SOAP Web Service Introduction or FAQ

1)What is Web service?

A simple definition for a web service is 

Web Service is an application or business logic that makes itself available over the internet and is accessible using standard Internet protocols

W3C definition for a web service is

A software system designed to support interoperable machine-to-machine interaction over a network

2)Why you need to learn web services?

 Interoperability

 It is not tied to any one operating system or programming language.A java based application on Windows can communicate with a .Net based one on Linux.Web services are browsers and operating system independent service, which means it can run on any browser without the need of making any changes. Web Services take Web-applications to the Next Level

Exposing the Existing Function on the network

Web services allows you to expose the functionality of your existing code over the network. Once it is exposed on the network, other application can use the functionality of your program

 Loosely Coupled:

 Each service exists independently of the other services that make up the application. Individual pieces of the application to be modified without impacting unrelated areas.

 

3)Different Favors of Web Services

There are two major categories of web services

1. SOAP (Simple Object Access Protocol) Web Service(JAX-WS)
2. REST (Representation State Transfer) Web  Service(JAXRS)

In simple words SOAP web services uses XML as a mechanism for exchanging of information (one way) between the two programs (end points) over any transport layer protocol. SOAP is not relying on any particular programming language or operating system as far as they understand XML and SOAP message

In REST, every resource is addressed or accessed by Unique URI (Uniform Resource Identifier) and data and functionality forms a resource

Instead of using XML for request/response, it relies on simple unique URI and that’s why it is also considered as lightweight alternative to SOAP. As it is rely on URI as a resource identifier, the standard CRUD operations of HTTP (GET, PUT, DELETE, POST, HEAD) can be performed on that URI/resource.

SOAP provides the response in XML format only while REST doesn’t have to use XML only. Though XML and HTML formats are often used for REST response, REST can also response in JSON, CSV and RSS formats

4)What are the few example for free and 

commercial implementations available for Web

 Services?

•      Axis 2 (Axis 1 is now obsolete)
•      JAX-WS Reference Implementation
•      JAX-RS Reference Implementation (Jersey ,RESTeasy.)
•      Apache CXF (formerly called XFire)
•      JBossWS
•      SpringWS

5)SOAP Web Services - Architecture

Role Based Architecture

There are three major roles within the web service architecture:

• Service Provider
• Service Requestor
• Service Registry

Service Provider

This is the provider of the web service. The service provider implements the service and makes it available on the Internet.

Service Requestor

This is any consumer of the web service. The requestor utilizes an existing web service by opening a network connection and sending an XML request.

Service Registory

This is a logically centralized directory of services.The registry provides a central place where  developers can publish new services or find existing ones.

Protocol Stack-Architecture

Second option for viewing the web service architecture is to examine the emerging web service protocol stack

• Service Transport-This layer is responsible for transporting messages between applications. Currently, this layer includesHyper Text Transport Protocol (HTTP),Simple Mail Transfer Protocol (SMTP),File Transfer Protocol (FTP) etc.
• XML Messaging-This layer is responsible for encoding messages in a common XML format so that messages can be understood at either end. Currently, this layer includes XML-RPC and SOAP.
• Service Description-This layer is responsible for describing the public interface to a specific web service. 
• Service Discovery-This layer is responsible for centralizing services into a common registry and providing easy publish/find functionality. Currently, service discovery is handled via Universal Description, Discovery, and Integration (UDDI).

6)Some jargons used in SOAP Web services

WSDL (Web Services Description Language)

Also maintained by the W3C, WSDL is an XML-based format for describing Web services.The WSDL document that describes a Web service acts as a contract between Web service client and server. By adhering to this contact the service provider and consumer are able to exchange data in a standard way, regardless of the underlying platforms and applications on which they are operating.Through the WSDL, a Web services client learns where a service can be accessed, what operations the service performs, the communication protocols the service supports, and the correct format for sending messages to the service.

Primary purpose to describe
The methods(service available)
The Parameter and data type
The network protocols
The location of the web service and how to access it

A WSDL file is an XML document that describes a Web service using six main elements:

Types – defines the data types (as defined in an XML Schema) used by theservice for sending messages between the client and server.

Message – describes the names and format of the messages supported by service for sending messages between the client and server.
Port type – groups and describes the operations performed by the service through the defined interface.

Service – It is a collection of ports

Port – specifies an address for a binding, i.e., defines a communication port.
Binding – defines the communication protocols supported by the operations provided by the service.

Simple Object Access Protocol(SOAP):

SOAP is an XML-based protocol from the W3C for exchanging data over HTTP.

SOAP is mechanism (protocol) for transferring information (messages) between applications which may be widely distributed.

Web services use SOAP to send messages between a service and its client(s). Because HTTP is supported by all Web servers and browsers, SOAP messages can be sent between applications regardless of their platform or programming language. This quality gives Web services their characteristic interoperability.

SOAP messages are XML documents that contain some or all of the following elements:

•     SOAP envelope: It is a root element and wraps the entire soap document
•      SOAP header: This optional element sometime contains application-specific information for the web service exposed like UsernameToken for security purpose.The header contains additional information to be passed for the web service exposed– like UsernameToken for security purpose
•     SOAP body: This is mandatory element of the SOAP envelope and contains the request information/message for the recipient/client-includes the message payload.
•     SOAP fault: This is used to report errors and child element of SOAP body, if present. Mostly we can see fault on the response message as either web service returns well-constructed response message or soap-fault

UDDI(Universal Description, Discovery and Integration )

•    UDDI is a standard of the Organization for the  Advancement of Structured Information Standards (OASIS)
•     Universal Description, Discovery and Integration (UDDI) is a directory service. Web services can register with a UDDI and make themselves available through it for discovery.
•    The UDDI allows clients to search this registry, find the intended service, and retrieve its details
•    The Home Page of the UDDI is http://uddi.xml.org/uddi-org

SOAP Web Service Design

There are two approaches in implementing a web service and they are bottom-up and top-down.

Contract last or Bottom Up Approach

Bottom up approach is where we first define the logic of a web service and then using that we will build the interface. Service code is written first and then the WSDL is created using the service code.

Contract first or Top Down Approach

Top down is the reverse of bottom up approach first .Here you first define web service contract. The complete service definition, message format, transport protocol, security and everything is described in WSDL. Then service is written after the WSDL. Using that wsdl the skeleton code is generated automatically and after that the service code is filled up.

A walkthrough examples using SOAP Web Service using JAX-WS 

1) Open Eclipse IDE

2) Create java project named " JAX-WSExample "

3) Create new package named " com.xyz.webservice.jaxws.example "

4) Create JAXWS Service Endpoint Interface. HelloWorld.java

5) Create JAXWS Service Endpoint implementation class.
HelloWorldImpl.java 

6) Create Endpoint publisher.
HelloWorldWSPublisher.java

6) Generated WSDL Verification