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186 KNOWLED GE AND DATA MANAGEME NT IN G RIDS1. Introduction

For the Semantic Grid vision [15] to become a reality, high quality of ser-vice must be offered to users and applications at all levels of the Grid fabric.In this paper, we concentrate on high quality of service in the provision ofresource discovery services in Semantic Grids. Resource discovery is an im-portant problem in Grids in general, and Semantic Grids in particular. Wediscuss how to achieve high-performance, scalability, resilience to failures,robustness and adaptivity in the provision of resource discovery services inSemantic Grids, and especially in OntoKit, the Semantic Grid toolkit currentlyunder development in project OntoGrid [24].

OntoGrid ( h t t p ://www o n t o g r i d . n e t ) is a Semantic Grid projectfunded by the Grid Technologies unit of the European Commission under thestrategic objective "Grid-based systems for Complex Problem Solving" of theInformation Society Technologies programme of FP6.Our basic assumption in this paper is that Semantic Grid resources (e.g., ma-chines, services or ontologies) will be annotated by RDF(S) metadata. Meta-data pervades the Semantic Grid and is used to describe Grid resources, theenvironment, provenance and trust information etc. [15]. The Resource De-scription Framework (RDF) and RDF Schema (RDFS) are frameworks for rep-resenting information about Web resources. RDF(S) consists of W3C recom-mendations that enable the encoding, exchange and reuse of structured meta-data, providing the means for publishing both human-readable and machine-processable information and vocabularies for semantically describing thingson the Web. Although RDF(S) was originally proposed in the context of theSemantic Web, it is also a very natural framework for representing informationabout Grid resources. As a result, it is used heavily in various Semantic Gridprojects e.g., mYGrid ( h t t p ://www m y gr i d. o r g .uk) or OntoGrid.We propose to view resource discovery in Semantic Grids as distributedRDF query answering on top of a P2P network of Grid resource providers andrequesters. Our proposal complements well-known Grid information servicessuch as MDS4 of GT4 in two ways:

We offer service providers and service requesters expressive semantics-based data models and query languages (i.e., RDF(S) and RQL insteadof XML and XPath).

We implement resource discovery using techniques from P2P systems.This allows us to achieve ull distribution, high-performance, scalability,resilience to failures, robustness and adaptivity. Related experimentalwork is presented in [26, 28,271.


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Semantic Grid Resource Discovery in Atlas 187In the context o f OntoGrid, our proposal is realized with the implementationof Atlas, a P2P system for the distributed storage and querying of RDF(S)metadata describing Semantic Grid resources.The rest of the paper is organized as follows. Section 6 briefly discussesrelated work at the crossroads of Grid and P2P computing research. Section3 gives a sh ort description of the various components and protocols of Atlas.Section 4 shows how to use Atlas for service discovery in OntoKit. Finally,Section 5 concludes the paper.

2. Related WorkOur research can be understood to lie at the intersection of P2P and Grid

computing. Although these computing paradigms have different origins andhave been developed largely independently, there has been a lot of interestingwork lately at the crossroads of these paradigm s [13, 34 , 111.Previous papers that explore connections among Grids and P2P networkscan be distinguished in the following categories:1 General papers that discuss the similarities and differences of P2P andGrid systems pointing out important areas where more work is needed[13,34, 111.2 Papers where ideas from P2P computing are used in Grid systems. Here,we can further differentiate as follows:

(a) Works where G rid computing problems are given as a primary mo-tivation, but the contributions are essentially in the P2P domainand can also be applied elsewhere. For example, [4, 23, 71 con-sider attribute-value data models that can be used to describe G ridresources (e.g., by specifying the CPU power, disk space capac-ity, operating system and location of a computer) and show how toevaluate queries in these models on top of DH Ts (e.g., I am lookingfor an idle PC that runs Linux and has CPU > 3GHz).(b) Works where P2P techniques are used to improve functionality inexisting Grid systems e.g., resource discovery [20, 18 , 191 andreplica location management in Globus [8] or flocking in Condor

[61.(c) Service-oriented application development frameworks that en-

hance existing frameworks for Web or Grid service computing [ I ,161 w ith P2P protocols.3 Papers where ideas from Grid computing are used in P2P systems. Forexample, [ lo ] shows how to implement a P2P data integration frame-work using OGSA-DAI [2].


