WEB SERVICES:Beyond Component-

Based Computing

by

Gaurav Sharma

Sri Harsha Reddy Meda

The Objective

B2B applications are expected to drive business processes that are no longer constrained by computer related or company related boundaries.

Limitations of existing OO middleware Middleware technologies like CORBA, COM, EJB create islands, which remain separated hindering the communication and collaboration between components over the Internet. The naïve combination of component-based middleware and Web technologies has proven insufficient for such integration. Enterprise Application Integration(EAI) solutions are proprietary, complex to use and not interoperable.

The solution: A Web-based Middleware

Overview of the common middleware architecture

Client and Servant Proxies Broker Architectures Generation of Glue

Client and Servant proxies Proxy is an intermediate layer between the client and the servant to enable distribution transparency. At runtime proxy acts as an interceptor. One proxy at the servant side and one at the client side for providing location transparency. Method invocation paradigm is used to represent the request, responses and faults. Message passing is used to handle method invocation. Problems:

Server activation and deactivation Hard coding of location information on proxies.

Broker Architectures The broker component is

Globally available Introduces a central repository to map logical object references to physical ports.

Multiple servants reside on the same server. Servers register with central broker component using shared library or central data store. Servers listen on port assigned by broker for incoming requests. Clients access the broker for the servant’s communication endpoint, a dedicated channel is then established for communication. Problems like

Generation of proxies Is a tedious process, error prone, time consuming.

Client Proxy

MarshalUnmarshal

Receive_resultService_P

Client Proxy

MarshalUnmarshal

Receive_resultService_P

Broker

Srv_registrationSrv_lookup

Transmit_message

Broker

Srv_registrationSrv_lookup

Transmit_message

Server Proxy

MarshalUnmarshalDispatch

Receive_request

Server Proxy

MarshalUnmarshalDispatch

Receive_request

Client

Call_service_PStart_task

Client

Call_service_PStart_task

Servant

Service_i

Servant

Service_i

Server

Start_upMain_loopDispatch

Server

Start_upMain_loopDispatch

Client Broker ServerClient Proxy Server Proxy

method call locate server

server port

marshal

marshal

unmarshal

unmarshal

dispatch

start_upassign port

register

Generation of Glue Generation of glue is automated by using tools. Express structural interface related information independent of platform, communication and also programming language. IDL is used to describe the interfaces in

a programming language using coding conventions or a programming language independent way.

Generator parses the interface definitions and automatically generates client side and server side proxies.

Why Web-based Middleware? The Broker pattern describes the common

architectural concepts behind various OO middleware. But this is implemented differently by each of them. An example of this limitation is that it is almost impossible to invoke a CORBA servant from a Web-based COM client without using proprietary solutions.

Technologies like COM, CORBA were not implemented with the Web in mind. Thus, additional efforts are required to transmit CORBA or COM method requests over HTTP.

Usage of static glue limits their ability to meet dynamic requirements of the Web.

Thus, integrating existing OO middleware into the Web is not the solution to achieve interoperability.

The Solution(Web-based Middleware)

Integrate the Web into OO middleware. Implement the Broker architecture using

Internet standard protocols and XML.

Steps required

Implementing the Communication Protocol

Implementing an Interface Definition Language (IDL)

Implementing a Service Directory

SOAP - The Communication Protocol

SOAP (Simple Object Access Protocol) is used to expose useful functionality through the Web.

Defines the syntax, semantics and order of messages exchanged between peers a decentralized, distributed environment.

Leverages HTTP. XML (a self-describing data representation) is

used as to structure and encode messages, which makes it easier to interpret messages on the fly.

SOAP MessageA SOAP message is based on XML and contains

the following parts Envelope: Top-level container to represent the message. Header: Generic container for added features to a SOAP

message. Body: Container for mandatory information intended for the

ultimate message receiver.

EnvelopeEnvelope

HeaderHeader

BodyBody

SOAP Message (contd)

<?xml version="1.0" encoding="UTF-8" ?><env:Envelope xmlns:env="http://www.w3.org/2001/09/soap-envelope"> <env:Header> <n:someattribute xmlns:n="http://example.org/someattribute"> <n:priority>1</n:priority> <n:expires>2001-06-22T14:00:00-05:00</n:expires> </n:someattribute> </env:Header> <env:Body> <m:someinfo xmlns:m="http://example.org/someinfo"> <m:msg>Please get the expert</m:msg> </m:someinfo> </env:Body></env:Envelope>

WSDL - The IDL

WSDL (Web Services Definition Language) defines an XML grammar for describing the services as collections of communication endpoints capable of exchanging messages.

Used to automatically generate the client-side & the server-side proxies.

WSDL structure

Types: Provides data type definitions used to describe the messages exchanged.

Message: Represents an abstract definition of the data being transmitted. A message consists of logical parts, each of which is associated with a definition within some type system.

PortType: Set of abstract operations. Each operation refers to an input message and output messages.

Binding: Specifies concrete protocol and data format specifications for the operations and messages defined by a particular portType.

Port: Specifies an address for a binding, thus defining a single communication endpoint.

Service: Used to aggregate a set of related ports.

UDDI – The Service Directory

UDDI (Universal Discovery, Description & Integration) provides all functionality of a service broker.

It is a specification for distributed Web-based information registries of Web Services.

UDDI registries are used to promote and discover these distributed Web Services.

Web Services Operation

UDDI Service Registry

UDDI Service Registry

ClientClientWeb Serverhosting the

Web Service

Web Serverhosting the

Web Service

Inquire the Web Service Publish the Web Service

Transfer WSDL

Request over SOAP

Response over SOAP

Web Services Operation (contd)

Back-end Enterprise

System

Back-end Enterprise

System

Web ServiceWeb Service

Web ServiceWeb Service

Connectors

Incoming Requests

Incoming Requests

Web Server

A Broker is not enough

For building sophisticated solutions it is not sufficient to provide a stack of protocols.

Information & services must be consumable from any device, any place, anytime. Can be achieved using a virtual execution system (JVM or MS CLR)

Web Services infrastructure should support context information such as transaction IDs, security & location.

Represent gateways to existing back-end servers. This integration can take advantage of the existing OO middleware technologies.

Reuse & integration are support by introducing core services.

Integration via Web Services

Include virtual execution engines that can be ported to new environments quickly.

SOAP, WSDL & UDDI help to easily integrate existing OO middleware into the Web. Web Services can be connected to CORBA, COM, EJB.

X-Schema, XSLT used for data representation & transformation across heterogeneous applications.

Drawbacks

Web Services is an immature technology.

Interoperability cannot be guaranteed since the used standards are still evolving.

Web Services lack OO concepts such as inheritance, polymorphism, or even the notion of objects.

Conclusion

Existing OO middleware may still be used to implement complex back-end services, but Web Services will be used to connect these islands across the Internet.

References

Michael Stal, “Web Services: Beyond Component-Based Computing”, Communications of the ACM, Oct 2002/Vol 45, No. 10

Philippe Mougin & Christophe Barriolade, “Web Services, Business Objects and Component Models”, Orchestra Networks, White Paper - July 2001

http://www.w3.org/TR/SOAP http://www.w3.org/TR/wsdl http://middleware.internet2.edu

Q&A

Thank You