Basics of Web Services

Web Services

Web Services can convert your application into a Web-application, which can publish its function or message to the rest of the world.

The basic Web Services platform is XML + HTTP.

Web Services can convert your applications into Web-applications.

Web Services are published, found, and used through the Web.

 

What You Should Already Know

Before you continue, you should have a basic understanding of the following:

  • HTML
  • XML

If you want to study these subjects first, find the tutorials on W3C website.

 What are Web Services?

  • Web services are application components
  • Web services communicate using open protocols
  • Web services are self-contained and self-describing
  • Web services can be discovered using UDDI
  • Web services can be used by other applications
  • XML is the basis for Web services

 

How Does it Work?

The basic Web services platform is XML + HTTP.

XML provides a language which can be used between different platforms and programming languages and still express complex messages and functions.

The HTTP protocol is the most used Internet protocol.

Web services platform elements:

  • SOAP (Simple Object Access Protocol)
  • UDDI (Universal Description, Discovery and Integration)
  • WSDL (Web Services Description Language)

 

Why web services?

A few years ago Web services were not fast enough to be interesting.

 

Interoperability has Highest Priority

When all major platforms could access the Web using Web browsers, different platforms couldn’t interact. For these platforms to work together, Web-applications were developed.

Web-applications are simply applications that run on the web. These are built around the Web browser standards and can be used by any browser on any platform.

 

Web Services take Web-applications to the Next Level

By using Web services, your application can publish its function or message to the rest of the world.

Web services use XML to code and to decode data, and SOAP to transport it (using open protocols).

With Web services, your accounting department’s Win 2k server’s billing system can connect with your IT supplier’s UNIX server.

 

Web Services have Two Types of Uses

Reusable application-components.

There are things applications need very often. So why make these over and over again?

Web services can offer application-components like: currency conversion, weather reports, or even language translation as services.

Connect existing software.

Web services can help to solve the interoperability problem by giving different applications a way to link their data.

With Web services you can exchange data between different applications and different platforms.

Web Services Platform Elements 

Web Services have three basic platform elements: SOAP, WSDL and UDDI.

 What is SOAP?

SOAP is an XML-based protocol to let applications exchange information over HTTP.

Or more simple: SOAP is a protocol for accessing a Web Service.

  • SOAP stands for Simple Object Access Protocol
  • SOAP is a communication protocol
  • SOAP is a format for sending messages
  • SOAP is designed to communicate via Internet
  • SOAP is platform independent
  • SOAP is language independent
  • SOAP is based on XML
  • SOAP is simple and extensible
  • SOAP allows you to get around firewalls
  • SOAP is a W3C standard

Read more about SOAP on W3C Website.

What is WSDL?

WSDL is an XML-based language for locating and describing Web services.

  • WSDL stands for Web Services Description Language
  • WSDL is based on XML
  • WSDL is used to describe Web services
  • WSDL is used to locate Web services
  • WSDL is a W3C standard

What is UDDI?

UDDI is a directory service where companies can register and search for Web services.

  • UDDI stands for Universal Description, Discovery and Integration
  • UDDI is a directory for storing information about web services
  • UDDI is a directory of web service interfaces described by WSDL
  • UDDI communicates via SOAP
  • UDDI is built into the Microsoft .NET platform

 

WSDL

What You Should Already Know

Before you continue you should have a basic understanding of the following:

  • XML
  • XML Namespaces
  • XML Schema

If you want to study these subjects first, find the tutorials on our Home page.

 

What is WSDL?

  • WSDL stands for Web Services Description Language
  • WSDL is written in XML
  • WSDL is an XML document
  • WSDL is used to describe Web services
  • WSDL is also used to locate Web services
  • WSDL is a W3C recommendation

 

WSDL Describes Web Services

WSDL stands for Web Services Description Language.

WSDL is a document written in XML. The document describes a Web service. It specifies the location of the service and the operations (or methods) the service exposes.

 

WSDL is a W3C Recommendation

WSDL became a W3C Recommendation 26. June 2007.

You can read more about the XML Schema standard in  W3C tutorial.

A WSDL document is just a simple XML document.

It contains set of definitions to describe a web service.

 

The WSDL Document Structure

A WSDL document describes a web service using these major elements:

Element Description
<types> A container for data type definitions used by the web service
<message> A typed definition of the data being communicated
<portType> A set of operations supported by one or more endpoints
<binding> A protocol and data format specification for a particular port type

The main structure of a WSDL document looks like this:

<definitions>

 

<types>

data type definitions……..

