsofotherprocesses. To nectedtothestart/endnodesofτ(n),resp. Ifnwasthestart/end
simplifythepresentationweomitthishere. nodeofp,thestart/endnodeofτ(n)nowtakesthisrole.]
Theimplementationfunctionispartialwhentheinternalstruc- Ifp→p →...→p wesaythatp isarefinementofp.
1 k k
ture of some compound activities is unknown (for instance when WesaythatS →S(cid:48)(w.r.t.τ)ifS(cid:48)isobtainedfromSbyreplac-
their providers do not wish to expose their specification). Recall ingtheimplementationpofsomecompoundactivitynodeninS
fromdefinition3.1thattheonlydifferencebetweenbusinessgraphs byarefinementp(cid:48) ofp. [Namely,acopyofp(cid:48) isaddedtoP,the
andbusinessprocessesisthatthelatterhavedistinguishedstartand mappingτ fornisupdatedtopointtoit,andτ isextendedtomap
end nodes. Systems of processes are used to model real life ap- compoundnodesinp(cid:48),tothesameBPsasinP. Finally,ifpisno
plications. Systems of graphs will prove useful to model query longertheimplementationofanynode,itisremovedfromP.]
answers. For brevity, since we will mostly be dealing with sys- IfS →S →...→S wesaythatS isarefinementofS. (cid:163)
1 k k
temsofprocesses,unlessstatedotherwisethetermsystemshould
beinterpretedassystemofprocesses. Note that if S is a system, then each of its refinements is also
Figure 9 shows a partial system. This is a partial description a system. Figure 10 shows a refinement of the system from Fig-
oftheTravelAgencysystemfromFigures1,2(forsimplicity,the ure 9, after one refinement step, in which the implementation of
dataandattributenodesareomitted). Thefullsystemshouldalso behaviorwasrefined:thenodelabeledsearchTriphasbeen
contain,forexample,theprocessesoftheairline,carreservation, ”zoomedinto”andreplacedbyitsimplementingprocess.
andhotelcompanies.
3.2 Simple Queries
System Refinement. Given a system S, some BP p in it, and
Wenowconsiderqueriesandtheiranswers. Forsimplicitywe
acompoundactivitynodeninp,amoredetaileddescriptionofp
consider first simple positive queries without negation and joins.
(andhenceofS)canbeobtainedbyzooming-inandreplacingthe
These,andotherextensions,areconsideredinSection3.4.
nodenbyitsimplementation.Wecallthisarefinement.
Queries. QueriesaremodeledusingBPpatterns. Thesegener-
DEFINITION 3.3. GivenasystemS =(P,τ)andaBPpinP,
alizeBPssimilarlytothewaytreepatternsgeneralizeXMLtrees.
wesaythatp → p(cid:48) (w.r.t. τ)ifp(cid:48) isobtainedfrompbyreplacing
Thelabelsofnodescanbespecified,orleftopenusing∗. Edges
somecompoundactivitynode ninpbyitsimplementationτ(n).
inagraphcanbeeithersingle-headed,inwhichcasetheyarein-
[Namely,nisdeletedfromp,andacopyoftheBPτ(n)isplugged
terpretedoveredges,ordouble-headed,inwhichcasetheyarein-
3Inanactualsystem,τ(n)canberepresentedbyattachingtonthe terpreted over paths. Similarly, nodes have a single or a double
peerandprocessid(WebserviceURL,operationname, etc.) for boundary, for searching only in the direct implementation of the
theimplementationofn. nodeorinallitsrefinements,resp.Wecalledgeswithdoublehead
(resp.nodeswithdoubleboundary)transitiveedges(nodes). finitesetofgraphs,wheregraphsmaycontainnon-terminalsym-
bols,andwheregrammarrulesallowtoreplaceanon-terminalby
DEFINITION 3.4. ABPpatternisatuple(p∗,T,R),where agraphfromagivenfinitecollection.
