properties of Semantic Web Services (SWS) and DHT-based Peer-to-Peer networks (DHT-P) and ... used for hosting services on both. Grid and Peer-to-Peer ...
Semantic Web Services and DHT-based Peer-to-Peer Networks: A New Symbiotic Relationship Senthil Ayyasamy, Chintan Patel, and Yugyung Lee School of Interdisciplinary Computing and Engineering University of Missouri – Kansas City {saq66, copdk4, leeyu}@umkc.edu
Abstract. In this short position paper, we will describe the intersecting properties of Semantic Web Services (SWS) and DHT-based Peer-to-Peer networks (DHT-P) and then propose an integrated architecture, which utilizes each other’s properties to solve their respective problems. Finally, we go beyond the architecture and list some open issues for effective deployment of such architectures.
THE CONFLUENCE Semantic Web Services [1] enable rapid use of e-commerce based applications. SWS can be best defined as a “service that will transform the web from a static collection of information into a distributed device of computation”[2]. It enables automatic discovery, selection, execution and monitoring of Web services by following either inter-organization business logic or mark-up based on first-order logic derivatives. Distributed hash tables [3] are benchmarked to introduce a new generation of large-scale Internet services. They provide a general mapping between an unstructured key and a location, thereby providing a location-independent routing substrate upon which SWS and other services can be built. Here, peers are organized into a well-defined structure that is used for routing queries The confluence of the “semantic” Web service and “decentralized” DHT-P helps in solving each other’s problems. A number of traditional techniques like mediator based systems, knowledge engineering and recent topics like stream processing, semantic and schema matching help in gluing the two different architectures. It is summarized in the following two tables DHT-P Needs:
SWS provides:
SWS Needs:
DHT-P Provides:
Meta- information
Upper ontology
Location information
Lookup (Key)
Complex Query processing Information Integration
Knowledge Base
Decentralized Broker
Semantic composition
Semantic Free referencing SWS Monitoring
Decentralized substrate Lookup (Key)
Meta search
Upper ontology
Management
Metadata Repository
Context aware applications
Latency Statistics Sensor interface
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Senthil Ayyasamy, Chintan Patel, and Yugyung Lee
ARCHITECTURE In this section, we will discuss the proposed integrated architecture based on the features mentioned above. The layered model has five main components: DHT component, data layer, metadata layer, semantic layer and SWS components. DHTs allow data request to be sent oblivious of where the corresponding items are stored. The target data is associated with a name, which then is hashed to a key in a virtual address space. Depending on the design of the routing algorithm [4], virtual space is partitioned and hosts are allotted to the sliced cell. One of the strong motivations for using DHT is its programming abstraction. For example, the sensor devices1 can also be considered as an underlying substrate. The data layer contains all query processing steps. Their main function is to disintegrate complex queries and pass onto the DHT for locating the resource. For applications involving continuous data, it has streaming processors. They can be used if the Web service includes sensor-based features etc. The metadata or management layer components include: knowledge base and Meta repository. All the query details and other relevant Meta information with respect to SWS are available from the repository. They help in the management of semantic information. The knowledge base maintains the neighborhood information and other domain specific ontologies. SWS1
SWS2
SWS n
…
Registry Layer
Agent Layer Metadata layer
Semantic Router
…
Semantic Router
Knowledge base Meta repository
Model-based mediator architecture Data layer Query stream processor
Query handler
Translator and optimizer
Dynamic hash tables Node
Node
…
Figure 1: Architecture
1
[5] proposes a geographic based DHT interface for sensors
Node
Semantic Web Services and DHT based P2P Networks
Model based mediation is a type of global as view information integration. They are employed in environments with widely different schema with no overlap in attributes. For example, they can be used for hosting services on both Grid and Peer-to-Peer substrate. It depends on the knowledge base of the metadata layer.