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188 KNOWLEDG E AND DATA MANAGEMENT IN G RIDSOur work should be classified in categories 2(b) and 2(c) above. Work withgoals similar to ours that uses description logics instead of RDF(S) is reportedin [17].

3. The P2P System AtlasIn Atlas, we use state of the art distributed hash table (DHT) technology[5] to implement a distributed system that will be able to scale to hundreds ofthousands of nodes and to large amounts of RDF(S) data and queries. Nodes in

an Atlas network are organized under the Bamboo DHT protocol [3 11. Bam-boo is a DHT based on Pastry [32] from where it takes the circular identifierspace and the routing algorithms. Bamboo improves on Pastry by being able towithstand very dynamic changes in network membership i.e., it is resilient tochurn [3 11. Like most implementations of DHTs, Bamboo offers a very sim-ple interface consisting of two operations: put ( ID, it em ) and get ( I D ) .The put operation inserts an item with key I D and value item in the DHT.The get operation returns a pointer to the DHT node responsible for key I D .Our operations for storing data and querying Atlas, described below, are basedon these simple operations offered by Bamboo.Atlas nodes can enter RDF(S) data into the network and pose RQL queries.Two kinds of querying functionality are supported by Atlas: one-time queryingandpublish/subscribe. Each time a node poses a one-time query, the networknodes cooperate to find RDF(S) data that form the answer to the query. In thepublish/subscribe scenario, a node can subscribe with a continuous query. Acontinuous query is indexed somewhere in the network and each time matchingRDF(S) data is published, nodes cooperate to notlfi the subscriber.The current implementation of Atlas (Atlas v0.6) supports a subset of thequery language RQL [22] as we explain in Section 3.4 below. The queryprocessing algorithm we use for one-time queries is an extension of the algo-rithm proposed in [9] for a smaller class of queries based on triple patterns[9]. Publishlsubscribe scenarios in Atlas are handled using the algorithms in[28,27] that are briefly discussed in Section 3.3 below but have not been fullyimplemented in Atlas v0.6. In the future, Atlas will also support the recentlyproposed RDF update language RUL for inserting, deleting and updating RDFmetadata [30].Atlas is used in OntoKit for realizing a fully distributed metadata service.A high level view of Atlas and the metadata service of OntoKit is shown inFigure 1.3.1 RDF Documents and Queries in Atlas

Atlas nodes provide their data in the form of RDF documents [25]. Thesedocuments are decomposed into RDF triples that are indexed in various nodes


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Semantic Grid Resource Discovery in Atlas 189

Lamboonetworlc layer

\ /QueryMetadata(R0LQuery) UpdateMetadata(RUL0peranon)-...---..--..--.-..

Metadata Service

Figure I . Atlas and the metadata serviceof the network. A triple represents a statement about a domain and has theform (subject,predicate, object)where subject and predicate are URIs andobject is a URI or a literal. We adopt the triple indexing algorithm presented in[9], where each triple is indexed on the DHT three times, once for its subject,once for its predicate and once for its object. For each of these storage opera-tions we make use of the pu t operation provided by the Bamboo DHT usingas key the subject, predicate or object value respectively. The key is hashed tocreate the identifier that leads to the appropriate node where the triple is stored.Atlas supports internally the query language TPQL (triple-pattern querylanguage) which allows the expression ofpositive (i.e., without negation) con-junctive queries where each conjunct is a triple pattern.A conjunctive query q is a formula of the form

where s l , . . . ,s,, pl, . . . ,p, are variables or URIs, 0 1 , . . . ,om are vari-ables, URIs or literals, ? X I , . . ,?xk are variables and {?x l , . , ? x k ) Ci s 1 , . . ,Sm,pl , . . . ,p,, 0 1 , . . . ,om). Variables will always start with the '?'character. The triple patterns (s l pl ,o l ) , . . ,(s,, p,, 0,) are the subqueriesof q. A query will be called atomic if it consists of a single conjunct.