1. p∗isaBPwherenodesarelabeledbyelementsfromL∪{∗}, Theintuitionisthat,forasystemS,theimplementationrelation-
2. T isadistinguishedsetofedgesandcompoundnodesinp∗ shipscorrespondtogrammarrules; thesystemrefinementscorre-
calledthetransitiveedgesandnodes,resp. spondtothegraphlanguagedefinedbythegrammar. Similarly,a
3. Risadistinguishedsetofedgesandnodesinp∗ calledthe queryqcanalsobeviewedasCFGGwhosegraphlanguageconsists
resultedgesandnodes,resp. ofallthegraphsthatsatisfythequeryconstraints(i.e. containthe
Asimplequeryq isasystemofBPpatterns(Q,τ),whereQisa patternsspecifiedbythequery). Thisintuitioncanbeextendedto
setofBPpatterns,andτ isanimplementationfunction. (cid:163) thequeryanswer:Insteadofconstructingexplicitlythepotentially
infinite set of results, one may construct a CFGG that represents
Toevaluateaquery,itspatternsarematchedtothoseof(refine- it. Specifically, thequeryanswercanbeviewedassomekindof
mentsof)thesystem.Amatchiscalledanembedding. “intersection” of the languages defined by the system and query
grammars(followedbya“projection”thatomitstheportionsthat
DEFINITION 3.5. Letq = (Q,τ)beasimplequeryandletS
werenotrequestedasoutput).
beasimplesystem. AnembeddingofqintoSisahomomorphism
Ingeneral,theintersectionoftwoCFGGlanguagesmaynotbea
ρfromthenodesandedgesinqtonodesedgesandpathsinsome
refinementS(cid:48) =(P(cid:48),τ(cid:48))ofSs.t. CFGGlanguage[21].(Thisgeneralizesthesamepropertyforstring
CFGs.) Inourparticularcase, however, thequeryspecificationis
1. (nodes)eachstart(resp.end)nodeinqismappedtoastart
sufficientlysimpletoguaranteetherequiredclosure:onecanshow
(resp. end)nodeinS(cid:48); eachconcretenodeinq ismapped
toaconcretenodeinS(cid:48)ofthesamekind;andanodewitha that it belongs to a restricted class of CFGGs called recognizable
sets[14],forwhichtheintersectionwithanotherCFGGisknownto
constantlabelismappedtoanodehavingthesamelabel.
yieldaCFGG. Thisimpliesthatinprincipleonecouldemploythe
2. (edges)each(transitive)edgefromnodemtonodeninq
intersectionalgorithmpresentedin[14]toconstructafiniterepre-
ismappedtoanedge(path)fromρ(m)toρ(n)inS(cid:48).
sentation for the query results. The problem, however, with this
3. (implementation)Foreachcompoundactivitynodenin directsolutionisthatthealgorithmof[14]isofhighcomplexity-
q,ρmapsthenodesand(transitive)edgesinτ(n)tonodes exponentialinthesizeoftheBPs4 -henceimpracticalforquery
and edges (paths) in τ(cid:48)(ρ(n)). If n is not transitive then evaluation. Animportantresultofthepresentworkwastodetect
τ(cid:48)(ρ(n))mustbeanoriginalBPofS(i.e.notarefinement). that BP-QL queries form a subclass of the recognizable sets for
whichPTIMEsolutionispossible,andtodesignsuchanalgorithm.
Thequeryresultdefinedbyρistheimageunderρofq,restricted OuralgorithmisbasedonamodularconstructionofaCFGGthat
toitsoutputnodesandedges.Ifthesamenode/edgeoccursseveral describesthequeryresults.Itreliesonthefollowingtwoideas.
timesintheimage,distinctcopiesareusedforeachoccurrence.(cid:163)
1. The first idea is that each query result is a combination of
Theresultassociatedwithanembeddingρis,ingeneral,asys- smallerresultsthatdescribehowonequeryprocess,saypq,
tem of graphs. As the number of such qualifying results may be ismappedtoonesystemprocess,saypS. Thecombination
large(possiblyinfinite)weprovideaconcise(andfinite)represen- mustsatisfythecondition(implementation).
