Global versus local descriptions

There are two styles to write type and element descriptions in XML Schema: global and local. So far, we have seen schemas in global style.

In global style, all elements and attributes of the schema are declared as immediate children of the root schema element with a type attribute that contains the corresponding type name. Moreover, both user-defined simple and complex types are also defined globally as immediate children of the root schema element with an attribute name for the type name. In the type definitions, elements and attributes are referred to by using an element or attribute element with an attribute ref for the referred name. A typical example follows:

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  
  <xs:element name="author" type="xs:string"/>
  <xs:element name="title" type="xs:string"/>
  <xs:element name="year" type="xs:gYear"/>
  <xs:element name="item" type="itemType"/>
  
  <xs:attribute name="isbn" type="xs:string"/>

  <xs:complexType name="itemType">
    <xs:sequence>    
      <xs:element ref="author" maxOccurs="unbounded"/>
      <xs:element ref="title"/>
      <xs:element ref="year"/>
    </xs:sequence>  
    <xs:attribute ref="isbn"/>    
  </xs:complexType>
  
</xs:schema>

In the local style (or inline style) of writing schemas, types are defined inside the corresponding element declaration. Moreover, these inlined type definitions contain element and attribute declarations (and not references) that are used to describe the type. The same example shown above is rewritten in local style in the following:

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  
  <xs:element name="item">
    <xs:complexType>
      <xs:sequence>    
        <xs:element name="author" type="xs:string" maxOccurs="unbounded"/>
        <xs:element name="title" type="xs:string"/>
        <xs:element name="year" type="xs:gYear"/>
      </xs:sequence>  
      <xs:attribute name="isbn" type="xs:string"/>
    </xs:complexType>
  </xs:element>
  
</xs:schema>

Notice that the element item is the only global description and has no type attribute. Its type is now anonymous and is defined inside the element declaration.

Which style to use is mainly a matter of personal preference. Normally, the style is neither fully global nor fully local, by it mixes global and local descriptions. However, there are some differences between the two styles:

a schema in global style is a flat, unnested XML document with a large fanout for the schema root node; a schema in local style is deeply nested and typically there is only one child of the schema root node, that is, the declaration for the instance root element.

As a consequence, global-styled schemas are more readable and in particular it is immediate to find the description of elements and attributes with a given name. However, local-styled schemas resemble closely the structure of the instance document;
two global elements cannot have the same name, even if they have different types. The same for attributes and types. However, an element or an attribute and a type may have the same name, and also an element and an attribute may share the same name. Hence, you have to invent different names for global descriptions of the same category.

On the other hand, local elements may share the same name. In particular, two local elements with the same name may have different types. This holds also for attributes as soon as they belong to different elements, because an element cannot have attributes with the same name;

This overloading of element and attribute names is useful when the instance document contains two or more elements or attributes with the same name but with different meanings (and usually different types). This is quite common for attributes;
since local type definitions are anonymous, they cannot be reused in other spots of the schema. Moreover, a local element or attribute declaration cannot be referred to. Hence, local declarations do not promote reuse;
only global elements may be root elements in the instance document. In schemas in global style, all elements are global and hence the instance root cannot be forced. On the other hand, schemas in local style typically has only one global element that corresponds to the instance root;
local and global elements behave differently with respect to namespaces. When used in the instance document, global element and attribute names must always be qualified with the namespace prefix whenever a namespace has been declared for the XML application. However, local names might be unqualified. Indeed, the schema element has two attributes called elementFormDefault and attributeFormDefault to control the qualification of names of local elements and local attributes. The values of these attributes may be qualified or unqualified. The value unqualified, which is the default, means that names for the locally declared elements and attributes must have no namespace prefix in the instance document.

Unqualified elements and attributes are resolved by looking at the surrounding global element. This is the expected namespace behaviour for attributes. However, this is not the case for elements; usually, unqualified element names are resolved by looking up to the relevant default namespace declaration.

The following two examples are aimed to clarify this issue. The following global-styled schema defines a target namespace and a corresponding b: prefix.
```
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://www.dimi.uniud.it/francesc/biblio"
           xmlns:b="http://www.dimi.uniud.it/francesc/biblio">
  
  <xs:element name="author" type="xs:string"/>
  <xs:element name="title" type="xs:string"/>
  <xs:element name="year" type="xs:gYear"/>
  <xs:element name="item" type="b:itemType"/>
  
  <xs:attribute name="isbn" type="xs:string"/>

  <xs:complexType name="itemType">
    <xs:sequence>    
      <xs:element ref="b:author" maxOccurs="unbounded"/>
      <xs:element ref="b:title"/>
      <xs:element ref="b:year"/>
    </xs:sequence>  
    <xs:attribute ref="b:isbn"/>    
  </xs:complexType>
  
</xs:schema>
```
By associating the schema with a target namespace, every global description has been implicitly associated with that namespace (it is not necessary to explicitly qualify the names of global descriptions). This is the reason why a reference to a global description must be qualified with the target namespace prefix, like in the type attribute of the element item or in the ref attributes of the element references in the definition of type itemType.

A valid instance follows. Notice that both elements and attributes are qualified since they have been declared globally in the schema above (only local elements might be unqualified):
```
<?xml version="1.0"?>
<b:item xmlns:b="http://www.dimi.uniud.it/francesc/biblio"
        b:isbn="0-596-00764-7">
  <b:author>Elliotte Harold</b:author>
  <b:title>XML in a nutshell</b:title>
  <b:year>2004</b:year>
</b:item>
```
The following schema introduces the following modifications: the author element and the isbn attribute have been inlined. Moreover, the attribute elementFormDefault has been added with value qualified:
```
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://www.dimi.uniud.it/francesc/biblio"
           xmlns:b="http://www.dimi.uniud.it/francesc/biblio"
           elementFormDefault="qualified">
  
  <xs:element name="title" type="xs:string"/>
  <xs:element name="year" type="xs:gYear"/>
  <xs:element name="item" type="b:itemType"/>
  
  <xs:complexType name="itemType">
    <xs:sequence>    
      <xs:element name="author" type="xs:string" maxOccurs="unbounded"/>
      <xs:element ref="b:title"/>
      <xs:element ref="b:year"/>
    </xs:sequence>  
    <xs:attribute name="isbn" type="xs:string"/>
  </xs:complexType>
  
</xs:schema>
```
The following is a valid instance with respect to the modified schema. The locally declared element author is still qualified, as requested by the elementFormDefault attribute. However, the locally declared attribute isbn is not qualified, as requested by the default value of the attributeFormDefault attribute (unqualified):
```
<?xml version="1.0"?>
<b:item xmlns:b="http://www.dimi.uniud.it/francesc/biblio"
        isbn="0-596-00764-7">
  <b:author>Elliotte Harold</b:author>
  <b:title>XML in a nutshell</b:title>
  <b:year>2004</b:year>
</b:item>
```

In summary, global descriptions enhance readability and reusability of the schema, but local declarations are sometimes handy. We recommend the following best practices:

use global descriptions as much as possible since they enhance readability of the resulting schema and promote reuse of schema fragments;
use local declarations for elements and attributes that must be overloaded. Moreover, in case your XML application employs namespaces, local declarations for attributes allow you to get rid of the prefixes for attributes as soon as the attribute attributeFormDefault is set to unqualified (notice that this is the default);
if your XML application employs namespaces and your schema uses local elements, set elementFormDefault to qualified to establish the normal behaviour for namespaces.