SOAP is a simple XML-based protocol to let applications exchange information over HTTP.
SOAP is a protocol for accessing a Web Service.
SOAP stands for Simple Object Access Protocol
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.
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
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
The mandatory SOAP Envelope element is the root element of a SOAP message.
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:
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.
The mandatory SOAP Body element contains the actual SOAP message.
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.
The optional SOAP Fault element is used to hold error and status information for a SOAP message.
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
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, 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
A SOAP Example
In the example below, a GetStockPrice request is sent to a server. The request has a StockName parameter, and a Price parameter will be returned in the response. The namespace for the function is defined in "http://www.example.org/stock" address.
The SOAP request:
POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: nnn
SOAP Summary
This tutorial has taught you how to use SOAP to exchange information between applications over HTTP.
You have learned about the different elements and attributes in a SOAP message.
You have also learned how use SOAP as a protocol for accessing a web service.
Now You Know SOAP, What's Next?
The next step is to learn about WSDL and Web Services.
WSDL
WSDL is an XML-based language for describing Web services and how to access them.
WSDL describes a web service, along with the message format and protocol details for the web service.
If you want to learn more about WSDL, please visit our WSDL tutorial.
Web Services
Web services can convert your applications into web-applications.
By using XML, messages can be sent between applications.