tationforthisset. 2. Thesecondideaisthatmanyembeddingssharethesameun-
derlyingnodemapping, anddifferonlyontheassignments
3.3 Compactrepresentationofqueryresults
to the, possibly transitive, edges. Thus, many results for a
Tounderstandtheconstructionofthisconciserepresentation,let pairpq,pS,mayberepresentedtogetherbyusingthisshared
usfirstlookatthetwomainfactorsthatcontributetolargeorinfi- node mapping (and, as we did above for flat BPs, repre-
niteanswers. Weconsiderfirstflatgraphs,i.e. BPswithnocom- sentingthedifferentpossiblepathassignmentsfortransitive
poundactivities,andthennestedBPs. edgesbetweenthenodesbyacopyofthesub-graphthatcon-
nectsthenodes.) Ofcourse,whenanodemappingfrompq
FlatBPsWhenaBPcontainscycles,thenumberofpathsthatmay
topS isconsidered,onemustensurethatitsatisfiesthecon-
matchagiven(transitive)queryedgemaybeinfinite.Observethat
ditions(nodes)and(edges).
evenwhenaBPisacyclic,thenumberofmatchingpathsmaybe
large. Forexampleiftheactivityflowforksintoseveralpathsand Thus, a CFGG representation for the query results is obtained
then joins back, forks and joins again, and so on, several times, fromacollectionofnodemappingsbetweendistinctpairsofquery
thenumberofpossiblepathsisexponentialinthenumberofforks. andsystemprocesses,thatsatisfytheconditionsabove.
Thesolutiontothisproblemiseasy: Wecanrepresentthesetof Wenextsketchthemainlinesofthisconstruction.Giveaquery
pathsbetweentwonodesbyacopyofthesub-graphthatconnects q andasystemS,considersomeBPpatternpq inq andaBPpS
thenodes. Onemightactuallysaythisiswhattheuserintended: inS.Theimportantobservationisthatanembeddingfrompq toa
toseethespecificationofthepathsbetweenthetwonodes,rather refinementofthesystemprocesspS consistsofseveralparts. The
thantheindividualpathsthemselves. first part maps some of the query nodes and edges of pq into pS
itself. Subsequentpartsmapadditionalnodesandedgesofpq into
NestedBPsThingsbecomemorecomplexinthepresenceofcom-
implementationsofcompoundactivitynodesofpS,andsoon.The
pound activities. A system may contain recursive activity imple-
nodesandedgesinthequerypatternpq thatarenotmappedtopS
mentations, hence have an infinite set of refinements. Since the
itself,butintoimplementationsofitscompoundnodes,musthave
resultsofaqueryareconstructedfromembeddingsintoallthere-
astructurethatfitssuchimplementations.Inparticular,theyshould
finements,theremaybeinfinitenumberofsuchpossibleresultsas
formasetJ ofdisjointsub-graphsofpq,eachwithasingleentry
well. Thesolutionhereisbasedonviewingsystemsandqueries
andexitnodes.
ascontextfreegraphgrammars[21],abbreviatedCFGG. (Confus-
ingly, these are also called in the literature regular graph gram- 4Furthermore,toourknowledge,noPTIMEalgorithmsforthisin-
mars. We will use only context free in this paper.) A CFGG is a tersectionproblemareknown.
Thus,theconstructionisrecursive. ForeachsuchsetJ itmodi- forthepositiveportionsofthequerygraphs,whichcannotbeex-
fiesthequerypatternpq,byreplacingeachsub-graphGinJ bya tendedtoincludethenegativeportions,areconsidered.