The class of conjunctive queries can be used to express many interestingrequests in P2P applications using RDF. For example, assume that a service


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190 KNOW LEDGE A ND DATA MANAGEMENT IN GRIDSrequester wants to discover a Web service for arranging the repair of a car.This request c an be expressed as a conjunctive query as follows:

? x , ? y : ( ? x , h a s S e rv i c eK e y w o r d , " C a r s " ) A

3.2 One-Time Query Processing in AtlasIn this section, we describe the algorithm for one-time query processingin Atlas using termino logy from relational databases. Each triple can be un-derstood to be a tuple in a relation T R I P L E ( S , P,0 )with attributes S for

subject, P for predicate and 0 for object. Then , conjunctive queries areselect-project-join queries over the database that consists simply of the rela-tion T R I P L E . The exact query processing algorithm of Atlas is as follows.Let n l be a node that wants to pose a conjunctive query q of the form intro-duced in Section 3.1. Node n l creates a message

p a r t i a l R e s u l t , v a r ia b l e s, r e t u r n A d d r e s s )and sen ds it to the node with identifier i d using the underlying Bamboo infras-tructure. In this message, tr i p l e p a t t e r n is the triple pattern of q which noden l chooses to be evaluated first', i d is the identifier obtained by hashing one ofthe constants in triple pattern t r ip le p at te rn , r e st T r i p le P a t t e rn s is the lis tof remaining triple patterns of q, p a rt ia lR e s u lt is a relation for partial results(see below) wh ich is initially empty, va ri ab le s is the list of answer variablesof q, and re tu rn A d d re ss is the IP address of node n l .When another node n2 receives the above message qu er y R eq u es t, it doesthe following. It first com putes the bindings of the variables included inthe given triple pattern by finding the triples in its local database that matcht r i p l e p a t t e r n . These bindings form a new relation R with attributes thevariables in question. If p a rt ia lR e s u lt is empty, then node n2 assignsR to p ar ti al R e su lt . Otherwise, n2 computes the natural join of R andpa r t i a l R e su l t (i.e., pa r t i a lR esu l t M R) and assigns it to partialResultt .Then, n2 creates a new message

pa r t i a l R esul t', va r i ab le s , r e tu rnA ddre s s )

his choice is crucial depending on the metric one wants to optimize; in Atlas v0.6, we simply pick thefirst triple pattedconjunct.


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Semantic Grid Resource Discovery in Atlas 191When this message is received by another node ns, the same procedure isfollowed. These nodes join the relation R of the bindings they retrieve locallywith the relation p a rt ia lR e s u lt and send a message to the next node. This pro-

cedure terminates in two possible ways. Either, the list r e s t T r i p l e P a t t e r n sbecomes empty or the relation p ar ti al R es u lt becomes empty. The lattermeans that the current triple pattern does not match with any triples storedlocally, and thus relation R becomes em pty and the join operation results in anem pty relation. In both cases, a response with the results should be returnedto node n l which issued the query. The field re tu rn A d d re s s is used for thispurpose; it remains unchanged throughout the whole procedure and refers tothe IP address of node n l .The node n, that determines that the query evaluation procedure is fin-ished computes the bindings of the answer variables ?xl,.. ,?xk. In or-der to do that, n, computes the projection of relation p a r t ia l R e s u l t on thevariables included in the list variables and inserts the results in the relationva r i ab leB in d ing s i.e .,

