SOAP (Simple Object Access Protocol)

SOAP is a simple XML-based protocol to let applications exchange information over HTTP.SOAP provides a way to communicate between applications running on different operating systems, with different technologies and programming languages.
RPC(Remote Procedure Calls) 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.

What is SOAP:-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 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

SOAP Envelope Element:-
The required SOAP Envelope element is the root element of 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>

"http://www.w3.org/2001/12/soap-envelope%22.The namespace defines the Envelope as a SOAP Envelope.
If a different namespace is used, the application generates an error and discards the message.

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.
<?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.

(i)mustUnderstand Attribute:-
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"

(ii)actor Attribute:-
The SOAP actor attribute is used to address the Header element to a specific endpoint.
Syntax
soap:actor="URI"
example:-
<soap:Header>
  <m:Trans xmlns:m="http://www.w3schools.com/transaction/"
  soap:actor="http://www.w3schools.com/appml/%22%3E234
  </m:Trans>
</soap:Header>

(iii)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.
Syntax
soap:encodingStyle="URI"

SOAP Body Element:-
The required SOAP Body element contains the actual SOAP 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.

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

SOAP HTTP Binding:-
HTTP communicates over TCP/IP. An HTTP client connects to an HTTP server using TCP.
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.