PeertoPeer Networking for Distributed Learning Repositories:

1
Peer-to-Peer Networking for Distributed Learning
Repositories

• The Edutella Network
• Diplomarbeit von Boris Wolf

2
Edutella Motivation

• P2P systems dont work without metadata.
• Shortcomings of todays successful P2P systems
  (Napster, LimeWire, Gnutella etc.) Poor
  expressiveness of metadata, focused on certain
  application or community.
• Edutella Specification and implementation of an
  extendable Open Source infrastructure for
  exchange and processing of any kind of RDF
  metadata (initial focus on educational
  resources).
• Edutella is Open Source, uses Open Source, builds
  on Open Standards.
• Part of the PADLR Project (Stanford, Stockholm,
  Uppsala, Hannover, Braunschweig, Karlsruhe).

3
Edutella Technology (RDF)

• Connects highly heterogeneous peers
  (heterogeneous in uptime, storage capability,
  performance, functionality/services offered,
  number of users etc.).
• Essential assumption Everything in Edutella can
  be described in RDF therefore
• Edutella peer RDF repository
• Edutella network huge RDF repository
  integrating all participating peer repositories.

• Goal making distributed nature of RDF
  repositories completely transparent to Edutella
  clients.

4
Edutella Technology (JXTA)

• Open Source project supported by Sun Microsystems
• JXTA set of protocol specifications to cover
  typical P2P functionality (all protocols based on
  XML).

• Java binding offers a layered approach to
  standardize P2P application development (core,
  services, applications).
• Framework for prototyping and developing P2P
  applications.
• Introduces well-known UNIX concepts to the P2P
  world peer groups, pipes, thin core libraries
  / services, JXTA shell.

5
Edutella Architecture

• Edutella Collection of JXTA Services.
• Each peer will be required to support a number of
  basic services and may offer an arbitrary number
  of advanced services.
• Edutella Query Standardized query and retrieval
  mechanism for RDF metadata stored in distributed
  RDF/XML repositories.
• Further Edutella Services Edutella Clustering
  (semantic routing), Edutella Replication
  (redundant storage), Edutella Mapping (different
  ontologies), Edutella Annotation (distributed
  annotation facilities).

6
Edutella Query

• RDF Query Exchange Language RDF-QEL (RDF can be
  easily analyzed utilizing existing RDF/XML
  parsers, and transferred as XML data).
• Edutella Common Data Model (ECDM) for internal
  represenation of queries and results (based on
  Datalog semantics).

7
Edutella Query Wrappers

• Wrappers translate between RDF based query
  languages (RDF-QEL-i, RQL, TRIPLE, etc.) and
  local query languages (SQL, Xpath, etc.).

8
Edutella Query Mediators

• Distributed query mechanism with Mediators/Hubs
  based on query indices Simple Mediators
  distribute entire queries to registered peers,
  advanced Mediators split queries into subqueries
  and reconcile their results.

9
Edutella Query Datalog

• Datalog Uses logic programming concepts (Prolog)
  for databases and query languages.
• Literals Predicate expressions describing any
  kind of relations between an arbitrary number of
  constants or variables, e.g.
• title(X,Artificial Intelligence)
• Database/Knowledge Base Set of ground facts
  set of literals with constant values only, e.g.
• title(http//www.xyz.com/ai.html,Artificial
  Intelligence)

10
Edutella Query Datalog

• Query Conjunction of literals, e.g.
• ?- title(X,Artificial Intelligence),
  type(X,Book).
• Rule Implication with one head and an arbitrary
  number of body literals, e.g.
• aibook(X) - title(X,Artificial
  Intelligence), type(X,Book).

11
Edutella Query Knowledge Base

• RDF only knows assertions RDF repository
  Knowledge base of ground (binary) facts.
• http//purl.org/dc/elements/1.1/title('http//www.
  xyz.com/ai.html','Artificial Intelligence')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/ai.html','Stuart Russel')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/ai.html','Peter Norvig')
• http//www.w3.org/1999/02/22-rdf-syntax-nstype('h
  ttp//www.xyz.com/ai.html','http//www.lit.edu/typ
  esBook')
• http//purl.org/dc/elements/1.1/title('http//www.
  xyz.com/sw.html','Software Engineering')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/sw.html','Helmut Balzert')
• http//www.w3.org/1999/02/22-rdf-syntax-nstype('h
  ttp//www.xyz.com/sw.html','http//www.lit.edu/typ
  esBook')
• http//purl.org/dc/elements/1.1/title('http//www.
  xyz.com/pl.html','Programming in Prolog')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/pl.html','William Clocksin')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/pl.html','Christopher Mellish')
• http//www.w3.org/1999/02/22-rdf-syntax-nstype('h
  ttp//www.xyz.com/pl.html','http//www.lit.edu/typ
  esAI-Book')
• http//purl.org/dc/elements/1.1/title('http//www.
  xyz.com/jv.html','Just Java')
• http//purl.org/dc/elements/1.1/author('http//www
  .xyz.com/jv.html','Peter van der Linden')
• http//www.w3.org/1999/02/22-rdf-syntax-nstype('h
  ttp//www.xyz.com/jv.html','http//www.lit.edu/typ
  esBook')

