Developing Distributed Semantic Systems

Building Distributed
Semantic Systems

Antonio Garrote Hernández

Monday, December 21, 2009

Deﬁnition

• Distributed processes
• Semantic meta data exposed as resources
• Triple space operations
• RESTful interface


Different abstraction
levels


Sample application


Exception logging
system
• Sensor processes monitoring generation of
exceptions in different nodes and languages/platforms

• Aggregator processes retrieving the exceptions and
creating Exception resources

• Exception RESTful semantic web service

• Web client displaying generated exceptions


Methodology


4 step process

1. Knowledge modeling
2. Formal description
3. Implementation of RESTful semantic
services
4. Implementation of client processes


1.Knowledge modeling


1.Knowledge modeling
• Building of an ontology for the problem domain

• Ontology = TBox in Description Logics

• Use of W3C’s standard languages: RDFS or OWL

• Assistance of graphical tools like Protégé


Sample application ontology

level message createdAt

Exception

generated
platform

Node Language

ipAddress domain dc:title formatPattern


Developing the sample application ontology with Protégé


Ontology(<http://semantic_rest/exceptions.owl>

Declaration(Class(Exception))
SubClassOf(Exception owl:Thing)
Declaration(Class(Node))
SubClassOf(Node owl:Thing)
Declaration(Class(ProgrammingLanguage))
SubClassOf(ProgrammingLanguage owl:Thing)

Declaration(ObjectProperty(platform))
ObjectPropertyDomain(platform Exception)
ObjectPropertyRange(platform ProgrammingLanguage)

Declaration(ObjectProperty(generatedIn))
ObjectPropertyDomain(generatedIn Exception)
ObjectPropertyRange(generatedIn Node)

Resulting ontology Declaration(DataProperty(level))
DataPropertyDomain(level Exception)

using OWL
DataPropertyRange(level rdfs:Literal)

Declaration(DataProperty(ipAddress))

Functional Notation
DataPropertyDomain(ipAddress Node)
DataPropertyRange(ipAddress xsd:string)

Declaration(DataProperty(formatPattern))
DataPropertyDomain(formatPattern Exception)
DataPropertyRange(formatPattern xsd:string)

Declaration(DataProperty(createdAt))
DataPropertyDomain(createdAt Exception)
DataPropertyRange(createdAt xsd:dateTime)

Declaration(DataProperty(content))
DataPropertyDomain(content Exception)
DataPropertyRange(content xsd:string)

Declaration(DataProperty(domain))
DataPropertyDomain(domain Node)
DataPropertyRange(domain rdfs:Literal)
)


2.Formal description


• Formal description of the whole distributed
system

• Concerned with ‘dynamic’ aspects of the
system

• Use of an extended version of the Pi-
Calculus


Formal description of the sample application

S(r)

GET subscribe n POST n

W E A TE

S(j)


Formal description of the sample application

Processes equations:


3.Implementation of
RESTful semantic
services


• 1 RESTful semantic service per OWL/RDFS
class in the ontology

• Use of the Plaza Framework developed for
building RESTful semantic web services


RESTful Semantic Web
Service

• OWL/RDFS class triples
• Exposed as a RESTful resource
• Conceptually equivalent to a Linda triple
space


Service’s Interface
Create POST

Read GET

Blocking read GET + xblocking header

Subscribe • GET + xsubscribe header
• Websockets
Update PUT

Destroy DELETE


RDF
Lifting Graph

SPARQL query

REST Resource

triple
resource
space

triple space op.

HTTP request Lowering

Plaza Framework


Plaza Framework
• Manages ‘Plaza Applications’: OWL ontolgy +
conﬁguration module + resources modules

• Declarative: almost no coding required

• Support for different Java RDF repositories

• Automatic managing of triples through the concepts of
write-tree and read-tree for the resources

• Support for the subscribe operation through chunked
HTTP responses and websockets


Sample application conﬁguration module


Sample application exceptions resource module


4.Implementation of
client processes


• Client processes are just REST clients
• Required libraries: HTTP and triples format
parser (RDF/XML, Turtle, JSON)

• Optional support for Websockets
• Clojure library included as part of the Plaza
Framework


Clojure implementation of the sensor and aggregator
processes


Javascript web client using websockets


Web client execution


Conclusions
• Semantic technologies enable rich description of
problem domains vs. relational modeling

• Formal description of distributed systems enables
reasoning about its validity and correctness

• RESTful semantic web services enable easy access
to semantic data

• Building RESTful semantic web services clients is
trivial due to the use of well known HTTP
standard

• Triple Space operations are powerful coordination
mechanisms for distributed processes


Developing Distributed Semantic Systems

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