distinctnewnodelabeled∗(denotedinthesequel∗ ),obtaining
a modified query graph pq/J. Then it constructs reGpresentations Labelpredicatesandregularpathexpressions. Thesim-
ple queries considered so far only allow nodes with a particular
forallresultsobtainedfromembeddingsfromthemodifiedquery
pq/J totheBPpS(groupingtogether,asexplainedabove,embed- labelor∗. Butsometimesonemaybeinterestedinsystemnodes
thatconformtocertainconditions.Forinstance,ratherthansearch-
dingsthatagreeontheirnodemappings).Assumethe∗ nodewas
G ingforthesearchFlighsactivity,wemaywanttoretrieveall
mappedtoacompoundactivitynode,sayA.Intherepresentation,
theactivitieswhosenamecontainsthestring“search”.Thiscanbe
this node is noted as A , and it serves as a non-terminal of the
G
achievedbyusinglabelpredicates.Inanembedding,aquerynode
grammar. Then,foreachGandA ,itrecursivelyconstructsrep-
G
labeledbyalabelpredicatemustbemappedtosystemnodewhose
resentations, sayR , forresultsobtainedfromembeddingsfrom
G
labelsatisfiesthepredicate.
G to the implementation of A. The grammar rule then allows to
Anotherusefulfeatureareregularpathexpressions. Transitive
replaceA byR .
G G
edges in the query may be annotated by regular expressions. In
Special care must be paid in the construction above to transi-
anembedding,suchedgesmustbemappedtopathssuchthattheir
tive edges. These may be mapped to arbitrarily long paths, that
maystartinonesystemBPpS,continueinanimplementationof labelsequenceformsawordinthecorrespondingregularlanguage.
acompoundactivitynodeofpS,andsoon,possiblythroughacy- Theconstructionofafiniterepresentationforthequeryanswer
extendsnaturallytosupportthesetwoextensions.
cleofimplementations.Suchpathsarebrokenintocomponentsby
introducing special dummy nodes into the query pattern pq, then Variables and joins. Together with label predicates and reg-
employingtheconstructionasdescribedabove. ular path expressions, one may also want to use label and path
Forlackofspace, weomitherethepresentationofthefullal- variablesandtestfor(in)equalityoftheassignedlabelsandpaths.
gorithmanditscorrectnessproof. (Theseareavailableinthefull The interpretation is that query nodes labeled by (un)equal label
versionofthepaper[6]).Notethatalmostallgraphsforwhichrep- variables are mapped to system nodes with (distinct)identical la-
resentations are constructed are sub-graphs of the query patterns bels;queryedgeslabeledby(un)equalpathvariablesaremappedto
pq. (Theintroductionofdummynodescomplicatesthisargument pathswhosesequencesoflabelsare(different)equalwords. While
abit.) Theconstructionterminateswheneachmappingfromeach theuseoflabelvariablesposesnoparticularproblem,forqueries
suchgraphtoeachsystemprocesshasbeenconsidered(orbefore withjoinsonpathvariables,ourconstructionmayfail;theanswer
that,ifembeddingsforsomesub-graphareneverrequired). tosuchqueriesmaynolongerberepresentableasafinitesystem.
Theimportantpointtoobserveisthefollowing. To understand why, recall that our systems may be viewed as
THEOREM 3.6. Thesizeoftherepresentationforthequeryre- CFGGs. Aquerythattestsforequalityofpathvariablesmayhave
sults,constructedbytheabovealgorithm,aswellastheconstruc- forananswersetsofgraphsthatarenotaCFGGlanguageandare
tiontime,arepolynomialinthesizeofthesystemS(withtheexpo- inherentlyhardertocompute,asillustratedbythefollowingtheo-
nentdeterminedbythesizeofq). rem. Thetheoremalsohighlightsthedifferenceincomputational
complexitybetweenthequeryingofflatandnestedgraphs.
Theintuitionisthat,intheworstcase,eachsub-graphofeachquery
pattern needs to be mapped into each system BP. The number of THEOREM 3.7. For queries with equality conditions on path
querysub-graphsoftheappropriateformisafunctionofthequery variables,theproblemoftestingwhetherthequeryanswerisempty
size alone. Since embeddings that agree on their node mappings onasystemisundecidable.
arerepresentedtogether,itsufficestocountthedistinctnodemap- The problem can be solved in exponential time if the system
pings. Foreachquerysub-graphandeachsystemBP,thisnumber to which the query is applied has no recursive activities. It is
ispolynomialinthesizeoftheBP(withtheexponentdetermined PSPACE-hardw.r.tthesystemsize,evenifthesystemBPsalsohave
bythesizeofthequerysub-graph.) nocycles.
