What should be in the XML header for validation?

Today i was looking for a way to let users configurate the application that i'm writing. The obvious way to do that is by having the users create an XML file with configuration information.

However, to avoid errors in these user-XML files, they should ve validated against a schema definition or .XSD file. The headers of the XML files i saw in the wild always looked a bit daunting to me: The `schema' element just has so many cryptic attributes. So here's an explanation of what they mean and how to set them.

First of all, you should create a schema definition for your XML files. In my case, i want to specify an XML format for configuring the `proxy' component of my application. Hence i call the main element `proxyConfig'. Below, you see a snippet from the XSD file with comments to clarify what the header lines mean.

<xs:schema     
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns="http://www.mycompany.com"
    targetNamespace="http://www.mycompany.com" 
    elementFormDefault="qualified">

    <xs:element name="proxyConfig">
        <xs:complexType>
                 [stuff omitted]
        </xs:complexType>
    </xs:element>
<xs:schema>

The first line xmlns:xs="http://www.w3.org/2001/XMLSchema" links namespace `xs' to the schema namespace http://www.w3.org/2001/XMLSchema. Now, if elements are prefixed with `xs:' they are assumed to refer to elements in the said namespace.

The second line xmlns="http://www.mycompany.com" defines the default namespace for THIS document. (I.e. the .xsd document you see above.) If an element used in this document does not have a namespace prefix, it is assumed to refer to the namespace http://www.mycompany.com.

The third line targetNamespace="http://www.mycompany.com" defines the namespace that the elements defined in this .xsd file should be placed in. This line is important, because we'll refer to it in our XML file below!

(Aside: You see that the target namespace and all referred namespaces here are URL's. As far as i understand, this is not actually necessary, it is merely a convention. This is a source for confusion, since it suggests that the URL's can be accessed to fetch information about syntax of the namespace. This is not true, it is just an arbitrary name.)

Finally, after creating the XSD file, we place it somewhere on the internet, e.g. on http://www.mycompany.com/download XML/proxyConfig.xsd .

Now for the XML file. A simple XML file is shown below:

<proxyConfig xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://www.mycompany.com"
    xsi:schemaLocation="http://www.mycompany.com http://www.mycompany.com/download/XML/proxyConfig.xsd">

  [stuff omitted]

</proxyConfig>

You see that in the first line xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" we say that namespace xsi refers to namespace "http://www.w3.org/2001/XMLSchema-instance". This namespace contains a number of attributes, among wchich the attribute xsi:schemaLocation. This attribute can be used to indicate the location for a namespace. Here, we use it in line 3:

xsi:schemaLocation="http://www.mycompany.com http://www.mycompany.com/download/XML/proxyConfig.xsd"

to indicate that the location of the .XSD file that defined namespace http://www.mycompany.com is http://www.mycompany.com/download/XML/proxyConfig.xsd. Remember that we set the namespace name (http://www.mycompany.com) and the xsd location http://www.mycompany.com/download /XML/proxyConfig.xsd ourselves when we created the .xsd file.

Finally, the second line xmlns="http://www.mycompany.com" defines the default namespace for the XML document.

Another source of confusion (to me) was the fact that your `own' namespaces need to be pointed at by a schemaLocation attribute. However, when referring to the http://www.w3.org/2001/XMLSchema or the http://www.w3.org/2001/XMLSchema-instance namespaces above, it is not necessary to explicitly give the location of the defining .XSD. This does not seem very consequent to me. (Presumably, this is because these latter namespaces are W3C namespaces somehow.) Maybe someone can explain this?