Tuesday, 31 May 2011

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 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, 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 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

 

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
<?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 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 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.

 

The SOAP Header Element

The SOAP Header element contains header information. 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>
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"

 

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"

<soap:Header>
  <m:Trans xmlns:m="http://www.w3schools.com/transaction/"
  soap:actor="http://www.w3schools.com/appml/">234
  </m:Trans>
</soap:Header>

 

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.
Ex:
<?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

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

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

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