Finally,forflatBPs,itcanbesolvedintimepolynomialinthe
3.4 ARichermodel
sizeofthesystem(withtheexponentdeterminedbythesizeofq).
So far, for the sake of simplicity, we used a very simple data
Proof:(sketch)Theundecidabilityandhardnessproofsarebyre-
modelandquerylanguage.Wenowpresentsomeusefulextensions
duction to the problem of testing whether the intersection of the
that enhance the expressive power, and facilitate the querying of
reallifebusinessprocesses. languages of two string context free grammars (CFGs) is empty.
Giventwo CFGsG1,G2, webuildasystemS withaBPthathas
Negation. Inaquerywithnegation,thepatternshavesomenodes twobranches. Thefirstcontainsacompoundactivitynodeg and
1
andedgesthataredistinguishedasnegative.Theintuitiveinterpre- the second a compound activity g . The implementation of g ,
2 i
tationisthatthequerysearchesforoccurrencesofthepositivepor- i = 1,2, (whichresemblesinspiritthegrammarrulesofG ), is
i
tionsofthepatterns,forwhichnoneofthenegativepartsco-occur. definedsuchthateachofitspossiblerefinementshasline-shaped
Moreformally,todefinethesemanticsofquerieswithnegation structure,representingawordinthecontextfreelanguageofG .
i
weextendthenotionofembedding:Givenaqueryqwithnegation, Theimplementationsaredefinedsuchthatg andg canberefined
1 2
thepositivepartofq, denotedpositive(q), isthequeryobtained toanactivitysequencewiththesameshapeiffthissequencerep-
from q by deleting all the negative edges and nodes, and all the resentsawordthatbelongstoboth G andG . Next, wedefine
1 2
edges incident on these nodes. The embeddings of q into S are queryqwithtwotransitiveedgese ,e thatmatch(therefinements
1 2
thoseembeddingsofpositive(q)whichcannotbeextendedtoan of)g andg resp.,andhaveanequalityconditionontheirattached
1 2
embeddingofanyqueryq(cid:48) obtainedfrompositive(q)byadding pathvariables. Itiseasytoseethatthequeryhasanonemptyre-
allthenegativenodesandedgesofsomeofitsBPpatterns. The sultiffthelanguagesintersectionisnotempty. Thisisknownto
answerofqisdefinedasbefore,basedontheaboveembeddings. beundecidableinthegeneralcase,andwasrecentlyprovedtobe
Afiniterepresentationforthequeryanswercanbeconstructed PSPACE-completefornon-recursivecontextfreelanguages[29].
essentiallyasabove. Theonlydifferenceisthatonlyembeddings Thepolynomialandexponentialalgorithmsworkasfollows.For
flat BPs, the algorithm considers all possible mappings of query
nodestotheBP.Testingjoinconditionshereamountstotestingif
theintersectionoftheregularlanguagesdefinedbythesub-graph
that connects the nodes is empty, which can be done in PTIME.
For nested, non-recursive, BPs, the algorithm enumerates all the
systemrefinements(possiblyanexponentialnumber)andtestsfor
theexistenceofalegalembeddinginasimilarway. (cid:163)
We have consequently decided to restrict the use of path vari-
ablesinBP-QLandallowjoinsonlyonlabelvariables.
Distributed systems and queries. Sofar, wehaveignored Figure11:BPELXML.
