=Paper=
{{Paper
|id=Vol-100/paper-12
|storemode=property
|title=An O-Telos Provider Peer for the RDF-based Edutella P2P-network
|pdfUrl=https://ceur-ws.org/Vol-100/Martin_Wolpers-et-al.pdf
|volume=Vol-100
|dblpUrl=https://dblp.org/rec/conf/ecai/WolpersNB02
}}
==An O-Telos Provider Peer for the RDF-based Edutella P2P-network==
https://ceur-ws.org/Vol-100/Martin_Wolpers-et-al.pdf
An O-Telos provider peer for the RDF-based Edutella
P2P-network
Martin Wolpers, Wolfgang Nejdl, Ingo Brunkhorst
Learning Lab Lower Saxony,
University of Hannover,
Expo Plaza 1, 30359 Hannover, Germany,
email: wolpers,nejdl,brunkhorst@learninglab.de
Abstract. The open source project Edutella is a Peer-to-Peer (P2P) oriented database manager that implements the meta data repre-
network for learning materials. It builds upon meta data standards de- sentation language O-Telos [6] [8]. In contrast to RDF, O-Telos is
fined for the WWW and aims to provide a RDF-based meta data in- an object-oriented meta-modeling language that provides facilities
frastructure for P2P applications. In this paper we discuss a prototype for unrestricted meta modeling levels. Furthermore the ConceptBase
provider peer for the Edutella network, which provides advanced rea- database implements a powerful query and reasoning (rules and con-
soning peers for RDF data. This prototype peer provides storage and straints) mechanism based on Datalog which we want to make ac-
querying services for which it uses the ConceptBase database with cessible to the Edutella network.
its powerful reasoning mechanism as storage and query facility. Con- The provider-peer provides two basic services which we will
ceptBase implements the meta data language O-Telos while Edutella present in this paper. On one hand it is designed to store RDF(S) data
uses RDF(S). Based on previous work in this area we present a mech- in the ConceptBase repository; the storage service. The data repre-
anism to translate the property-centered meta modeling language sented in RDF(S) is translated to O-Telos which is used by the repos-
RDF to the object-centered meta modeling language O-Telos. Thus itory. The translation uses the RDF-O-Telos axioms as described in
we are able to provide a Datalog based reasoning facility for the [9].
P2P Edutella network. On the other hand the provider-peer serves as a query interface to
As the Edutella network also uses the query exchange language the RDF data stored in the ConceptBase repository; the query ser-
RDF-QEL we also describe in detail the peer’s query service. It vice. Queries formulated in the RDF exchange language RDF-QEL
translates RDF-QEL queries to O-Telos queries and the respective [10] are posed to the peer that translates them into O-Telos queries.
O-Telos answers to RDF-QEL answers. The O-Telos queries than are answered by the ConceptBase database.
The peer translates the ConceptBase answers from O-Telos into RDF
and returns these RDF statements to the network.
1 Introduction Combining the storage and query service with ConceptBases’
Datalog reasoning facilities enables us to reason about the meta
Within the last year we have been working on a project towards
data used in the Edutella network.
implementing an extendable open source meta data-based peer-to-
This paper is structured as follows: the next chapter describes how
peer infrastructure called Edutella for the exchange of distributed
RDF(s) data is stored in ConceptBase. With the help of a simple
resources. Resources initially considered are learning materials but
example our translation algorithm is explained in more detail. The
the infrastructure is agnostic to the specific type of resources to be
third chapter deals with the querying facility provided by this peer.
exchanged. In our case we have large collections of learning mate-
We explain how we are able to use the advanced querying facilities of
rial and we are working on annotating this material with RDF [7]
ConceptBase for querying RDF(S) data. The fourth chapter describes
and RDFS [1]. This will enable us to use the Edutella network ser-
briefly the technical issues concerned with this provider peer. A short
vices like storage, exchange, query and processing services which
summary closes the paper.
are based on RDF(S).
In its current state the Edutella network consists of client- and
provider-peers. A client-peer poses queries to the Edutella network 2 Storing RDF meta data in ConceptBase
and displays the answers returned from the network. The provider-
peers receive the queries from the network and answer them if pos- The resources exchanged in the Edutella network are basically learn-
sible. Thus a peer must realize a storage and query service for the ing materials, each learning object is described by RDF(S) data. In
Edutella network in order to function as a provider-peer. For a com- this paper we describe a way to provide an Edutella peer to store and
plete description of Edutella see the Edutella White Paper [10] and retrieve RDF(S) data. The example used throughout this paper is a
the projects’ home page [3]. small database containing the RDF description of some books. This
In this paper we describe a provider-peer and its services for database is taken from the Edutella [3] examples library. A part of
the Edutella network, which provides extended reasoning capabil- this database is represented in figure 1.
ities. The provider-peer uses the ConceptBase database [5] as a In order to store the RDF meta data in the ConceptBase database
repository for storing meta data. ConceptBase is a deductive object- the peer has to translate RDF to O-Telos. O-Telos itself is an object-
oriented modeling language defined by 32 axioms. In order to trans-
late RDF to O-Telos it is necessary to modify and eventually extend
RDF(S). Therefore we have developed a translation based on the ax-
ioms from O-Telos-RDF. For a description of O-Telos-RDF and its
defining axioms please see [9].
