Simple Object Access Protocol

SOAP is a messaging protocol specification for exchanging structured information. The protocol was originally developed by Microsoft.

The message format is exclusively based XML.

The underlying protocol is most likely HTTP (It also operates over SMTP, TCP, UDP, JMS etc). HTTP is preferred because it easily tunnels through firewalls and proxies.

SOAP, WSDL and UDDI (Universal Description, Discovery, and Integration) can be seen as the three pillars of web services.

Structure of a message

An envelope defines the message structure (header, body and fault).
A set of encoding rules
A convention for representing procedure calls and responses.

The envelope and the body are required, header and fault are not.

Major characteristics

Extensible (security and WS-Addressing)
Neutral (in respect to the underlying protocol)
Independent (in respect to the programming language used)

Concepts

nodes
roles
protocol binding
features
module

Encapsulation

message
envelope
header block
header
body
fault

Example message

HTTP request

POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: 299
SOAPAction: "http://www.w3.org/2003/05/soap-envelope"

<?xml version="1.0"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:m="http://www.example.org/stock/Reddy">
  <soap:Header>
  </soap:Header>
  <soap:Body>
    <m:GetStockPrice>
      <m:StockName>GOOG</m:StockName>
    </m:GetStockPrice>
  </soap:Body>
</soap:Envelope>

HTTP response

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>