distribution. Inadistributedsetting,eachpeerholdsasetBPsand
4.1 DesignConsiderations
may provides (resp. use) activities to (of) remote peers. If the
serviceprovidersmaketheirspecificationavailabletotheircoop- BP-QL is based on an intuitive, conceptual model of BPs, an
erating organizations (say via a web service), users may wish to abstractionoftheBPELspecification,allowingforsimpleformu-
zoom-inontheseremotecomponentsaswelltoquerytheservice lation of queries. over this model. When we considered the im-
specification. plementation,thefollowingproblemhadtobeaddressed:Asmen-
The data model extends naturally to this setting, associating a tionedinSection1,theBPELXMLformatwasdesignedwithease
peeridwitheachprocessandeachactivitynode.Queriesmaythen ofautomaticcodegeneration,ratherthanquerying,inmind.Activ-
annotategraphpatternsandactivitynodesbypeerids, restricting itiesandedgesaredefinedseparately,asdistinctactivityandlink
thesearchtothespecifiedpeers. Inparticular,whena(transitive) elements. The process flow is only recorded by associating with
activity node in a query is annotated by a peer id, the search is eachactivityelementtheidsofitsincomingandoutgoingedges,
restrictedtoimplementationssuppliedbythespecifiedpeer(resp. representedresp.bytargetandsourcechildrenofthenode.Thisis
refinementsconsistingonlyofinvocationsofactivitiesofthespec- illustratedinFigure11,whichshowstheBPELXMLrepresenta-
ifiedpeer).Moregenerally,queriesmayusepredicatesonpeerids tion6oftheTravelAgencybusinessprocessfromFigure1.Conse-
torestrictthesearchtoaspecificfamilyofpeers. quently,tocheckwhetherflowpathsofagivenprocesssatisfythe
Remark: While the extension of the formal model to a distrib- conditionsdetailedinaBP-QLquery,alarge,possiblyunbounded,
utedsettingisratherimmediate,implementation-wise,distribution numberofjoinoperationsinvolvingedgeidsbetweenactivityand
posessignificantchallengesintermsofqueryevaluation. Specifi- edgeelementsneedstobeperformed. Whilethisisexpressiblein,
cally,wewouldliketoevaluateaqueryina“lazy”manner,sothat say,XQuery,e.g.withtheuseofrecursivefunctions,theexcessive
onlythosepeerswhoseprocessesandactivitiesareindeedrelevant numberofjoinsbecomesaperformancebottleneck.
tothequeryareconsulted. Furthermore, itisdesirableto“push” Todrasticallyreducethenumberofjoins,wedecidedtostorea
partsofthequery,whenpossible,tothepeersholdingtherelevant processspecificationinastructuremoresimilartoitsgraphview.
process information. Our implementation, described in the next In XML terms, the parent child relationships in the XML repre-
section,addressestheseissues. sentationofaprocessshouldreflectthe“followedby”relationship
ofnodesintheprocessgraph.ThiswouldallowtheuseofXPath’s
Summary. ThedesignofBP-QLwasdirectedbythespecialre-
”/”and”//”operatorsforqueryingflowpaths,avoidingmanyjoins.
quirementofqueryingspecificationswithazoom-infeatureatdif-
But,sinceatypicalBPisagraph,ratherthanatree,wealsouse
ferentlevelsofgranularityandtheretrievalofqualifyingexecution
XMLidrefstocapturethegraphstructure.
paths. As explained above, this required a careful design of the
Anotherfundamentaldecisiontobemadewaswhichofthefol-
languagetoavoidfeaturesthatmightseemtobeworthyofinclu-
lowingtwooptionstochoose:(1)toimplementawholenewquery
sion in the language, such as joins on path variables, but incur a
engineforourmodelfromscratch,or(2)torelyonsomeexisting
prohibitivelyhighcomputationalcost.
queryenginetoperformasmuchaspossiblefromthecomputation,
ThecharacterizationoftheexactexpressivepowerofBP-QLis
andcompletetheprocessingofthemissingfeaturesbyanadequate
anon-goingresearch.OurinitialresultsindicatethatBP-QLcanbe
pre and post processing of queries and query results. We opted
characterizedasaparticularsubclassofFO(TC)5.Inparticular,for
forthesecondoption. Theissuestobeconsideredinselectingan
flatBPs BP-QLcaptures powersimilar tothat ofthe conjunctive
enginewerethefollowing:
partofXPathandcoreXQuery,includingnegation,whenconsid-
• Ourquerylanguageallowstoretrievepaths,whereastypical
eredinthecontextofgraphs. Duetospacelimitationsthisisnot
existingXML/graphquerylanguagesonlyretrievenodes.
presentedhere.