The algorithm ensures that RDF(S) data is translated without any
loss of information. At first we are changing the representation from
XML serialization to its representation as triple statements. Each
triple is provided with an unique id. Furthermore the quadruples built
are eventually supplemented with additional quadruples capturing
further facts of the actual namespace to fulfill the axioms of O-Telos
and O-Telos-RDF. The sum of compound and generated quadruples
form the O-Telos database where each quadruple represents an O-
Telos statement.
An O-Telos quadruple statement has the form
(sid,subject,literal,object). Sid, subject and object are unique
statement identifiers. Sid identifies the statement that connects the
subject statement with the object statement using the predicate
literal. Sid and subject can hold the same identifier only if sid,
subject and object are the same identifier, so that the statement
represents an individual with the literal as name. O-Telos declares
several predefined literals like the In literal stating instantiation, the
IsA literal stating inheritance, etc.
As an example for the translation process we will focus on just
one RDF resource description. The translation of the other resource
descriptions of the example book database follows basically the same
procedure varying in small details only. The RDF statements for the
example resource http://www.xyz.com/jv.html describe a
book with its properties “Just Java” of type dc:title and “Peter van
der Linden” of type dc:author. The resource itself is of the rdf:type
http://www.lit.edu/ types#Book.
The RDF-triple representation of this resource description is given
in table 1.
Nr. triple
1 http://www.xyz.com/jv.html,
http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
http://www.lit.edu/types#Book
2 http://www.xyz.com/jv.html,
http://purl.org/dc/elements/1.1/author,
”Peter van der Linden”
3 http://www.xyz.com/jv.html,
http://purl.org/dc/elements/1.1/title,
”Just Java”
Figure 1. RDF graph of the example book database Table 1. RDF triples of the book ”Just Java”
Nr. Quadruple
1 sid1, sid1, http www xyz com jv html, sid1
2 sid2, sid1, In, #Book
3 sid3, sid1, In, #DCElements11
4 sid4, sid1, title1, ”Just Java”
5 sid5, sid4, In, #DCElements11.title
6 sid6, sid1, author1, ”Peter van der Linden”
7 sid7, sid6, In, #DCElements11.author
8 sid8, sid1, namespace1, ”http://www.xyz.com/”
9 sid9, sid8, In, #Book.namespace
Table 2. O-Telos quadruples of the book ”Just Java”
The O-Telos quadruples that describe the example re-
source http://www.xyz.com/jv.html are given
in table 2. Statement #1 represents a individual name
http www xyz com jv html. This individual is instance
of a class #Book and a class #DCElements11 (statements #2 and
#3). The individual has an attribute named title1 connecting it to the
object ”Just Java”. The attribute is instance of #DCElements11.title.
The individual is also connected to the object ”Peter van der
Linden” with the author1 attribute. This attribute is instance of
#DCElements11.author. Also the individual is connected via the
namespace1 attribute to ”http://www.xyz.com/” which is an instance
of #Book.namespace.
The translation from RDF triple to O-Telos quadruple representa-
tion employ the axioms from O-Telos-RDF. Especially the axioms
describing instantiation, property and type translation are used. Note
Figure 2. O-Telos graph of the example book database
that each RDF statement obtains a unique id to form the O-Telos
statement. The ids are the statement identifiers really and as such
are unique globally (except when exactly the same statements are
made in two different places. In our example the ids are represented in the ConceptBase database. Thus figure 2 shows the O-Telos graph
by a short form sidx to enhance readability which would expand of the same part of the book database which is shown as RDF graph
to namespace:resourcename, e.g. here sid1 is the abbreviated in figure 1
form for http://www.xyz.com:jv.html. The comparison of both graphs of the book database (figure 1 for
If a RDF statement includes elements from other than the current RDF and figure 2 for O-Telos) shows their equivalence. They are dif-
schema, identified by a different than the current namespace, these ferent representations of the same book database using various dif-
elements are grouped in specially created O-Telos classes. For exam- ferent notations and serializations (triple/quadruple and XML/frame
ple the properties of the “Just Java” books’ RDF description dc:title syntax). Thus by employing the translation described above it is pos-
and dc:author, originating in the Dublin Core schema, are grouped sible to store RDF(S) data in the O-Telos meta data language. The
in an O-Telos class called DCElements11 (see table 2). Note that the next chapter shows how the lossless transformation of the RDF(S)
sids prefixed with ’#’ are stated for simplicity reasons and denote data from its O-Telos representation is realized.
the respective class names. In reality these are again statements with
own ids which would enlarge the example without adding any new
insight. We therefore use the shortcut with the ’#’ prefix and the class 3 The peer query service
name instead.