</types>

 

<message>

definition of the data being communicated….

</message>

 

<portType>

set of operations……

</portType>

 

<binding>

protocol and data format specification….

</binding>

 

</definitions>

A WSDL document can also contain other elements, like extension elements, and a service element that makes it possible to group together the definitions of several web services in one single WSDL document.

 

WSDL Ports

The <portType> element is the most important WSDL element.

It describes a web service, the operations that can be performed, and the messages that are involved.

The <portType> element can be compared to a function library (or a module, or a class) in a traditional programming language.

 

WSDL Messages

The <message> element defines the data elements of an operation.

Each message can consist of one or more parts. The parts can be compared to the parameters of a function call in a traditional programming language.

 

WSDL Types

The <types> element defines the data types that are used by the web service.

For maximum platform neutrality, WSDL uses XML Schema syntax to define data types.

 

WSDL Bindings

The <binding> element defines the data format and protocol for each port type.

 

WSDL Example

This is a simplified fraction of a WSDL document:

<message name=”getTermRequest”>

<part name=”term” type=”xs:string”/>

</message>

 

<message name=”getTermResponse”>

<part name=”value” type=”xs:string”/>

</message>

 

<portType name=”glossaryTerms”>

<operation name=”getTerm”>

<input message=”getTermRequest”/>

<output message=”getTermResponse”/>

</operation>

</portType>

In this example the <portType> element defines “glossaryTerms” as the name of a port, and “getTerm” as the name of an operation.

The “getTerm” operation has an input message called “getTermRequest” and an output message called “getTermResponse”.

The <message> elements define the parts of each message and the associated data types.

Compared to traditional programming, glossaryTerms is a function library, “getTerm” is a function with “getTermRequest” as the input parameter, and getTermResponse as the return parameter.

 

The <portType> element is the most important WSDL element.

 

WSDL – The <portType> Element

The <portType> element defines a web service, the operations that can be performed, and the messages that are involved.

<portType> defines the connection point to a web service. It can be compared to a function library (or a module, or a class) in a traditional programming language. Each operation can be compared to a function in a traditional programming language.

 

Operation Types

The request-response type is the most common operation type, but WSDL defines four types:

Type Definition
One-way The operation can receive a message but will not return a response
Request-response The operation can receive a request and will return a response
Solicit-response The operation can send a request and will wait for a response
Notification The operation can send a message but will not wait for a response

 

One-Way Operation

A one-way operation example:

<message name=”newTermValues”>

<part name=”term” type=”xs:string”/>

<part name=”value” type=”xs:string”/>

</message>

 

<portType name=”glossaryTerms”>

<operation name=”setTerm”>

<input name=”newTerm” message=”newTermValues”/>

</operation>

</portType >

In the example above, the portType “glossaryTerms” defines a one-way operation called “setTerm”.

The “setTerm” operation allows input of new glossary terms messages using a “newTermValues” message with the input parameters “term” and “value”. However, no output is defined for the operation.

 

Request-Response Operation

A request-response operation example:

<message name=”getTermRequest”>

<part name=”term” type=”xs:string”/>

</message>

 

<message name=”getTermResponse”>

<part name=”value” type=”xs:string”/>

</message>

 

<portType name=”glossaryTerms”>

<operation name=”getTerm”>

<input message=”getTermRequest”/>

<output message=”getTermResponse”/>

</operation>

</portType>

In the example above, the portType “glossaryTerms” defines a request-response operation called “getTerm”.

The “getTerm” operation requires an input message called “getTermRequest” with a parameter called “term”, and will return an output message called “getTermResponse” with a parameter called “value”.

 

 

WSDL bindings defines the message format and protocol details for a web service.

 

Binding to SOAP

A request-response operation example:

<message name=”getTermRequest”>

<part name=”term” type=”xs:string”/>

</message>

 

<message name=”getTermResponse”>

<part name=”value” type=”xs:string”/>

</message>

 

<portType name=”glossaryTerms”>

<operation name=”getTerm”>

<input message=”getTermRequest”/>

<output message=”getTermResponse”/>

</operation>

</portType>

 

<binding type=”glossaryTerms” name=”b1″>

<soap:binding style=”document”

transport=”http://schemas.xmlsoap.org/soap/http” />

<operation>

<soap:operation soapAction=”http://example.com/getTerm”/>

<input><soap:body use=”literal”/></input>

<output><soap:body use=”literal”/></output>

</operation>

</binding>

The binding element has two attributes – name and type.

