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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Introducing SOAP

SOAP lets Web applications send data back and forth in messages in a platform-independent, language-independent way. SOAP was designed so that distributed applications could communicate through corporate firewalls, allowing applications to work with objects across all kinds of boundaries. In SOAP we send XML-based messages by using HTTP; because it uses widely available HTTP, SOAP has become very popular. SOAP messages can provide a backbone for distributed applications, using existing technologies to connect the parts of those distributed applications. We can also send attachments by using SOAP messages, as you'll see today.

SOAP is a general-purpose XML application that is helping to standardize the way people handle data on the Internet. There are plenty of SOAP resources on the Internet; here's a starter list, including the W3C's documentation on it:

http://www.w3.org/TR/SOAP— The W3C SOAP 1.1 documentation
http://www.w3.org/TR/SOAP-attachments— The W3C "SOAP Messages with Attachments" documentation
http://www.javaworld.com/javaworld/jw-03-2001/jw-0330-soap.html— Part 1 of a SOAP tutorial
http://www.javaworld.com/javaworld/jw-04-2001/jw-0427-soap.html— Part 2 of a SOAP tutorial
http://www.javaworld.com/javaworld/jw-06-2001/jw-0601-soap.html— Part 3 of a SOAP tutorial
http://www.javaworld.com/javaworld/jw-07-2001/jw-0706-soap.html— Part 4 of a SOAP tutorial
http://www.w3schools.com/SOAP/default.asp— A SOAP tutorial
http://www.xml.com/pub/a/2000/02/09/feature/index.html— An article on SOAP from XML.com
http://xml.apache.org/soap/faq/index.html— The Apache project's SOAP FAQ

We'll start digging into SOAP now, beginning with the syntax that makes SOAP work.

Understanding SOAP Syntax

A SOAP message is simply an XML document that uses the SOAP syntax rules. There are three parts of a valid SOAP message:

Envelope— The envelope contains the message itself.
Header— This optional part contains information about and descriptions of the message.
Body— The body contains the actual SOAP message.

As a simple example, here's a SOAP message, complete with an envelope and a body. This message indicates the number of laptops in stock at a warehouse (note that soap-env is the usual envelope namespace prefix):

<soap-env:Envelope
    xmlns:soap-env="http://www.w3.org/2001/12/soap-envelope"
    soap:soap-enc="http://www.w3.org/2001/12/soap-encoding">
    <soap-env:Header>
        <m:Name xmlns:m="http://www.XMLPowerCorp.com">
            XMLPowerCorp
        </m:Name>
    </soap-env:Header>
    <soap-env:Body>
        <m:numberAvailable xmlns:m="http://www.XMLPowerCorp.com">
            <m:laptops>216</m:laptops>
        </m:numberAvailable>
    </soap-env:Body>
</soap-env:Envelope>

The default namespace setting for a SOAP envelope is "http://schemas.xmlsoap.org/soap/envelope/". The default namespace setting for the document encoding and data types is " http://schemas.xmlsoap.org/soap/encoding/ ".

So what elements are already defined in SOAP?

Introducing the SOAP Elements

The root element in a SOAP message is the <Envelope> element. There are three possible child elements—<Header>, <Body>, and <Fault>. These elements have to use the names Header, Body, and Fault, respectively, although they can have child elements with any names. For example, here's what a SOAP envelope might look like:

<soap-env:Envelope
    xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"
    soap-env:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    .
    .
    .
</soap-env:Envelope>

The next element, the <Header> element, is optional. It holds information about the SOAP message, as in the following example, where we're setting the language to U.S. English (<m:locale> and <m:language> are elements we've defined):

<soap-env:Envelope
    xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"
    soap-env:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <soap-env:Header>

           <m:locale xmlns:m="http://www.XMLPowerCorp.com">

               <m:language>en-us</m:language>

           </m:locale>
    </soap-env:Header>
    .
    .
    .
</soap-env:Envelope>

Next, the <Body> element holds the actual SOAP message:

<soap-env:Envelope
    xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"
    soap-env:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <soap-env:Header>
        <m:locale xmlns:m="http://www.XMLPowerCorp.com">
            <m:language>en-us</m:language>
        </m:locale>
    </soap-env:Header>

    <soap-env:Body>

           <m:numberAvailable xmlns:m="http://www.XMLPowerCorp.com">

               <m:laptops>216</m:laptops>

           </m:numberAvailable>

       </soap-env:Body>
</soap-env:Envelope>

Note that the <Body> element may also contain a <Fault> element, which can hold any errors that occurred:

<soap-env:Envelope
    xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"
    soap-env:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <soap-env:Header>
        <m:locale xmlns:m="http://www.XMLPowerCorp.com">
            <m:language>en-us</m:language>
        </m:locale>
    </soap-env:Header>

    <soap-env:Body>
        <m:numberAvailable xmlns:m="http://www.XMLPowerCorp.com">
            <m:laptops>216</m:laptops>
        </m:numberAvailable>

        <soap-env:Fault>

               <faultcode>1166</faultcode>

               <faultstring>Batteries are low</faultstring>

           </soap-env:Fault>
    </soap-env:Body>
</soap-env:Envelope>

<Fault> elements can have these subelements:

<faultcode> — Contains an error code.
<faultstring> — Contains an error string.
<faultactor> — Specifies the error's source.
<detail> — Contains the details of the error.

A number of fault codes are already defined for use in the <faultcode> element:

VersionMismatch — Means that the namespace for the SOAP <Envelope> element was not correct.
MustUnderstand — Means that an element's content must be understood if this attribute is set to "1".
Client — Means there was a problem with the message as sent from the client.
Server — Means that there was a problem with the server.

In addition, SOAP elements can support various attributes, which we'll look at now.

Introducing the SOAP Attributes

The possible attributes we can use in SOAP elements are actor, encodingStyle, and mustUnderstand. For example, the actor attribute lists a URI corresponding to the group that is using the SOAP message, like this:

<soap-env:Header>
    <m:data xmlns:m="http://www.XMLPowerCorp.com"
        soap-env:actor="http://www.XMLPowerCorp.com/philosophy" />
        <m:language>en-us</m:language>
    </m:data>
</soap-env:Header>

The encodingStyle attribute indicates the data types used in the message. The W3C provides a default schema for SOAP types, which you can find at http://schemas.xmlsoap.org/soap/encoding/:

<soap-env:Envelope
    xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"
    soap-env:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    .
    .
    .
</soap-env:Envelope>

The mustUnderstand attribute is used in the <Header> element, and it indicates whether the software interpreting a message must understand a header element or cause a fatal error if it does not. You can set it to a Boolean value, "1" for true or "0" for false:

<soap-env:Header>
    <m:data xmlns:m="http://www.XMLPowerCorp.com"
        soap-env:mustUnderstand="0" />
        <m:language>en-us</m:language>
    </m:data>
</soap-env:Header>

Now it's time to see SOAP at work.

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