The chapter will provide some insight in how the data stored in an O-
Also the O-Telos attribute namespace is introduced to hold the
Telos provider-peer is regained using RDF queries. The RDF queries
namespace of each RDF resource and property. Usually the names-
are posed to the peer by using the RDF-QEL exchange language as
pace is part of the unique id of each statement as hinted in the exam-
described in [10]. Based on Datalog [11] RDF-QEL abstracts the
ple 2.1 but unfortunately the O-Telos frame syntax and parser forbids
various RDF storage layer query languages (e.g. SQL) and user level
the possible special characters like dot, slash, dash, backslash, etc. of
query languages (e.g. RDQL) thus providing the syntax and seman-
URLs in the element name. Therefore we introduce the namespace
tics for an overall standard query interface across heterogeneous peer
attribute that is assigned to each element and attribute. This attribute
repositories for any kind of RDF meta data.
holds the namespace or resource URL respectively as a work around.
RDF-QEL does not distinguish between data and schema levels
thus enabling the querying of different modeling levels. In doing
Individual http___www_xyz_com_jv_html
so RDF-QEL conforms with the RDF(S) schema definitions and
in Book, DCElements
the more recent RDF model theory [4]. In order to describe and
with
handle different query capabilities of this particular peer, we defined
title
several RDF-QEL-i exchange language levels with increasing
title1 : "Just Java"
expressiveness: RDF-QEL-1 (conjunctive queries) is expressed
author
as unreified RDF graphs while the higher levels use reified RDF
author1 : "Peter van der Linden"
statements for increased expressiveness (e.g. RDF-QEL-3 covers
attribute
relational algebra, RDF-QEL-4 incorporates full Datalog). In
namespace : "http://www.xyz.com/"
this paper we will use examples in RDF-QEL-1 only for sim-
end
plicity reasons. More advanced working examples can be found at
http://cip1-s.cip1.uni-hannover.de:3120/rdf2cb
Example 2.1: O-Telos frame representation of the book ”Just Java”
where advanced features like negation are demonstrated.
The RDF-QEL queries are translated into O-Telos queries which
The frame representation of the O-Telos quadruples of table 2 is than are posed to and answered by the ConceptBase database. The
stated in example 2.1. O-Telos answers are translated back to RDF in XML serialization. In
From the small example of the book ”Just Java” in its RDF and order to clarify the translation further we will stick with the exam-
O-Telos descriptions we have gained an insight on the translation ple from above. There the RDF description of the resource with the
of RDF to O-Telos. When applied to the whole book database the URL http://www.xyz.com/jv.html and the title “Just Java”
respective complete O-Telos representation is generated and stored is stored in the database.
QueryClass EduQuery isA Individual with
namespace : String
varX : $ exists Y/Individual
and A(this,title,"Just Java") $
]> Example 3.2: The O-Telos frame representation of the query example
namespace
nam1 : "http://www.xyz.com/"
end
Example 3.3: Frame syntax of ConceptBases’ answer for the query
The answer consists of RDF statements declaring that there is a
resource with URL http://www.xyz.com/jv.html. This re-
source is the domain of a property named title which has the range
The above example completes our presentations on how RDF(S)
can be translated to O-Telos and back. The previous chapter intro-
duced the ability of translating RDF to O-Telos thus enabling the
storing capabilities of the meta database ConceptBase. This chapter
retrieved and presented in RDF.
4 The peer’s implementation
Example 3.1: The RDF-QEL XML representation of the example The Edutella peer described is fully implemented in Java as is the
query whole EDUTELLA-Project. The actual implementation consists of
three parts: the servlet enables a simple user interface, the Edutel-
laPeer implementation realizes the Edutella peer service and the
Now we want to know from the book database which books have EdutellaProvider provides the actual query service.
the title “Just Java”. The respective query expressed in RDF-QEL-1 The demo and test servlet running at
and XML is stated in example 3.1. http://www.cip1.uni-hannover.de :3120/rdf2cb/
The query asks for at least one resource in the database which runs within a SUN JavaWebServer 2.0. The servlet itself realizes the
has a property called dc:title with a value “Just Java”. The resulting user interface for testing purposes only thus enabling users to pose
resource(s) are returned in XML notation. The peer constructs from RDF-QEL queries in the same way the Edutella peer service will do
this RDF representation the respective O-Telos representation. Thus
the query is reformulated in O-Telos so that it now states that there
is at least one resource in the database which has a property called
Individual thus has as answers other instances of the class Individ-
String. The constraint varX declares that all instances of the answer
The namespace of the title attribute is derived from ConceptBase
with a second query. For simplicity reasons we omit this query here.