• Ourquerylanguageoffersazoom-infacility.
• Businessprocessestypicallyoperateinacross-organization,
4. IMPLEMENTATION
distributed environment. The specifications of the services
Thequerylanguagepresentedabovehasbeenfullyimplemented participating in process may reside on distinct peers. Dis-
andtestedintheBP-QLpeer-to-peersystem.Thesystemprovides tributedqueryprocessingthusbecomesessential.
persistentstorageforBPELspecifications, allowsuserstodesign A natural candidate was to use a standard XQuery engine, en-
newprocesses,andtoqueryexistingspecifications. joying the benefits of indexing and optimization offered by such
ThevisualinterfaceofthesystemisimplementedasanEclipse engines.7 However,XQuerydoesnotsupporttheretrievalofpaths,
[20]plug-in. Itallowsto:designnewbusinessprocessesandstore distribution,orzoom-inqueries;nordoesit“traverse”idrefs.Nec-
theirspecificationsintherepository; importexistingBPELdocu- essarily,allofthesewouldhavetobeimplementedbypreandpost
mentstotherepository;formulatequeries,runthemandviewthe
6For simplicity, the figure provides an abstraction of the actual
results. Therestofthesectionisdevotedtothemaincomponent
BPELXMLfilestructure,withmanydetailsomitted.
—thequeryengine.
7AnalternativeviablesolutiontothegraphshapeofBPscouldbe
5FirstOrderLogicaugmentedwithTransitiveClosure. touseanativegraphqueryengine.
processing. Consequently, wedecidedtobaseoursolutiononan
extensionofXML,calledActiveXML(AXMLforshort).AXML
is essentially a middleware system that includes an XQuery-like
querylanguage,butoffersadditionalfacilitieswhichprovidebetter
supportforaddressingsomeoftheaboveissues. Additionalben-
efitsincludecertainoptimizationtechniquesthatareimplemented
intheAXMLsystem,asexplainedbelow.
AbriefoverviewofActiveXML. ActiveXML(AXML,for
short)isadeclarativeframeworkthatharnessesWebservicesfor
data integration, and works in a peer-to-peer architecture[3]. An
AXMLdocumentisanXMLdocumentwheresomedataisgiven
extensionally,asregularXMLelements,whileotherdataisgiven
intensionally,bymeansofcallstoWebservices[3],andcanbema-
terializedbyinvokingtheservices.AXMLemploysthequerylan-
guageXOQL,anXQuery-likequerylanguageasitsqueryengine.
Figure12:AXMLtree.
WhenaqueryisevaluatedonanAXMLdocument,theservicecalls
whose answer may be relevant for the query are identified; only Thusqueryevaluationcanaccessthereturnedsubtreeasifit
these calls are invoked. Additionally, (sub-)queries are pushed, actuallytraversedthe“pointer”.
whenpossible,totheserviceproviders,thusreducingthecostsof • A getOperation service call retrieves the specification
datamaterializationandtransfer. Recursivecallsaretracked,and ofaremotecompoundactivityandconvertsit,whenneeded,
onlytherelevantdataismaterialized(see[2]fordetails). fromBPELformattoanAXMLrepresentation.Azoom-out
elementisattachedtoitsfinalstate, sothatitpointstothe
Insummary,BP-QLusestheAXMLsystem[3]asanimplemen-
followingactivityintheflow.
tationplatform.ThefacilitiesofferedbyAXMLareusedtoaddress
our needs, as follows: Intentional data, implemented by service Toillustratethefirsttypeofcall,thegetActivity("join")
calls,areusedinourimplementationto(1)retrieve,whenneeded, nodes below the searchFlights and searchRooms, in the
thespecificationsofremoteprocesses,thussupportingdistributed middleofFigure12,pointtothejoinnodebelowsearchCars.
