SOAP

SOAP is a simple XML based protocol to let applications exchange information over HTTP. SOAP is a protocol for accessing a Web Service.

Why SOAP?
It is over HTTP which will not be blocked by most of the firewalls or proxy servers.

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

• A required Envelope element that identifies the XML document as a SOAP message
• An optional Header element that contains header information
• A required Body element that contains call and response information
• An optional Fault element that provides information about errors that occurred while processing the message

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 encodingStyle attribute is used to define the data types used in the document.

A SOAP message may travel from a sender to a receiver by passing different endpoints along the message path. Not all parts of the SOAP message may be intended for the ultimate endpoint of the SOAP message but, instead, may be intended for one or more of the endpoints on the message path. The SOAP actor attribute may be used to address the Header element to a particular endpoint.

<?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 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 must fail when processing the Header.

<?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 SOAP Body Element

<?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. 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
An error message from a SOAP message is carried inside a Fault element.
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.