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 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 will be developed as a W3C standard

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.

Microsoft and SOAP

SOAP is a key element of Microsoft's .NET architecture for future Internet application development.

SOAP 1.1 was Proposed to W3C

UserLand, Ariba, Commerce One, Compaq, Developmentor, HP, IBM, IONA, Lotus, Microsoft, and SAP proposed to W3C, in May 2000, the SOAP Internet protocol that they hope will revolutionize application development by connecting graphic user interface desktop applications to powerful Internet servers using the standards of the Internet: HTTP and XML.

W3C is Working on SOAP 1.2

SOAP Building Blocks

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

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

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

  ...
  ...

  ...
  ...
  
    ...
    ...
  

The SOAP Envelope Element

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

Note the use of the xmlns:soap namespace. It should always have the value of:

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

and it defines the Envelope as a 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
  ...

The xmlns:soap Namespace

A SOAP message must always have an Envelope element associated with the "http://www.w3.org/2001/12/soap-envelope" namespace.

If a different namespace is used, the application must generate an error and discard the message.

The encodingStyle Attribute

The SOAP 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"

Example

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 Header Element

The optional 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.

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

xmlns:m="http://www.w3schools.com/transaction/"
soap:mustUnderstand="1">234

...
...

The example above contains a header with a "Trans" element, a "mustUnderstand" attribute 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: actor, mustUnderstand, and encodingStyle. The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.

The actor Attribute

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.

Syntax

soap:actor="URI"

Example

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

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

...
...

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 must fail when processing the Header.

Syntax

soap:mustUnderstand="0|1"

Example

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

xmlns:m="http://www.w3schools.com/transaction/"
soap:mustUnderstand="1">
234

...
...