Table Of ContentThe WebContent XML Store ∗
BenjaminNguyen SpyrosZoupanos
PRISM–UVSQ ProjectLEO–INRIASaclay-ˆIle-de-France
ProjectSMIS–INRIA UniversityofParis-Dauphine
Abstract 2 Storage services
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Inthisarticle,wedescribetheXMLstoragesystemusedin CentralizedStore: Forstorageonasinglemachine,we
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0 theWebContentproject.Webeginbyadvocatingtheuseof canuseeitherMonetDBorMSSQLServer. Inbothcases,
2 anXMLdatabaseinordertostoreWebContentdocuments, theWebContentdocumentsarestoredintheirnativeXML
andwepresenttwodifferentwaysofstoringandquerying format,andcanbequeriedviaXQuery.Anissuewithsuch
n
a these documents : the use of a centralized XML database queriesisthattheymayreturnresultsofany(XML)type.
J andtheuseofaP2PXMLdatabase. Therefore,wehavedevelopedaspecificWebContentquery
7 interfacethatonlyallowsqueriesreturningWebContentre-
2 1 Context sources,whichmaybeplacedinthewarehouse.
] Overview: The WebContent platform 1 proposes a spe- P2P storage service: The P2P storage service is imple-
B
cificUMLschematobeusedbyallitsservices. Through mented jointly by several peers, so that the exact location
D
a canonical transformation, this schema can be converted ofapieceofdataistransparenttotheuser.TheP2Pstorage
.
s intoanXMLSchema. Thisisextremelyusefullsincethe servicealsosupportsindexingfacilities. ADHTserviceis
c WebServicesparadigmusesXMLdocumentstocommu- implemented on top of a distributed hash table (or DHT,
[
nicate with each other. It seems therefore straightforward forshort[4]). TheDHT,adistributedsoftwarerunningon
1 to manage all the documents inside the WebContent plat- all peers, provides the connectivity of the network. It as-
v
forminXMLformat,whichwillpresentadvantageswhen signsuniqueidentifierstopeersandallowsthemtoeasily
0
storingthemandqueryingthem.Inthisarticle,wedescribe join and leave the network3. Indexing is supported using
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8 two ways of managing the storage and querying of such adistributeddatastructurebasedonthesimpleabstraction
4 documents, by using a centralized and distributed (P2P) of(key,value)pairs(withtwoservices,namelyput(k,v)and
. XML database. These storage-service modules conform get(k)).Withoutdelvingintothedetails,theDHTstoresall
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0 totheWebContentinterfaceforstorage. Themainreason values associated to a given key k, on a particular peer in
0 for chosing to use XML databases over a simple file stor- chargeofthatkey.
1 age format is twofold : performance and expressivity of DifferentDHTsmayhavedifferentalgorithmicproperties,
:
v queries, since as we will see, it is possible to express any interestingfromdifferentperformanceviewpoints. Forin-
Xi sortofXQueryonaWebContentdocument. stance, a DHT may guarantee that two keys k1 and k2,
“close” by some distance measure, are managed by peers
r WebContentStorageServicesInterface: Theplatform
a that are “close” in some sense. To take advantage of the
definesaninterfaceforastorageserviceandconsequently
good properties of distinct DHTs, several DHTs may co-
a query service, to access the data that is stored. These
exist in a WebContent deployment architecture. Thus, a
interfaces are generic. To illustrate their flexibility, we
peer p belonging to the DHTs dht ,dht ,... is an end-
have provided two implementations [6, 1]. The first one 1 2
pointfortheservicesjoin ,leave ,put ,get ,butalsofor
provides storage and querying on top of existing single- 1 1 1 1
join ,leave ,put ,get etc. We have successfully inte-
site (centralized) XML database servers using an existing 2 2 2 2
gratedsofartwoDHTs[1]: FreePastry[7]fromMIT,in-
XMLQueryengine: MonetDB2.Second,wehaveimple-
cludingourownextensionsforrobustscalableXMLindex-
mentedaresourcestoredistributedoverthenetworkpeers,
ing [2]; and PathFinder [5], specially tuned to support in-
and similarly a query service implemented jointly by all
tervalsearchqueries(whichFreePastrydoesnotsupport).
thepeersinthenetwork. Westressthatmovingfromone
The Active XML 4 engine is responsible to interact with
implementationofthestorageservicetoanotheristotally
theavailableDHTssincetheirpresenceandqueryprocess-
transparenttotheuser,andsimilarlyforthequeryservice.
ingperformedbyeachofthemshouldbetransparenttothe
user.
∗This research was supported by the French National Research
Agency(ANR)throughtheRNTLprogram,andtheSystem@ticParis-
Re´gioncluster. 3Rememberthatinahybridarchitecture,allparticipantsneednotbe
1http://www.webcontent-project.org/ partoftheP2Pnetwork.
2http://monetdb.cwi.nl/XQuery 4http://www.activexml.net
1
3 Query services
Peer p1
XQuery: WebContentresourceexploitationreliesonad- P2Pquery(SPARQL) SomeRDFS
vanced query processing capabilities. To this end, we
use XML query services. In its centralized (one-site) im- P2Pquery(XQuery) OptimAX TGV MonetDB query(XQuery)
plementation, an XML query service takes as input an
Tree pattern Tree pattern
XQuery 5, and returns its results as evaluated by the un- query processor 1 query processor 2
index/lookup index/lookup
derlyingXMLDB.Observethatinthiscontext, itisonly index1(res) module DHT1 module DHT2 index2(res)
meaningfultosolicitthequeryserviceonthemachinethat join1 join2
leave1
leave2
stores the queried document(s). XQuery is an extremely
DHT1
powerful language, and it is possible to write many com-
plex queries in particular to restructure or perform joins DHT2
onthedocuments. WhiletheWebContentinterfaceallows p5
p2
such queries to be written, the main functionality of the p3 p4
store is to provide access to any resource that is stored in
thedatabase. Recallthataresourcecanbeanythingfrom
a document to one of its atomic resources such as a para- Figure1: OutlineofP2Pservices.
graph. SuchqueriesaremuchsimplerthanXQueries,and
are implemented in the centralized service using an index
onalltheresourceelements. Itisthereforepossibletofind
therecompositionquery.
intimeO(1)anyresourcestoredinthedatabase(serializa-
Finaly,eachpeeriscapableofprecessingsemanticqueries
tioncostisofcoursefunctionofthesizeoftheresource).
over RDF data, expressed in a conjunctive subset of the
P2P Query engine: The implementation of our P2P SPARQL6language.
XML query service is more evolved. This service is pro-
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