SOAP Tutorial

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

The first public Working Draft on SOAP was published from W3C in December 2001. To read more about the SOAP activities at W3C please visit our W3C tutorial.

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
...

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
...
...

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
...
...

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. SOAP defines one element inside the Body element in the default namespace ("http://www.w3.org/2001/12/soap-envelope"). This is the SOAP Fault element, which is used to indicate error messages.

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

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 standard.

A SOAP response could look something like this:

xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
   
      1.90

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.

The SOAP Fault element has the following sub elements:

Sub Element

Description


A code for identifying the fault


A human readable explanation of the fault


Information about who caused the fault to happen


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

The HTTP Protocol

HTTP communicates over TCP/IP. An HTTP client connects to an HTTP server using TCP. After establishing a connection, the client can send an HTTP request message to the server:

POST /item HTTP/1.1
Host: 189.123.345.239
Content-Type: text/plain
Content-Length: 200

The server then processes the request and sends an HTTP response back to the client. The response contains a status code that indicates the status of the request:

200 OK
Content-Type: text/plain
Content-Length: 200

In the example above, the server returned a status code of 200. This is the standard success code for HTTP.

If the server could not decode the request, it could have returned something like this:

400 Bad Request
Content-Length: 0

SOAP HTTP Binding

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.

The HTTP POST request specifies at least two HTTP headers: Content-Type and Content-Length.

Content-Type

The Content-Type header for a SOAP request and response defines the MIME type for the message and the character encoding (optional) used for the XML body of the request or response.

Syntax

Content-Type: MIMEType; charset=character-encoding

Example

POST /item HTTP/1.1
Content-Type: application/soap+xml; charset=utf-8

Content-Length

The Content-Length header for a SOAP request and response specifies the number of bytes in the body of the request or response.

Syntax

Content-Length: bytes

Example

POST /item HTTP/1.1
Content-Type: application/soap+xml; charset=utf-8
Content-Length: 250

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