12
Edutella Query Example Query

• Return all resources which are of type AI-Book
  or which are of type Book and furthermore have
  the title Artificial Intelligence
• aibook(X) - http//purl.org/dc/elements/1.1/title
  (X,'Artificial Intelligence'), http//purl.org/
  dc/elements/1.1/type(X,'http//www.lib.org/typesB
  ook').
• aibook(X) - http//purl.org/dc/elements/1.1/type(
  X,'http//www.lib.org/typesAI-Book').
• ?- aibook(X).
• Query literals are evaluated one by one.
• Matching rule heads are substituted and their
  variables unified.
• Disjunction as rules with identical head.

13
Edutella Query Results

• Results as tuples of variable bindings.
• aibook(http//www.xyz.com/ai.html)
• aibook(http//www.xyz.com/pl.html)

14
Edutella Query RDF-QEL Design criteria for an
RDF Query Exchange Language

• Standard semantics and sound RDF serialization.
• Expressiveness (simple queries should look simple
  while complex queries should be possible).
• Adaptibility (QBE/simple graph query, relational
  calculus, inference engines).
• Transformability.

15
Edutella Query RDF-QEL-i

• Solution Several language levels RDF-QEL-i (1-4)
  with increasing
• expressiveness
• RDF-QEL-1 purely conjunctive queries, follows
  QBE paradigm (corresponds to Datalog query
  without rules).
• RDF-QEL-2 disjunctive queries, requires
  reification, AND/OR-tree, suitable for query
  visualization and reuse (corresponds to Datalog
  with non-recursive rules).
• RDF-QEL-3 allows negation, covers relational
  calculus (corresponds to Datalog with
  non-recursive rules and negated literals).
• RDF-QEL-4 allows recursion to express transitive
  closure and linear recursive query definitions.

16
Edutella Query Example Query in RDF-QEL-3
17
Edutella Query Example Query

• aibook(X) - http//purl.org/dc/elements/1.1/title
  (X,'Artificial Intelligence'), http//purl.org/dc/
  elements/1.1/type(X,'http//www.lib.org/typesBook
  ').
• aibook(X) - http//purl.org/dc/elements/1.1/type(
  X,'http//www.lib.org/typesAI-Book').
• ?- aibook(X).

18
Edutella Prototype Functionality

• Edutella Query Wrapper
• Edutella network interface Import/export of
  RDF-QEL-1, RDF-QEL-3 and Datalog queries.
• Storage layer interface Export of SQL2 queries.
• Internal Query Representation
• Edutella Common Data Model (ECDM).
• Edutella Query Mediator
• Maintains Query Index with entries of the form
  ltnamespacegt, ltnamespace, property_namegt or
  ltnamespace, property_name, property_valuegt.

19
Edutella Prototype ECDM Model
20
Edutella Prototype Demonstration
21
Edutella Prototype SQL

• SELECT STMT0.PREDICATE STMT0_PREDICATE,
  STMT0.ARG0 STMT0_ARG0
• FROM
•   (SELECT 'http//www.lib.orgaibook' PREDICATE,
  STMT0.SUBJECT ARG0
• FROM STATEMENTS STMT0
• WHERE STMT0.PREDICATE'http//www.w3.org/1999/0
  2/22-rdf-syntax-nstype'
• AND STMT0.OBJECT'http//www.lit.edu/typesAIBoo
  k'
• UNION
• SELECT 'http//www.lib.orgaibook' PREDICATE,
  STMT0.SUBJECT ARG0
• FROM STATEMENTS STMT0, STATEMENTS STMT1
• WHERE STMT0.PREDICATE'http//purl.org/dc/elemen
  ts/1.1/title'
• AND STMT0.OBJECT'Artificial Intelligence'
• AND STMT1.PREDICATE'http//www.w3.org/1999/02/2
  2-rdf-syntax-nstype'
• AND STMT1.OBJECT'http//www.lit.edu/typesBook'
  
• AND STMT0.SUBJECTSTMT1.SUBJECT ) STMT0

22
Edutella Prototype Future Work

• Further testing, enhanced error handling.
• Support for further Query Languages RDF-QEL-2,
  RQL, TRIPLE, RDF/XML, etc.
• Query Optimizer.
• Enhanced Distributed Query Mechanism.