Then, n, sends a response message queryResponse(variab1eBindings) onode n l , where v ar ia b le B in d in g s is a relation with the answer to the query.The key idea in the algorithm we described above is that we split a conjunc-tive query to the triple patterns that is consists of and evaluate each one at adifferent node of the network. In this way, we try to distribute the responsi-bility of answ ering a query to several nodes. Intermediate results flow throughthese nodes and finally the last one delivers the results back to the node thatsubmitted the query. N otice that in order to determine which node will evaluatea triple pattern the algorithm uses one of the constants contained in it. Finally,the distributed query plan is created once, i.e., at the time that the query issubmitted.In [ 2 6 ] ,we propose an improved algorithm for the evaluation of conjunctiveRDF queries on top of DHTs. In this algorithm, the distributed query plan iscreated dynamically by exploiting the values of matching triples found whileprocessing the query incrementally. This time we use com bination of constantsin a triple pattern to determine which will be the node to evaluate it. By enrich-ing the triple patterns w ith new values we have more com binations to use. Inthis way, this algorithm distributes the responsibility of evaluating a query tomore nodes than the previous one. Our initial experiments show a significantimprovement o n load distribution but, on the other hand, there is an overheadin network traffic.


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192 KNOWLEDGE AND DATA MANAGEMENT IN GRIDS3.3 Publish/Subscribe in Atlas

In [28, 271, we propose two distributed algorithms for pu bl ish hb sc rib e ontop of DHT s when publications are RDF triples and subscriptions are conjunc-tive multi-predicate queries.In ou r algorithms, when a continuous query is submitted, it is indexed some-where in the network and waits for triples to satisfy it. Each tim e a new triple isinserted, the network nodes cooperate to determine what queries are satisfied,compute their an swers and create notifications for the subscribers. The caseof conjunctive queries is an interesting one, since a single triple may satisfj, aquery q on lypa rtia lly by satisfying a subquery of q. In other words, more thanone triples m ay be needed to answer a query. M oreover, since the appropriatetriples do not necessarily arrive in the network at the same time, the networkshould "remember" the queries that have been partially satisfied in the past(e.g., by keeping intermediate results) and create notifications only when allsubqueries of a given query are satisfied.We could index queries to a globally known node or set of nodes, but thiswould eventually overload these nodes. In a P2P environment, we want asmany nodes as possible to contribute some of their resources (storage, cpu,bandwidth, etc.) for achieving the overall network functionality. The resourcecontribution of each node w ill obviously depend on its capabilities, its gainsfrom participating in the network, etc. In our work, we m ake the simplifyingassum ption that all nodes are altruistic, with equivalent capabilities, and, thus,can con tribute to query evaluation in identical ways.Let us now discuss the issues involved in pu bl ish hb sc rib e with conjunctivequeries. We first consider an atomic query q = ( ? s l , p l , ?o l). We can simplyassign q to the successor node x of H a s h (p 1 ) by using the constant part p l ofthe query. Triples that have predicate value equal to p l will be indexed to xtoo, where they will meet q. Assume now the atomic query q' = ( ? s2 ,pa , 02).We can index q' either to node x l = Su cce sso r (H ash (p2 ) ) or to nodex2 = S u cc es so r( H as h (0 2 )) . We prefer the second option since intuitivelythere will be m ore object values than predicate values in an instance of a givenschem a, which will allow us to distribute queries to a greater number of nodes.Another solu tion is to index q' to the node 23 = S u c c e s s o r ( H a s h ( p 2 + 02)).We use the operator + o denote the concatenation of string values. This is thebest option because the possible combinations of predicate and object valueswill be greater than the num ber of object values alone, so this will lead to aneven better distribution of queries.Th e difficulty with arbitrary conjunctive queries is that they demand morethan one conditions to be satisfied before the whole query can be satisfied.As an example, consider the query q = ql A q2 A q3. Our approach is tospli t the query to the subqueries that it consists of, and to index each subquery


