SOAP Notes

Introduction

SOAP Stands for Simple Object Access Protocol.

SOAP is a communication protocol.

SOAP is for communcation between applications.

SOAP is a format for sending messages.

SOAP is designed to communicate via the 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.

SOAP is a key element of Microsoft's .NET architecture.
Syntax

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 error information that occurs while processing the message.

All the elements above are declared in the default namespace for the SOAP envelope.
- 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>
            
```

SOAP Envelope

The mandatory SOAP envelop element is the root element of a SOAP message.

The SOAP Envelope Element

It defines the XML document as a SOAP message.

It should always have the value:

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

It defines the Envelope as a SOAP Envelope:

                <?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>

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 it's child elements.

A SOAP message has no default encoding.

Syntax

                  soap:encodingStyle="URI"

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>

SOAP Header

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.

All immediate child elements of the Header element must be namespace qualified.
```
              <?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 example above contains a header with a "Trans" element, a mustUnderstand" attribute value of "1", and a value of 234.

SOAP defines 3 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:

                  <?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 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:umstUnderstand="0|1"

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

                  ...
                  ...

                  </soap:Envelope>

SOAP Body

The SOAP Body Element

The 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.

              <?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.

The m:GetPrice and the Item elements above are application specific elements.

They are not part of the SOAP standard.

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>

SOAP Fault

The SOAP Fault Element

Error messages from a SOAP message are carried inside Fault elements.

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.

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
<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 be processed

SOAP HTTP Binding
- The HTTP Protocol
  
  HTTP communicates over TCP/IP. An HTTP client connects to an HTTP server using TCP. After establishing a connection, teh 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 requrest 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
                
```

SOAP Example

In the example below, a GetStockPrice request is sent to a server. The requst has a StockName parameter, and a Price parameter will be returned in the response.The namespace for the function is defined in http://www.stock.org/stock* address.

          POST /InStock HTTP/1.1
          Host: www.stock.org
          Content-Type: application/soap+xml; charset=utf-8
          Content-Length: nnn

          <?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 xmlns:m="http://www.stock.org/stock">
              <m:GetStockPrice>
                <m:StockName>IBM</m:StockName>
              </m:GetStockPrice>
            </soap:Body>

          </soap:Envelope>

A SOAP response:

          HTTP/1.1 200 OK
          Content-Type: application/soap; charset=utf-8
          Content-Length: nnn

          <?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 xmlns:m="http://www.stock.org/stock">
              <m:GetStockPriceResponse>
                <m:Price>34.5</m:Price>
              </m:GetStockPriceResponse>
            </soap:Body>

          </soap:Envelope>

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