Research Issues in Web Services CS 4244 Lecture Zaki Malik Department of Computer Science Virginia Tech

Document Web Application Web Service Web - Sharing research documents among scientists - Proprietary & closed systems - ad hoc interactions - Emergence of XML-based standards for description, invocation, discovery, … - Ubiquitous protocols (HTTP, …) Web Services time Evolution of the Web

The Web Service Model Web Service Service Registry Service Consumer Service Provider Publish Find message exchange has defines Service Description UDDI Publication UDDI Inquiry SOAP WSDL UDDI RegistryWeb Browser

Motivation Exploratory –The process of selecting Web services is non-deterministic –Web services are a priori unknown –Services should be determined dynamically Volatile –A Web service answering a request at a given time may not be available to answer the same request in the future –Services may become unavailable in the interval between selection and invocation Dynamic –Web service content provided by the operations may change frequently –Change may affect the overall execution of the request

Scenario

Issues –Select Web services with appropriate functionality (Querying) –Must be discovered in a reasonable time (Querying) –Multiple services may provide same functionality (Querying, Reputation) –More than one service might have to be selected (Composition) –Web services may change over a period of time (Change)

Issues Web Services Query Optimization Web Services Trust Web Services Composition Web Services Change Management

Web Services Query Optimization Check the Needed Operations Locate Web Services that Provide the Operations Potential Hindrance –Large Service Space

Web Services Query Optimization Reduce Web Service Search Space Filter out Inefficient Services Choose the Best Web Services

Web Services Query Optimization Need: –Organization of the Large Web Service Space –Compare the Service Execution Plans –Select the Needed Web Services, Based on QoS Quality of Service (QoS) –Set of characteristics of a system, necessary to achieve the required functionality QoS Attributes –Reputation: Objective evaluation of trust –Response Time: Delay between request and response –Availability: Probability of system accessibility –Service Fee: Price demanded for service

Trust: the belief that a service consumer has about the intention and ability of a service provider to act as expected Web Services Trust

Challenges Web Services Trust

Alternatives for Trust Establishment

Reputation Reputation Management –A services reputation within a community of consumers is a perception shared by some or all of the members of that community about that service Approach –Consumers rate Web services –Consumers collaborate to assess services reputation –Trust in services derives from their reputation –Reputation-based service selection and composition

Reputation Collection –Automatic Rating of Web Services –Rating Collection –Bootstrapping Assessment –Metrics and Models Storage Dissemination –Push-based –Pull-based Robustness –Prevention and Detection of Reputation Tampering Trust-based Selection and Composition

Web Services Composition Two or more services come-together to provide a value- added functionality Understanding the semantics of Web services Composability: Can Web service be composed together? Generate composite service descriptions

Web Services Composition RequirementImpact on Service Composition Dynamics- On the fly business relationships - Adaptability to changes Large Size- Fast composition - Scalability (number of services, etc.) Peer-to-Peer- No central authority to manage interactions among component services Autonomy- Component services interact based on their own terms Heterogeneity- Different component service capabilities (communication, content, and business logic)

Declarative Composition –Describing the composite service Composers input (component services, operations,…) Recursive composition –Generating composition plans –Selection of a composition plan Support of Dynamic Relationships –Specification of dynamic relationships –Selection of candidate services (selection policies) –Mapping to selected services (messages, operations, …) Web Services Composition

Web Services Change Management Change Management definition: –Detection Awareness that a change has occurred Subsequent identification of its cause –Propagation Informing all concerned entities in the system that a change has occurred –Reaction Executing a compensatory process that brings the system back to safe execution mode Categories of changes: –Internal Changes that occur inside a Web service Example: change in the content provided by a Web service –External Changes that occur outside of a Web service Example: temporary or permanent unavailability of a service or its operations

Change Management Mechanisms Detection –Service unavailability - agents send frequent alive messages to participant services –Change to operations - compare service descriptions in the registries with the ones in the system –Change in content - periodic invocation of the an operation and comparing the subsequent results Propagation –Web services participating in a service request are registered with a participant list –Participant list is maintained by agents Agents initially add Web service descriptions to the list The Participant list is consulted before a service is invoked Agents remove the service description from the list if change occurs Reaction –Selection of alternate Web services to fulfill a request –Request cancellation if no alternate service is available –Reaction to internal change is in the form of reconsolidating the result

Conclusion A complete Web Services Management System is needed –Services treated as first-class objects –Optimization, Trust, Composition, and Change Management seen as major components