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Semantic Grid Resource Discovery in Atlas 193separately. Then, three usually different nodes will be responsible for queryprocessing regarding q . Each one will be responsible for a single subquery ofq, e.g., nodes r l , r 2 and r3 will be responsible for ql, 92 and q3 respectively.These nodes will form the query chain of q, denoted by chain(q). Each oneof these nodes will monitor the satisfaction of only the subquery that it is re-sponsible for. To determine the satisfaction of q, we have to allow some kindof com munication between these three nodes. In this way, as triples arrive andsatisfy a subquery e.g., in node rl, rl will forward partial results of q to r2.Node r 2 will forw ard partial results that also satisfy the second subquery to r3and r3 will realize tha t the whole query is satisfied and create a notification.The first algorithm that we present in [28] creates a single query chain foreach conjunctive query while the second one creates multiple query chainsfor a single query to achieve a better query processing load distribution. Thefirst algorithm of [28] is essentially identical to the one-time query processingalgorithm discussed in Section 3.2 except that, in the publish/subscribe case,it is executed in a reactive manner as matching triples arrive in the network.In [28], the two algorithms presented are experimentally evaluated for con-junctive multi-predicate queries (i.e., queries where the sub ject of all the triplepatterns is the same variable ?s and predicates pl, . . . , , are all constant).How ever, the general idea of these algorithms is easily extensible to supportthe full class of conjuctive queries as we show in the forthcoming paper [27].3.4 The RQL-to-TPQL Translator

Atlas offers to users the ability to write queries in TPQL or in the well-known RDF query language RQL. RQL [22], which stands for RDF QueryLanguage, is a declarative language which relies on a formal graph m odel thatcaptures the RD F m odelling primitives. The novelty of RQ L lies in its ability tocombine schema and data querying sm oothly while exploiting the taxonomiesof labels and multiple classification of resources. The syntax of RQ L includesa set of basic queries (e.g. Resource, SubClassOf ) etc.) as well asSQL-like select f rom-where ueries to iterate over RD F collections andintroduce variables2 .Consider the schema of Figure 2 which describes information about Webservices in R DF S. This exam ple is part of the core services data model usedin project , y ~ r i d ~ . uppose we want to find a Web service for arranging therepair of the car. W hat follows is an appropriate RQL query:SELECT XFROM { ~ } n s : h a s ~ e r v i c e ~ e s c r i p t i o n { ~ }

'RQL is implemented in ICS-FORTH's Suite h t t p : //139.91.183.30: YOYO/RDF/' ttp://www.mygrid.org.uk


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1 9 4 K N O W L E D G E A N D D A TA M A N A G E M E N T I N G R I D S

h a s l n p u V O u l p u l

ns:hl tp: / /www,mygr id.org,uWontology#h a s S e r v i c e N a m e ~ hsOpe'r@ti_on I

g W S D L - o p e t a l i o n

~ ~ ' i \ ~ " / ~ - - - - - - ~ h a s S er vi ce N a m e \ ,& Re pairCar-serv iee

" C a r R e p a i r S e r v ic e r

Figure 2.

o,a,,L i t e r a l

RDFS schema for W eb Services

Resourcei~ Property

. . . . . ~ I ns t anceO f

W H E R E Y l i k e " * c a r * "U S I N G N A M E S P A C E n s = & h t t p :/ /w ww . m y g r i d , o rg . u k / # o n t o l o g y

I n o r d e r t o s u p p o r t RQ L q u e r i e s i n A t l a s , w e h a v e i n t r o d u c e d a m o d u l er e s p o n s ib l e f o r m a p p i n g a q u e r y e x p r e ss e d in R Q L to a q u e r y i n T P Q L , w h i c hi s t h e q u e r y l a n g u a g e s u p p o r t e d i n t e r n a l ly b y A t la s a n d d e s c r i b e d i n S e c t i o n3 .1 . I n A t l a s v 0 .6 , w e d o n o t s u p p o r t th e f u ll f u n c t i o n a l i ty o f RQ L b u t o n l ydata que ries with f i l ter ing conditions.Re c a l l t h e R Q L q u e r y p r e s e n t e d e a r li e r, a b o u t th e d i s c o v e r y o f s e r v ic e f o ra r r a n g i n g t h e r e p a i r o f t h e c ar. T h e e q u i v a le n t c o n j u n c t i v e q u e r y i s t h e f o l lo w -ing:

