SOAP Introduction

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 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>