SOAP stands for Simple Object Access Protocol. It is a format for sending and receiving messages. It is an application communication protocol and platform independent. It is important for web applications to be able to communicate over the Internet. The best way to communicate between applications is over HTTP, because HTTP is supported by all Internet browsers and servers. SOAP was created to accomplish this.
It is based on XML. It must contain the following elements:
- An Envelope element that identifies the XML document as a SOAP message.
- xmlns:soap Namespace defines the Envelope as a SOAP Envelope.
- The encodingStyle attribute is used to define the data types used in the document. This attribute may appear on any SOAP element, and applies to the element’s contents and all child elements.
soap:encodingStyle="URI" - A Header element that contains header information. 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). The example below contains a header with a “Trans” element, a “mustUnderstand” attribute with a value of 1, and a value of 234.
- SOAP defines three attributes in the default namespace. These attributes are: mustUnderstand, actor, and encodingStyle. The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.
- 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 recognise the element it will fail when processing the Header.
soap:mustUnderstand="0|1" - A SOAP message may travel from a sender to a receiver by passing different endpoints along the message path. However, not all parts of a SOAP message may be intended for the ultimate endpoint, instead, it may be intended for one or more of the endpoints on the message path. The SOAP actor attribute is used to address the Header element to a specific endpoint.
soap:actor="URI"
skeleton of SOAP message is shown below:
<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2003/05/soap-envelope/"
soap:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
<soap:Header><m:Trans xmlns:m="https://www.w3schools.com/transaction/"
soap:mustUnderstand="1">234
</m:Trans>
</soap:Header>
<soap:Body>
...
<soap:Fault>
...
</soap:Fault>
</soap:Body>
</soap:Envelope>
- A Body element that contains call and response information. 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:Body>>
<m:GetPrice xmlns:m="https://www.w3schools.com/prices">
<m:Item>Apples</m:Item>
</m:GetPrice>
</soap:Body
- 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. The response for this request shown above is:
<soap:Body>
<m:GetPriceResponse xmlns:m="https://www.w3schools.com/prices">
<m:Price>1.90</m:Price>
</m:GetPriceResponse>
</soap:Body>
- A Fault element containing errors and status information. 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. |
- 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 important syntax rules are :
- A SOAP message MUST be encoded using XML.
- A SOAP message MUST use the SOAP Envelope namespace.
- A SOAP message must NOT contain a DTD reference.
- A SOAP message must NOT contain XML Processing Instructions.
SOAP Bindings
SOAP bindings are mechanisms which allow SOAP messages to be effectively exchanged using a transport protocol namely HTTP or SMTP. HTTP is synchronous and widely used. A SOAP HTTP request specifies at least two HTTP headers: Content-Type and Content-Length.
In the example below, a GetStockPrice request is sent to a server. The request has a StockName parameter, and a Price parameter that will be returned in the response. The namespace for the function is defined in “http://www.example.org/stock”.
SOAP Request Example:
POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: nnn
<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2003/05/soap-envelope/"
soap:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
<soap:Body xmlns:m="http://www.example.org/stock">
<m:GetStockPrice>
<m:StockName>IBM</m:StockName>
</m:GetStockPrice>
</soap:Body>
</soap:Envelope>SOAP Response Example:
HTTP/1.1 200 OK
Content-Type: application/soap+xml; charset=utf-8
Content-Length: nnn
<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2003/05/soap-envelope/"
soap:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
<soap:Body xmlns:m="http://www.example.org/stock">
<m:GetStockPriceResponse>
<m:Price>34.5</m:Price>
</m:GetStockPriceResponse>
</soap:Body>
</soap:Envelope>