What is Service Oriented Computing?

Service OrientedComputing

Burr Watters

Tasha Wells

April 5, 2004

Overview

What Service Oriented Computing?

Service Oriented Architecture

Web Service Componentization

Service Composition

BPEL4WS

Coordination

Contracts and Port Types

Web Service Transactions

What is Service OrientedComputing?

It is the paradigm for distributed computing toenable application networks to be built usingservices as fundamental elements fordeveloping applications.

Service Oriented Architecture

Management Level

Operations Management:

Support critical applications that require organizations to manage the serviceplatform, the applications and the deployment of services.

The organization responsible for these functions is known as the serviceoperator.

Market Management:

Administration of the marketplace and maintenance to guarantee theadministration is open for business.

The creator/maintainer of a marketplace is known as the market-maker.

Composition Level

Allows consolidation of multiple services into a single compositeservice using the following functionality:

Coordination:

Controls the execution of component services and manages thedataflow between them.

Monitoring:

Monitors events or information produced by component services.

Conformance:

Ensures consistency of composite services by matching parametertypes with parameters of component services

imposes constraints on component services to enforce businessrules.

Quality of Service (QoS) composition:

Combines the component’s QoS to determine the composite QoSsuch as overall cost, performance and security.

Description/Integration Level

Creation of composite service through the following functions:

Service description:

Communicates the service’s capabilities/functionalities, interface,behavior, cost and quality of service.

Service discovery:

Locates appropriate services using requirements and selection criteria.

Service delivery:

Invocation – request for specified service according to agreed terms andconditions.

Provision – the agreed-upon service is supplied within the period agreedto in the contract.

Suspension – establish point where the service will no longer be needed.

Service composition:

Composes service from lower-level services in which the previous threefunctions are known.

Description/Integration Level

Service Protocols

Simple Object Access Protocol (SOAP)

Provides a format for communicating between webservices.

Web Services Description Language (WSDL)

Provides the definition interface for web componentsto communicate.

Universal Description, Discovery, and Integration(UDDI)

Provides a directory that can be used to locateavailable services and learn their details.

Web ServiceComponentization

Web ServiceComponentization

Service Composition

Componentized model becomes a naturalarchitecture

Services become the building blocks of an application.

Service compositions become the “glue” which binds theservices.

Web services must provide standards on:

An interoperability communication protocol,

Mechanisms for service description, discovery, andcommunication, and

Quality of Service

The Web Services Stack

Business Process Execution Language for WebServices (BPEL4WS)

BPEL (for short)

Standardized by the Organization for theAdvancement of Structured Information Standards(OASIS).

Provides a mechanism for defining servicecompositions in the form of choreographies of Webservices.

A choreography consists of the aggregation ofservices to certain business rules.

BPEL Interactions

BPEL composition are called processes.

Services that interact with each other are partners.

Business Protocolsin BPEL

BPEL composition defines the messageexchange between partners.

Partnerships are declared through the WSDLinterfaces that are specified in the BPELprocess.

WSDL provides the abstraction between servicedescription and service deployment.

Provides multi-protocol access to services.

Business Protocolsin BPEL

Facilities the message exchange between theprocess and its partners.

Complex algorithm are created using

Primitive Activities

Signaling faults, Terminating process execution,Manipulating data

Structured Activities

Sequence, Switch, While, Pick, Flow

Message Correlation

Tracks messages received from one partner to another in aparticular conversation.

Fault Handling and Compensationin BPEL

Provides a fault handling to deal with errorsthat may occur in an application.

Similar to a “try-catch” block.

Compensation is the process of rolling backactions that have been completed before afault occurred.

Scopes determine how fault handling /compensation are resolved in a process.

Closed tied to WS-Transactions.

WS-Coordination

Framework for pluggable coordination models.

Allows a set of independently distributed services toreach a consistent and defined outcome.

Elements of WS-Coordination

CoordinationContext – extensible context representing thecoordination that is propagated to the distributedparticipants.