The name attribute (you can use any name you want) defines the name of the binding, and the type attribute points to the port for the binding, in this case the “glossaryTerms” port.

The soap:binding element has two attributes – style and transport.

The style attribute can be “rpc” or “document”. In this case we use document. The transport attribute defines the SOAP protocol to use. In this case we use HTTP.

The operation element defines each operation that the portType exposes.

For each operation the corresponding SOAP action has to be defined. You must also specify how the input and output are encoded. In this case we use “literal”.

 

 

UDDI

Universal Description, Discovery and Integration (UDDI) is a directory service where businesses can register and search for Web services.

 

What is UDDI

UDDI is a platform-independent framework for describing services, discovering businesses, and integrating business services by using the Internet.

  • UDDI stands for Universal Description, Discovery and Integration
  • UDDI is a directory for storing information about web services
  • UDDI is a directory of web service interfaces described by WSDL
  • UDDI communicates via SOAP
  • UDDI is built into the Microsoft .NET platform

 

What is UDDI Based On?

UDDI uses World Wide Web Consortium (W3C) and Internet Engineering Task Force (IETF) Internet standards such as XML, HTTP, and DNS protocols.

UDDI uses WSDL to describe interfaces to web services

Additionally, cross platform programming features are addressed by adopting SOAP, known as XML Protocol messaging specifications found at the W3C Web site.

 

UDDI Benefits

Any industry or businesses of all sizes can benefit from UDDI.

Before UDDI, there was no Internet standard for businesses to reach their customers and partners with information about their products and services. Nor was there a method of how to integrate into each other’s systems and processes.

Problems the UDDI specification can help to solve:

  • Making it possible to discover the right business from the millions currently online
  • Defining how to enable commerce once the preferred business is discovered
  • Reaching new customers and increasing access to current customers
  • Expanding offerings and extending market reach
  • Solving customer-driven need to remove barriers to allow for rapid participation in the global Internet economy
  • Describing services and business processes programmatically in a single, open, and secure environment

 

How can UDDI be Used

If the industry published an UDDI standard for flight rate checking and reservation, airlines could register their services into an UDDI directory. Travel agencies could then search the UDDI directory to find the airline’s reservation interface. When the interface is found, the travel agency can communicate with the service immediately because it uses a well-defined reservation interface.

 

Who is Supporting UDDI?

UDDI is a cross-industry effort driven by all major platform and software providers like Dell, Fujitsu, HP, Hitachi, IBM, Intel, Microsoft, Oracle, SAP, and Sun, as well as a large community of marketplace operators, and e-business leaders.

Over 220 companies are members of the UDDI community.

 

 

SOAP

 

SOAP is a simple XML-based protocol to let applications exchange information over HTTP.

Or more simply: SOAP is a protocol for accessing a Web Service.

 

What You Should Already Know

Before you study SOAP you should have a basic understanding of XML and XML Namespaces.

What is SOAP?

  • SOAP stands for Simple Object Access Protocol
  • SOAP is a communication protocol
  • SOAP is for communication between applications
  • SOAP is a format for sending messages
  • SOAP communicates via Internet
  • SOAP is platform independent
  • SOAP is language independent
  • SOAP is based on XML
  • SOAP is simple and extensible
  • SOAP allows you to get around firewalls
  • SOAP is a W3C recommendation

 

Why SOAP?

It is important for application development to allow Internet communication between programs.

Today’s applications communicate using Remote Procedure Calls (RPC) between objects like DCOM and CORBA, but HTTP was not designed for this. RPC represents a compatibility and security problem; firewalls and proxy servers will normally block this kind of traffic.

A better way to communicate between applications is over HTTP, because HTTP is supported by all Internet browsers and servers. SOAP was created to accomplish this.

SOAP provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.

 

SOAP is a W3C Recommendation

SOAP became a W3C Recommendation 24. June 2003.