processing, and (2) account for the graph structure of the speci- Theyrepresentthefactthatthethreesearchesarefollowedbythat
fication (service calls play here role similar to XML idrefs, with samejoinoperation.8
theadvantagethattheyaretraversedautomaticallyinqueryevalu- ThegetOperation("searchRooms,"join")inthefig-
ation). BPELdocumentsarewrappedandrepresentedasAXML ureillustratesthesecondtypeofcall. Itretrievesthespecification
documents;BP-QLqueriesarepre-processedandcompiledintoa ofthesearchRoomsprocess,andsetitszoom-outtothefollowing
setofXQuery-likequeriesoversuchdocuments. Postprocessing “join” operation. Here again, AXML invokes getOperation
isemployedtocompletethecomputation,e.g. tovalidatezoom-in calls for the remote activities whose specification is judged to be
relationships,toextractpathsandtoconstructacompactrepresen- relevant for query evaluation. As mentioned above, it may also
tationfortheresult. “push”(sub-)queriestocapableserviceproviders,suchasBP-QL
peersthat“understand”BP-QLqueries.
From BP-QL to AXML. Here is a brief description of the DataelementsanddataflowarerepresentedinAXMLtreeina
AXML representation of a BP-QL business process. The repre- similar manner: The tree contains both data and activity element
sentation consists of three parts: Process properties (such as the nodes. getData and getActivity service calls are used as
serviceprovider,theservicetypeandcapabilities)aremaintained “references”betweentreedataandactivitynodes,resp.
as UDDI entries in a (standard) XML document. The other two, TogeneratetheAXMLrepresentation,theBP-QLgraphistra-
namelytheprocessactivitiesandexecutionflow,andthedataele- versedinadepth-firstorder,buildingAXMLtreesasdeepaspossi-
mentsandthedataflow,aremaintainedintwoAXMLdocuments. ble.Localcompoundactivitiesarethenzoomed-inandtheirgraphs
TheuseoftwoAXMLtrees, ratherthanone, allowsforefficient aresimilarlydetailed, recursively. Requeststoremoteoperations
evaluationofBP-QLquerieswithdoubleheadededges: itallows are represented by getOperation service calls. Web services
adoublyheadedactivity(resp. data)flowedgetobemappedtoa aregeneratedforprovidedoperations,exposingtheirspecification
“//”operatoronthecorrespondingAXMLdocument. totherequestingpeers.
Forexample,Figure12describes(partof)theAXMLtreeforthe With this representation, both the path-based and the zoom-in
Alpha-Toursactivitiesandflow.(Hereagain,forsimplicity,onlyan axisconditionscanbeevaluatedusingXOQLqueriesontheAXML
abstractionoftheactualAXMLtreeisprovided,withmanydetails documents.Somepostprocessingisneverthelessrequiredtomatch
omitted.) EachactivityisrepresentedbyanXMLelementnodein upthecomponents,extracttherequestedpaths(XOQL,likemost
thetree. Theparentchildrelationshipsreflecttheflow. Eachnode XQueryengines, returnsonlydocumentelementsnotpaths), and
representingacompoundactivityistheroot(labeledbyzoom-in)of constructacompactrepresentationoftheresult. Weomitthede-
asubtreethatdescribestheinternalstructureoftheactivity.Nodes tailsforspaceconstraints.
with bold labels are special elements that represent calls to Web
4.2 Trade-offs
services.Twotypesofsuchcallsareembeddedinthedocument:
Asexplainedabove,wehavedecidedtostoreBPsinastructure
• A getActivity service call plays a role similar to that of an
closetotheBPgraphshape,ratherthanintheBPELformat.Obvi-
XML idref, “pointing” to acertain node inthe tree. When
ously,thisreducesthenumberofjoinoperationsrequiredinquery
a query is evaluated, the relevant calls are detected and in-
voked. (Cycles are detected and cut by AXML). For each 8In the implementation, the input to the getActivity is a
uniqueidentifierforthepointedactivity,consistingofBPandac-
call, thereturneddata(acopyofthesub-tree“pointedto”)
tivityids.Itisabstractedhere,forbrevity,bytheactivityname.
isinsertedinplaceoftheservicecall,readytobeaccessed.