Introduction SOAP (Simple Object Access Protocol)

This chapter describes:

• What is SOAP?
• A SOAP communication example.

What Is SOAP?

SOAP is an XML based protocol intended for exchanging structured information in a distributed application environment.

SOAP 1.2 specification is divided into two parts.

SOAP part 1 defines the SOAP messaging framework, which contains the following key concepts:

• SOAP Message Construct - How a SOAP message should be constructed.
• SOAP Protocol Binding Framework - How a SOAP message should be bound a communication protocol to carry it from the sender to the receiver.
• SOAP Processing Model - How a SOAP message should be processed by SOAP receivers.
• SOAP Extensibility Model - How SOAP can be extended to add SOAP features and SOAP modules.

SOAP part 2 defines a set of adjuncts that can be used to extend the SOAP messaging framework:

• SOAP Data Model - How data structures and values should be represented as a graph of nodes.
• SOAP Encoding - How data presented in SOAP Data Model should be encoded as XML data.
• SOAP RPC Presentation - How Remote Procedure Call (RPC) should be modeled with SOAP.
• SOAP Convention for Describing Features and Bindings - How SOAP Features and Bindings should be described.
• SOAP Supplied Features - Commonly used SOAP extensions.
• SOAP HTTP Binding - How a SOAP message is bound to HTTP.

SOAP is actually a communication protocol that defines how a message should be constructed, transmitted from one node to another, and processed by each node as shown in the following diagram:

SOAP node          
 |
 | Construct an initial SOAP message
 | Bind the initial message for transmission
 | Transmit the initial message
 v
 ----------> SOAP node
              |  
              | Receive the initial message
              | Process some part of the initial message
              | Reconstruct an intermediate SOAP message
              | Bind the intermediate message for transmission
              | Transmit the intermediate message
              v
              ----------> SOAP node
                           |
                           | ...
                           v
                           ----------> SOAP node 
                                        |
                                        | Receive the final message
                                        | Process the final message
                                        v
                                        Done 

SOAP Communication Example

Before going into details of the SOAP specification, let's try a very simple SOAP example. There are 3 basic steps involved in a SOAP communication:

1. Creating a SOAP message. This is easy. Just use any text editor to enter the following SOAP message, hello.msg:

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
 <env:Header>
  <h:control xmlns:h="http://herong.com/header">
   <h:sender>Herong</h:sender>
  </h:control>
 </env:Header>
 <env:Body>
  <b:greeting xmlns:b="http://herong.com/body">
   <b:msg>Hello there!</b:msg>
  </b:greeting>
 </env:Body>
</env:Envelope>

2. Transmitting the message from a SOAP node, my machine, to another SOAP node, your machine. A simple way to do this is for me to send hello.msg to you as an email, so you will get something like this:

From: herong@my.com
To: you@your.com
Subject: Greeting

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">
 <env:Header>
  <h:control xmlns:h="http://herong.com/header">
   <h:sender>Herong</h:sender>
  </h:control>
 </env:Header>
 <env:Body>
  <b:greeting xmlns:b="http://herong.com/body">
   <b:msg>Hello there!</b:msg>
  </b:greeting>
 </env:Body>
</env:Envelope>

3. Processing the message. Once you got my SOAP message, you should be able to read it, and do whatever you want to do with it.