The query answers are than translated to RDF as shown in exam- Example 3.4: The XML serialization of the query answer expressed
ple 3.4. in RDF
and inspecting the respective answers. [5] M. A. Jeusfeld, M. Jarke, H. W. Nissen, and M. Staudt. Concept-
Figure 3 shows that the Servlet as well as the Edutella peer Base - Managing Conceptual Models about Information Systems. In
P. Bernus, K. Mertins, and G. Schmidt, editors, Handbook on Architec-
pose EduQuery-objects to the Edutella provider implementation. tures of Informations Systems. Springer Verlag, 1998.
The EduQuery-objects contain the queries. They are built using the [6] B. M. Kramer, V. K. Chandhri, M. Koubarakis, T. Topaloglon, H. Wang,
classes of the Edutella software package, especially employing the and J. Mylopoulos. Implementing Telos. SIGART Bulletin, 2(3), June
parser that is based on an adaptation of the Jena RDF parser API [2]. 1991.
[7] O. Lassila and R. Swick. W3C Resource Description framework (RDF)
Model and Syntax Specification, 2001. http://www.w3.org/TR/REC-
rdf-syntax/.
[8] J. Mylopoulos, A. Borgida, M. Jarke, and M. Koubarakis. Telos: A
language for representing knowledge about information systems. ACM
Transactions on Information Systems, 8(4), 1990.
[9] W. Nejdl, H. Dhraief, and M. Wolpers. O-Telos-RDF: a re-
source description format with enhanced metamodeling functionali-
ties based on O-Telos. In Workshop on Knowledge Markup and Se-
mantic Annotation at the First International Concerence on Knowl-
edge Capture (K-CAP’2001), October 2001. http://www.kbs.uni-
hannover.de/Arbeiten/Publikationen/2001/kcap01 final.pdf.
[10] W. Nejdl, B. Wolf, C. Qu, S. Decker, M. Sintek, A. Naeve, M. Nils-
son, M. Palmér, and T. Risch. EDUTELLA: a P2P Networking
Infrastructure based on RDF. In WWW 11 Conference Proceed-
ings, Hawaii, USA, May 2002. http://edutella.jxta.org/reports/edutella-
whitepaper.pdf.
[11] J. D. Ullmann. Principles of database and knowledgebase systems.
Principles of computer science series. Computer Science Press, Inc.,
Maryland, USA, 1988.
Figure 3. Schematic representation of the architecture
As shown in figure 3 the ProviderAdapter receives EduQuery ob-
jects. In a first step these are translated to O-Telos query objects by
the QueryWorker. In a second step the ResponseWorker establishes
a connection to the ConceptBase database, poses the query, receives
the answer and returns the EduResult objects constructed from Con-
ceptBases’ answer. The ProviderAdapter itself returns the EduResult
objects to either the servlet or the Edutella peer which in turn process
them as according to their respective needs.
5 Summary
The evolving Edutella P2P network combines provider and client
peers of various kinds. In this paper we have described a prototypi-
cal implementation of a provider peer which provides extended rea-
soning capabilities for RDF data. The provider peer uses the Con-
ceptBase database which implements the meta modeling language O-
Telos. Based on this database the basic Edutella services storage and
querying are realized: a repository for storing RDF data and query
facilities for the stored data.
The storage service uses a translation from RDF(S) to O-Telos
and vice versa where property-centered RDF(S) data is translated to
and from object-centered O-Telos data. Based on this translation the
query service translates RDF-QEL queries to O-Telos query classes
and translates the answers to these queries, instances of O-Telos
query classes, back into RDF(S) graphs.
REFERENCES
[1] D. Brickley and R. V. Guha. Resource Description Framework (RDF)
Schema Specification 1.0, 2002. http://www.w3.org/TR/rdf-schema.
[2] Hewlett-Packard Company. The Jena Semantic Web Toolkit, 2001.
http://www.hpl.hp.com/semweb/jena-tob.html.
[3] Edutella Project Homepage. http://edutella.jxta.org.
[4] P. Hayes. RDF model theory. W3C working draft, February 2002.
http://www.w3.org/TR/rdf-mt/.