SOAP Envelope Element

The SOAP envelope indicates the start and the end of the message so that the receiver knows when an entire message has been received. The SOAP envelope solves the problem of knowing when you’re done receiving a message and are ready to process it. The SOAP envelope is therefore basic ally a packaging mechanism
SOAP Envelope element can be explained as:

• Every SOAP message has a root Envelope element.
• Envelope element is mandatory part of SOAP Message.
• Every Envelope element must contain exactly one Body element.
• If an Envelope contains a Header element, it must contain no more than one, and it must appear as the first child of the Envelope, beforethe Body.
• The envelope changes when SOAP versions change.
• The SOAP envelope is specified using the ENV namespace prefix and the Envelope element.
• The optional SOAP encoding is also specified using a namespace name and the optional encodingStyle element, which could also point to an encoding style other than the SOAP one.
• A v1.1-compliant SOAP processor will generate a fault when receiving a message containing the v1.2 envelope namespace.
• A v1.2- compliant SOAP processor generates a VersionMismatch fault if it receives a message that does not include the v1.2 envelope namespace.

Example for v1.2 is given below-

<?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

Notice the xmlns:soap namespace in the example above. It should always have the value of: “http://www.w3.org/2001/12/soap-envelope“.

The namespace defines the Envelope as a SOAP Envelope.

If a different namespace is used, the application generates an error and discards the message.
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 applies to the element’s contents and all child elements.

A SOAP message has no default encoding.

Following example illustrates the use of a SOAP message within an HTTP POST operation, which sends the message to the server. It shows the namespaces for the envelope schema definition and for the schema definition of the encoding rules. The OrderEntry reference in the HTTP header is the name of the program to be invoked at the www.dineshonjava.com Web site.

POST /OrderEntry HTTP/1.1
Host: www.dineshonjava.com
Content-Type: application/soap; charset="utf-8"
Content-Length: nnnn
<?xml version="1.0"?>
<SOAP-ENV:Envelope
xmlns:SOAP-ENV="http://www.w3.org/2001/12/soap-envelope"
SOAP-ENV:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
  ...
  Message information goes here
  ...
</SOAP-ENV:Envelope>

References
1. Wikipedia for SOAP