In our SOAP tutorial, you will learn what SOAP is, and how it uses XML to exchange information between applications.
SOAP is a simple XML-based protocol to let applications exchange information over HTTP.
Or more simply: SOAP is a protocol foraccessing a Web Service.
What You Should Already Know
Before you study SOAP you should have a basic understanding of XML and XML Namespaces.
If you want to study these subjects first, please read our XML Tutorial.
What is SOAP?
* SOAP stands forSimple Object Access Protocol
* SOAP is a communication protocol
* SOAP is forcommunication between applications
* SOAP is a format forsending messages
* SOAP communicates 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 is a W3C recommendation
Why SOAP?
It is important forapplication 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 forthis. 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 is a W3C Recommendation
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 forthe SOAP envelope:
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>
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.
<?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 with a 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: mustUnderstand, actor, and encodingStyle.
The attributes defined in the SOAP Header defines how a recipient should process the SOAP message.
The mustUnderstand Attribute
The SOAP mustUnderstand attribute can be used to indicate whether a header entry is mandatory or optional forthe 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 will fail when processing the Header.
Syntax
soap:mustUnderstand="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>
The actor Attribute
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 forthe ultimate endpoint, instead, it may be intended forone or more of the endpoints on the message path.
The SOAP actor attribute is used to address the Header element to a specific 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 encodingStyle Attribute
The 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"
The SOAP Body Element
The required SOAP Body element contains the actual SOAP message intended forthe ultimate endpoint of the message.
Immediate child elements of the SOAP Body element may be namespace-qualified.
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: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. Note that the m:GetPrice and the Item elements above are application-specific elements. They are not a part of the SOAP namespace.
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>
The SOAP Fault Element
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 foridentifying 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
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 forthe 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 forHTTP.
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 fora SOAP request and response defines the MIME type forthe message and the character encoding (optional) used forthe 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 fora 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
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