An Ontological Framework for

Web Service Processes

By Claus Pahl and

Ronan Barrett

Introduction – Why the Paper was written??

Increase in attention of developing semantic based services on the Web There is a need to allow for the composition of

services based on abstract descriptions (by use of Ontologies)

Aim of paper to give semantics to the framework of process

composition to develop an ontological framework for service

process description and discovery

Contents

What is a Service Process How a Process is Composed Operation Framework presented in the

paper Ontological Framework presented in

the paper

What is a Service Process?

Models the dynamic behavior of business or workflow systems

Composition of individual services Service is a set of coherent operations provided at a

certain location Services are made available through abstract interfaces

where users can locate and invoke a desired service

Web Services Framework (WSF) is a platform that allows services to be invoked in a once-off basis

Service Composition Two forms of service composition

Orchestration Specifies the internal and external services to perform its

task Focuses on the execution order through the use of message

exchanges Controlled by one agent in the system Known as the Process Model

Choreography Specifies the interactions and collaboration between services Specifies the synchronization and the exchange of data

between the services Involves multiple agents where each agent describes its own

part in the interaction Known as the Interaction Model

Example of a Web Process

Service Composition

Orchestration:

Each process implements many services

Describes process of online shopping user starting at login, then repeatedly buying products and finally logging out

Choreography:

Each invocation results in an interaction – message exchange

Service Composition

Process Assembly is a two step process: 1) Discovery

Clients search for suitable services in registry Based on abstract descriptions (formalised on

Ontologies) 2) Assembly and Usage

Composition of matching services Interaction

Service Composition

Services & Processes – An Operational Framework

Introduce the abstract language that forms the Operational Framework for Description and Composition By use of pi-calculus

Operational Framework serves to capture requirements and forms an underlying layer for the Ontological Framework

Orchestration and Choreography Formal Description

Identifies Core Requirements for Orchestration Description Notation(based on the Web Services Process Execution Language (WS-BPEL)) Basic Elements: message-based actions

Invocations for external services Receive/Reply actions if service is available to others

Process Language Service Process Combinations Sequence, choice, iteration, concurrency

Abstraction Interface Process can be provided as a Web Service

State & Data Needed variables and parameters for actions

Orchestration and Choreography Formal Description

Orchestration Defines a Process in

terms of actions and control flow

Describe the external, interaction pattern that a service can engage in

Orchestration and Choreography Formal Description Orchestration Language

Orchestration Language now allows for an Abstract Process expression to be defined

Orchestration and Choreography Formal Description

Identifies Core Requirements for Choreography Description Notation (based on Web Services Choreography Interface (WSCI) and Web Services Choreography Description Language (WS-CDL))

Basic Activities Request and Response Activities for local activities Invoke to call operations to external services

Structured Activities Loop, sequence, choice, concurrency

Infrastructure Channels (connections between ports) Ports – represent services and their operations

Orchestration and Choreography Description

Focus on interaction behavior, not execution order

Based on Fixed Connections Shared channel that

forms a connection between two service processes

Orchestration and Choreography Formal Description

Choreography Language

Composition Support Description and matching are design

activities Need to also integrate aspects of

deployment in a life cycle of a process Each service is a family of ports

Sc – Contract Port Interface to capture abstract properties

Si – Connection Port for interaction Handles service invocation and input

Sr – Connection Port for interaction Handles the service reply

Composition Support

Defining the three ports allows the Notation of a Service Life Cycle Client

Provider

Request Service s using port Sc (requirements)

Invoke Action using port Si and specify result to be sent thru Sr

Result of action y is then received on port Sr

Provides Service s using port Sc (requirements)

Executes action received thru Si

Returns result of action f(a) to specified port Sr

Composition Support

Composition and Interaction Protocol Activities captured in a standard life cycle form

Composition Support

Matching of a Process Provider needs to meet the expected request pattern of the client

Defined using the notion of transition graph

Client

Provider

Composition Support

If the pairs of service operations match based on their individual descriptions Catalog/CatalogBrowse, Quote/QuoteProd Purchase/ProdPurch

then the provider matches the requested process

Composition Support Connection & Interaction

Successful matching results in a contract between two ports Contract Phase- both processes form a contract based

on matching abstract descriptions

Once Contract Port is established, processes can then interact through the a connector channel Connection Phase

Composition Support

Formal definition of Contract and Connector Rules

Services & Processes –an Ontological Framework Ontology Framework Defined

Data model used to represent a set of concepts within a domain and the relationships between these concepts

Enables the shared representation of knowledge Services Service Process Descriptions Reasoning about this knowledge

Needed to support the discovery and matching of processes through the use of descriptions

Represented through use of Description Logic i.e. Resource Description Framework (RDF)

XML-based language

Semantic Web Services is a current available framework that allows descriptions and searches but only at a service-level and not a process level

Services & Processes –an Ontological Framework

A Basic Process Ontology Defined Concepts

Classes of objects with same properties Represent states in a process

Roles Relations between concepts

<Subject, Property, Object> <Component, hasAuthor, Author>

Two Types Transitional roles

Represent Service Operations Description roles

Properties of a state Pre & post Conditions, Service Name & Description

Constructors Allow complex concepts to be constructed in form of concept description - interpreted as reaching a new state C through role R

Services & Processes –an Ontological Framework

Service Process Ontology Concepts (States) & Roles (Transitional, Descriptive) preState & postState (describe states in terms of conditions)

Services & Processes –an Ontological Framework

Notational for Ontology Language

Services & Processes –an Ontological Framework

Importance of incorporating Role Constructors Role constructors allow composition of services to service

processes

Role constructors allow for data and process parameters

Services & Processes –an Ontological Framework Now with the complete ontological language specified, a service

process can now be specified

Services & Processes –an Ontological Framework Matching Ontologies of Service Processes

Through use of Subsumption Defined as a Subset-relationship Incorporating something into a more generalized class

Subsumption of Concepts

Subsumption of Roles

More Input/Output oriented

Subsumption in terms of simulation Looking at the internal states of a composite role

expression

Services & Processes –an Ontological Framework Example of Matching

UserLogin matches requirements of Login Weaker, less restricted precondition (synax(id) -> True) Stronger postcondition (valid(id) v invalid(id) -> valid(id) v

invalid(id) v unknown())

Services & Processes –an Ontological Framework

Ontological Matching of Service Processes Requires some additional rules

Reachability of a concept A concept description

with composite transitional role P is reachable if the set of transitions is not empty

Consistency of a role A composite role

is consistent if the last state is reachable through transitions.

Conclusion

Paper presented a layered approach to service process composition Presented an Operational Framework Presented an Ontological Framework

These frameworks are created through the use of Pi-Calculus Allows for the mobility of interations by allow the

sending of ports for communication

Thank You!

Questions?