?x : (?x, http ://ww w.myg rid.org.uk/ontology~hasServiceDe scription, ?y)A ?y like " * car * "

T o d e s i g n t h e R Q L - t o - T P Q L t r a n s l a t o r w e h a v e f o l l o w e d t h e R Q L I n t e r -p r e t e r a r c h i t e c t u r e d e v e l o p e d b y I CS - F O RT H [ 1 4] ( s e e F i g u r e 3). O u r im p l e -m e n t a t i o n h a s b e e n d o n e i n J a v a u s i n g t h e J a v a Co m p i l e r Co m p i l e r ( J a v a CC)[3] p a r se r gene ra to r .

T h e syntax analyser m o d u l e r e c e iv e s as i n p u t a s tr in g , r e p r e s e n t i n g a n R Q Lq u e r y , a n d r e t u r n s t h e c o r r e s p o n d i n g CN F s y n t a x t re e ( i f t h e q u e r y i s va li d ).T h e s y n t a x t r e e i s p a s s e d t o t h e graph constructor m o d u l e , w h i c h c r e a t e s ag r a p h c o r r e s p o n d i n g to t h e s e m a n t i c r e p r e s e n t a t i o n o f t h e q u e ry . T h e s e t w om o d u l e s a re b a s ed o n t h e c o d e o f R Q L I n t e rp r et er . T h e translator m o d u l et a k e s a s i n p u t t h e s y n t a x t r e e an d g r a p h o f a n RQ L q u e r y a n d r e t u r n s t h e


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Semantic Grid Resource Discovery in Atlas

Mappingiuncfions

RO L4 Tr

equivalent expression in TPQL, as a list of triple patterns and constraints. Itconsists of a list of mapping functions, which implement the mapping rulesbetween RQL and TPQL presented in [2 11. The main module contains either aJNI-client and a standalone application for the management of the RQL querytranslator or directly creates the triple patterns data structures to be passed tothe rest of the Atlas modules for query processing.4. Atlas in Operation: Service Discovery in OntoKit

(1)

In this section, we show how Atlas can be used in OntoKit during serviceannotation and discovery [24]. The whole scenario is depicted in Figure 4.OntoGrid is developing annotation technology for Grid services 1331; thistechnology is deployed as the annotation service of OntoKit. For the purposesof this section, it is also important to mention another service of OntoKit, theontology service [12]. The current version of the ontology service providesa Grid interface to an RDFS store where RDFS ontologies are stored (e.g.,service ontologies or domain ontologies etc.).An ontology for services and various domain ontologies are needed in or-der to create a service annotation. Let us suppose that the annotation servicechooses to search for an ontology about cars in order to annotate a car-repairservice (the example comes from a car insurance use case studied in OntoGrid).The annotation service can pose an RQL query to the metadata service and get

Query

LexicalSynfacticaianalysisSynlacl cal Tree

CNFalgorithmI

Oue%(string)

plesPdttern

(2) Syntactical tree underCNF(3) 4

Ma in lunclion

A

Evaluation01dependenciesand

ISeman tical Representation (query graph)

Graph conslrucfionlacforirafion +-D

(6)

(5) C

Triples PatternTranslalion

I


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KNOWLEDGE AND DATA MANAGEMENT IN GRIDS

1. lwant a

about cars

Metadata Service FROM (X)ns:harSewiceDeocriptbnMWHERE Y like "can'USING NAMESPACEm - 8 h~ J h .m y p l .o r g . IWont o loe v I