SOAP Building Blocks

A SOAP message is an ordinary XML document containing the following elements:

  • An Envelope element that identifies the XML document as a SOAP message
  • A Header element that contains header information
  • A Body element that contains call and response information
  • A Fault element containing errors and status information

All the elements above are declared in the default namespace for the SOAP envelope:

http://www.w3.org/2001/12/soap-envelope

and the default namespace for SOAP encoding and data types is:

http://www.w3.org/2001/12/soap-encoding

 

Syntax Rules

Here are some important syntax rules:

  • A SOAP message MUST be encoded using XML
  • A SOAP message MUST use the SOAP Envelope namespace
  • A SOAP message MUST use the SOAP Encoding namespace
  • A SOAP message must NOT contain a DTD reference
  • A SOAP message must NOT contain XML Processing Instructions

 

Skeleton SOAP Message

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Header>

</soap:Header>

 

<soap:Body>

<soap:Fault>

</soap:Fault>

</soap:Body>

 

</soap:Envelope>

 

The SOAP Envelope element is the root element of a SOAP message.

 

The SOAP Envelope Element

The required SOAP Envelope element is the root element of a SOAP message. This element defines the XML document as a SOAP message.

Example

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

Message information goes here

</soap:Envelope>

 

The xmlns:soap Namespace

Notice the xmlns:soap namespace in the example above. It should always have the value of: “http://www.w3.org/2001/12/soap-envelope“.

The namespace defines the Envelope as a SOAP Envelope.

If a different namespace is used, the application generates an error and discards the message.

 

The encodingStyle Attribute

The encodingStyle attribute is used to define the data types used in the document. This attribute may appear on any SOAP element, and applies to the element’s contents and all child elements.

A SOAP message has no default encoding.

Syntax

soap:encodingStyle=”URI

Example

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

Message information goes here

</soap:Envelope>

 

 

The SOAP Header element contains header information.

 

The SOAP Header Element

The optional SOAP Header element contains application-specific information (like authentication, payment, etc) about the SOAP message.

If the Header element is present, it must be the first child element of the Envelope element.

Note: All immediate child elements of the Header element must be namespace-qualified.

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Header>

<m:Trans xmlns:m=”http://www.w3schools.com/transaction/

soap:mustUnderstand=”1″>234

</m:Trans>

</soap:Header>

</soap:Envelope>

The example above contains a header with a “Trans” element, a “mustUnderstand” attribute with a value of 1, and a value of 234.

SOAP defines three attributes in the default namespace (“http://www.w3.org/2001/12/soap-envelope“). These attributes are: mustUnderstand, actor, and encodingStyle.

The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.

 

The mustUnderstand Attribute

The SOAP mustUnderstand attribute can be used to indicate whether a header entry is mandatory or optional for the recipient to process.

If you add mustUnderstand=”1″ to a child element of the Header element it indicates that the receiver processing the Header must recognize the element. If the receiver does not recognize the element it will fail when processing the Header.

Syntax

soap:mustUnderstand=”0|1″

Example

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Header>

<m:Trans xmlns:m=”http://www.w3schools.com/transaction/

soap:mustUnderstand=”1″>234

</m:Trans>

</soap:Header>

</soap:Envelope>

 

The actor Attribute

A SOAP message may travel from a sender to a receiver by passing different endpoints along the message path. However, not all parts of a SOAP message may be intended for the ultimate endpoint, instead, it may be intended for one or more of the endpoints on the message path.

The SOAP actor attribute is used to address the Header element to a specific endpoint.

Syntax

soap:actor=”URI

Example

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Header>

<m:Trans xmlns:m=”http://www.w3schools.com/transaction/

soap:actor=”http://www.w3schools.com/appml/“>234

</m:Trans>

</soap:Header>

</soap:Envelope>

 

The encodingStyle Attribute

The encodingStyle attribute is used to define the data types used in the document. This attribute may appear on any SOAP element, and it will apply to that element’s contents and all child elements.

A SOAP message has no default encoding.

Syntax

soap:encodingStyle=”URI

 

The SOAP Body element contains the actual SOAP message.

 

The SOAP Body Element

The required SOAP Body element contains the actual SOAP message intended for the ultimate endpoint of the message.

Immediate child elements of the SOAP Body element may be namespace-qualified.

Example

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Body>

<m:GetPrice xmlns:m=”http://www.w3schools.com/prices“>

<m:Item>Apples</m:Item>

</m:GetPrice>

</soap:Body>

 

</soap:Envelope>

The example above requests the price of apples. Note that the m:GetPrice and the Item elements above are application-specific elements. They are not a part of the SOAP namespace.