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TRUST SWS ONTOLOGY RDFS (Schema) RDF URI
UNICODE
SFR
PROGRAMMING ABSTRACTION
The semantic layer consists of DHT-P SUBSTRATE array of semantic routers and model based mediators. The Figure 2: Layer Structure function of semantic router includes: semantic routing by means of neighbor discovery (using the knowledge base), meta search, receiving and forwarding packets depending on the queries, semantic interoperability and help with semantic matching of advertisement and request. Semantic routing does the task of mapping complex data query from a peer to result in composition of Semantic Web Services. It consists of three phases: first, the complex query posed by a peer is mapped to the semantic space. Then, information or task location in a peer is identified. Finally, express routing solutions (like SOAP router2 etc.) can do semantic composition based on the task specification. It can either convert the task specification into a work flow problem or support automatic composition. It should be noted that querying is equivalent to routing at application layer. The SWS component includes the registries, the actual service and the agents. As of this writing, the possible options for the registries include UDDI, semantic matchmaker etc. The agent layer makes possible multi-agent communication. This again depends on the knowledge base for accessing dynamic ontology. The Meta repository maintains records regarding the performance of Web service. Hence, they may indirectly serve as a Quality of Service monitor in web composition process. .
DISCUSSION Motivated by the proposed architecture, this section focuses on the open issues to be debated for realizing an effective deployment and inter-operation between Semantic Web, Web Services, Grid and Peer-to-Peer networks (not just our architecture.) Internet and Web are the two remarkable things that have happened ever. The success behind the Internet is its simple hourglass model and incremental deployment 2
Note that current SOAP router doesn’t have semantic composition and automatic execution capability.
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Senthil Ayyasamy, Chintan Patel, and Yugyung Lee
plan by taking its roots from a testbed. Web’s success lies in availability of simple authoring utilities and application development kits. We will try to utilize them into our solution space. One of the important problems in introducing disruptive technologies is lack of deployment. Many technologies - i.e. agents, ontology management, natural languages and active networks – that have been precursors to the present innovation mostly remained within research labs. Recently, one can see mushrooming 3 technology specific testbed initiatives (like planet lab – Peer-to-Peer, Mangrove, 4 5 6 Haystack - Semantic Web, DAML/SWWS initiative – SWS, Globus, Grid bus - Grid) to seed the incremental deployment. As mentioned in the starting section, there are lots of benefits if they inter-operate. Hence, need for creation of inter-domain gateway models at various levels is necessary. Most of the current deployments (Peer-to-Peer and Grid) follow overlay architecture but at last, depend on the layered approach. But, increasing complexity at the application level with no regard for the underlying layers is harmful. So, application level local sub-layering optimizations have to be considered. Although ours is a layering approach, alternatives like role-based assignments would be more appropriate. The amalgamation of the different technologies introduces new applications. With so much technology push, the visible demand-pull seems to be from e-commerce only. Hence, options to utilize traditional knowledge, data and computational intensive applications along with web-based services in all spheres have to be examined. With particular reference to our work, a lot remains undone. The future course of work includes implementation of the semantic router, semantic interoperability, accommodating location and content flux, complex query processing and deployment feasibility across planet lab.
REFERENCES [1] S.A. McIlraith, D. L. Martin, “ Bringing Semantics to Web Services,” IEEE Intelligent Systems, Jan-Feb 2003. [2] C. Bussler, D. Fensel, A. Maedche, “ A Conceptual Architecture for Semantic Web Enabled Web Services,” SIGMOD Record, Dec 2002. [3] H. Balakrishnan, F. Kaashoek, D. Karger, R. Morris, I. Stoica, “ Looking up data in P2P systems,” Communications of the ACM, Feb 2003. [4] S. Ratnasamy, S. Shenker, I. Stoica, “ Routing Algorithms for DHTs: Some Open Questions," IPTPS 02, March 2002. [5] S. Ratnasamy, B. Karp, L. Yin, F. Yu, D. Estrin, R. Govindan, S. Shenker, “GHT: A Geographic Hash Table for Data-Centric Storage,” WSNA ’02, Sept 2002.
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www.planet-lab.org http://www.cs.washington.edu/research/semweb/, http://haystack.lcs.mit.edu 5 www.daml.org/services, http://swws.semanticweb.org/ 6 http://www.globus.org/, http://www.gridbus.org/ 4