Figure 4. Using Atlas for Service Annotation and Discovery

information about such ontologies e.g., the location and description of a par-ticular ontology - et us call it c a r - r e p a i r o n t o l o g y . After discoveringinformation about c a r - r e p a i r -o n t o l o g y , the annota tion service can re-trieve it from the ontology service.If the annotation service does not know the ontology for annotating services,it has to search for such an ontology as well. A n exam ple ontolog y describingservices that could be found in this case is the mYGrid service ontology [29].We sho uld mention here that this step may be unnecessary if a specific serviceontology has been selected for annotating services in On toKit.Using these ontologies, the annotation service can complete the service an-notation process. The result of the annotation process will be stored in Atlasby cal l ing the UpdateMe t a d a t a operation (see Figure 4). The ontologyused fo r describing the serv ice should have been stored previously in Atlas bycalling the S t o r e o n t01 g y operation.Let us suppose now that an OntoKit user wants to discover a service forrepairing cars. This is accomplished by submitting RQL queries using appro-priate service and dom ain ontologies (see Figure 4).Finally, notice tha t after an annotation is stored, it migh t be necessary to beable to update it. An appropriate update operation can be expressed in RULand executed in Atlas.


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Semantic G ri d Resource Discover y in A t las5. Conclusions

We have argued that resource discovery services for Semantic Grids canbe m ade scalable, fault-tolerant, robust and adaptive, by exploiting distributedRD F query processing algorithms implemented on top of DHTs. We havediscussed the implementation of our ideas in the system Atlas and its role inthe S emantic Grid toolkit OntoKit. Th e implementation of Atlas was started atthe Technical University of Crete and is currently continued at the National andKapodistrian University of Athens. More information on the current versionof Atlas is available in [21]. Although we have stressed performance issues,we have not provided any measurements o r experimental results in this paper.Experimental results based on simulations can be found in [28] and moreexperimentation is underway [27, 261. Finally, we expect to be ab le to analysethe performance of Atlas soon on real-world wide-area networks using thePlanetLab infrastructure.References

[ I ] jxta. http://www.sun.com/software/jxta/.[2] Open Grid Services Architecture Data Integration (OGSA -DAI).http://www.ogsadai.org.uW.[3] Java Com piler Compiler(JavaCC). https://javacc.dev.java.net/,2004.[4] A. Andrzejak and Z. Xu. Scalable, Efficient Range Queries for Grid InformationServices. In the second IEEE International Conference on Peer-to-Peer Computing(P2P2002), Linkoping, Sweden , 5-7 September 2002.[5] H. Balakrishnan, M. Frans Kaashoek, D. R. Karger, R. Morris, and I. Stoica. Looking updata in P2P systems. Comm unications of the ACM, 46(2):43-48,2003.[6] A. Raza Butt, R. Zhang, and Y. Charlie Hu. A Self-organiz ing Flock of Condors. In

Procee dings of Supercomputing Conference (SC), Phoenix, Arizona, November 2003.[7] M. Cai, M. Frank, and P. Szekely. MAAN: A Multi-Attribute Addressable Network forGrid Information Services. In Pm ceedings of the 4th International Workshop on Gr id

Computing (Grid2003), 2003.[8] M. Cai, A. Chervenak, and M. Frank. A Peer-to-Peer Replica Location Service Based

on A Distributed Hash Table. In the 2004 ACMIEEE Conference on Supercomputing(SC2004), Pittsburgh, November 2004.[9] M. Cai, M. R. Frank, B. Yan, and R. M. MacGregor. A Subscribable Peer-to-Peer RDFRepository for Distributed Metadata Management. Journal of Web Semantics: Science,Services and Agents on the World Wide Web,2(2): 109-1 30, De cem ber 2004.

[ lo ] D. Calvanese, G. De Giacomo, M. Lenzerini, R. Rosati, and G. Vetere. Hyper: A Frame-work fo r Peer-to-Peer Data Integration on Grids. In Proceedings of the InternationalConference on Semantics of a Networked World: Semantics for Gr id Databases (ICSNW2004), pages 144-157,2004.

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