A SOAP response could look something like this:

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Body>

<m:GetPriceResponse xmlns:m=”http://www.w3schools.com/prices“>

<m:Price>1.90</m:Price>

</m:GetPriceResponse>

</soap:Body>

 

</soap:Envelope>

 

 

 

The SOAP Fault element holds errors and status information for a SOAP message.

 

The SOAP Fault Element

The optional SOAP Fault element is used to indicate error messages.

If a Fault element is present, it must appear as a child element of the Body element. A Fault element can only appear once in a SOAP message.

The SOAP Fault element has the following sub elements:

Sub Element Description
<faultcode> A code for identifying the fault
<faultstring> A human readable explanation of the fault
<faultactor> Information about who caused the fault to happen
<detail> Holds application specific error information related to the Body element

SOAP Fault Codes

The faultcode values defined below must be used in the faultcode element when describing faults:

Error Description
VersionMismatch Found an invalid namespace for the SOAP Envelope element
MustUnderstand An immediate child element of the Header element, with the mustUnderstand attribute set to “1”, was not understood
Client The message was incorrectly formed or contained incorrect information
Server There was a problem with the server so the message could not proceed

 

 

The HTTP Protocol

HTTP communicates over TCP/IP. An HTTP client connects to an HTTP server using TCP. After establishing a connection, the client can send an HTTP request message to the server:

POST /item HTTP/1.1

Host: 189.123.255.239

Content-Type: text/plain

Content-Length: 200

The server then processes the request and sends an HTTP response back to the client. The response contains a status code that indicates the status of the request:

200 OK

Content-Type: text/plain

Content-Length: 200

In the example above, the server returned a status code of 200. This is the standard success code for HTTP.

If the server could not decode the request, it could have returned something like this:

400 Bad Request

Content-Length: 0

 

SOAP HTTP Binding

A SOAP method is an HTTP request/response that complies with the SOAP encoding rules.

HTTP + XML = SOAP

A SOAP request could be an HTTP POST or an HTTP GET request.

The HTTP POST request specifies at least two HTTP headers: Content-Type and Content-Length.

 

Content-Type

The Content-Type header for a SOAP request and response defines the MIME type for the message and the character encoding (optional) used for the XML body of the request or response.

Syntax

Content-Type: MIMEType; charset=character-encoding

Example

POST /item HTTP/1.1

Content-Type: application/soap+xml; charset=utf-8

 

Content-Length

The Content-Length header for a SOAP request and response specifies the number of bytes in the body of the request or response.

Syntax

Content-Length: bytes

Example

POST /item HTTP/1.1

Content-Type: application/soap+xml; charset=utf-8

Content-Length: 250

 

A SOAP Example

In the example below, a GetStockPrice request is sent to a server. The request has a StockName parameter, and a Price parameter that will be returned in the response. The namespace for the function is defined in “http://www.example.org/stock“.

A SOAP request:

POST /InStock HTTP/1.1

Host: www.example.org

Content-Type: application/soap+xml; charset=utf-8

Content-Length: nnn

 

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Body xmlns:m=”http://www.example.org/stock“>

<m:GetStockPrice>

<m:StockName>IBM</m:StockName>

</m:GetStockPrice>

</soap:Body>

 

</soap:Envelope>

The SOAP response:

HTTP/1.1 200 OK

Content-Type: application/soap+xml; charset=utf-8

Content-Length: nnn

 

<?xml version=”1.0″?>

<soap:Envelope

xmlns:soap=”http://www.w3.org/2001/12/soap-envelope

soap:encodingStyle=”http://www.w3.org/2001/12/soap-encoding“>

 

<soap:Body xmlns:m=”http://www.example.org/stock“>

<m:GetStockPriceResponse>

<m:Price>34.5</m:Price>

</m:GetStockPriceResponse>

</soap:Body>

 

</soap:Envelope>

 

 

SOAP Summary

This tutorial has taught you how to use SOAP to exchange information between applications over HTTP.

You have learned about the different elements and attributes in a SOAP message.

You have also learned how use SOAP as a protocol for accessing a web service.

 

Now You Know SOAP, What’s Next?

The next step is to learn about WSDL and Web Services.

WSDL

WSDL is an XML-based language for describing Web services and how to access them.

WSDL describes a web service, along with the message format and protocol details for the web service.

Web Services

Web services can convert your applications into web-applications.

By using XML,  messages can be sent between applications.

 

 

 

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