Activiation – used by clients to create a coordinationcontext.

Registration – used by participants to register resources forinclusion in specific coordination protocols.

Coordinating BPEL Web Services

Behavioral Aspects of Service

Connectivity is great, but most of thecomplexity comes with interoperatingservices.

Web services have the autonomy to runindependently of each other.

The heterogeneity of the web causesprocesses to be written in any languageunder any assumptions.

Port Types

A service can send any message to a port aprocess may be running on and expect aresponse.

The port must be able to understand themessage.

Thus, port types describes to the requestingservice what messages the port will respondto.

Convenient because programmers do not have tocode for erroneous messages.

Contracts

Port types do not describe:

Quality of Service

Timeliness

Execute Order

These are prescribed in a contract

Sets the communication protocol for using the port.

Disciplines the channels of communication betweenservice and port.

Allows the service to eliminate ports whose serviceagreements do not conform to its needs.

WS-Transaction

Specifies two types of coordination activities.

Atomic Transaction

Short-running transactions.

Allows the transaction to hold the resources being used by theservice operations.

All resources must be committed before transaction succeeds.

Similar to the two-phase commit protocol.

Business Activity

Long-running business transactions.

Resources are not held; business logic provides error handling.

Allows for participants to register and leave the activity at will.

Participants can declare its outcome before being asked.

Web Service Transactions

Example of Two-Phase Commit Protocol

Are ACID transactions too strong?

Could hold a service until the transaction is completed.

Thus, making it available to other services.

Web Service Transactions

Long running activities

vs.

“Logical” long running activities

Each transaction is confirmed and release when done.

In the event of a rollback, certain compensation activitiesmust may be necessary to get the system back to aconsistent state.

Business Transaction Protocol(BTP)

Developed by Oasis

Gives the appearance of a two-phase commitprotocol without have ACID characteristics

Closed-top commit protocol

Executes entire two-phase commit protocol beforereturning an outcome

Verbs: begin, commit, and rollback

Open-to commit protocol

Allows the application to control the time between the two-phase commit protocol

Verbs: prepare, confirm, and cancel

Business Transaction Protocol(BTP)

BTP introduced two types of extended transactions

Atom Transactions

Traditional method of performing transactions.

All participants see the same outcome.

Cohesion Transactions

Allows business logic to confirm/cancel transaction.

Sets a “cohesion” scope for entire transaction.

Creates Atom scopes for services to “enroll” in.

These Atom scopes come with caveat.

That it may be “canceled” or “prepared”.

That it could take some time before arriving at its “confirm-set”.

Once the “confirm-set” has been reached, then the cohesioncollapses to a atom and the participates see the resultingoutcome.

Summary

Service Oriented Architecture

Management, Composition, and Description/IntegrationLevel

Web Service Componentization

Service Composition

BPEL4WS

Interactions, Protocols, Fault Handling and Compensation

Coordination

Contracts and Port Types

Transactions

Web Service Transactions and BTP

Resources

Papazoglou, M. & Georgakopoulos, D. Service Oriented Computing. Communications of theACM. October 2003. p 25.

Curbera, F., Khalaf, R., Mukhi, N., Tai, S., & Weerawarana, S. The Next Step in Web Services.Communications of the ACM. October 2003. p 29.

Yang, J. Web Service Componentization. Communications of the ACM. October 2003. p 35.

Meredith L. & Bjorg, S. Contracts and Types. Communications of the ACM. October 2003.p 41.

Little, M. Transactions and Web Services. Communications of the ACM. October 2003. p 49.

Tuner, M., Budgen, D., & Brereton, P. Turning Software into a Service. IEEE Computer.October 2003. p 38.

Curbera, F., Duftler, M., Khalaf, R., Nagy, W., Mukhi, N., & Weerawarana, S. Unraveling theWeb Services Web: An Introduction of SOAP, WSDL, and UDDI. IEEE InternetComputing. March/April 2003. p 86.

Questions?

Thank you!