12.2.2.1. Block attribute

The first two values (restriction and extension) control any substitution through xsi:type or substitution groups, and block the substitution by datatypes that are derived by restriction or extension from the datatype defined in the element declaration. The third (substitution) is specific to the substitution groups, and defines if an element from the substitution group (for which the element is the head) is allowed. Since only global elements can participate in a substitution group, the last value is clearly meaningful for global definitions only.

The fact that the block attribute is used both for type substitution and for substitution groups can be misleading, especially with the values restriction and extension, which act on both aspects. A simple example makes this more concrete. Let's say that we have a complex type definition (personType) describing a person as having a name, a mandatory birth date, and optional death date and qualification:

<xs:complexType name="personType">
  <xs:sequence>
    <xs:element ref="name"/>
    <xs:element ref="born"/>
    <xs:element ref="dead" minOccurs="0"/>
    <xs:element ref="qualification" minOccurs="0"/>
  </xs:sequence>
  <xs:attribute ref="id"/>
</xs:complexType>

We can also derive by extension from this complex type a datatype that describes an author as a person with a list of books:

<xs:complexType name="authorType">
  <xs:complexContent>
    <xs:extension base="personType">
      <xs:sequence> 
        <xs:element name="book" type="xs:token" minOccurs="1"
          maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

We can derive by restriction a datatype that describes a character as a person with no death date and a mandatory qualification:

<xs:complexType name="characterType">
  <xs:complexContent>
    <xs:restriction base="personType">
      <xs:sequence>
        <xs:element ref="name"/>
        <xs:element ref="born"/>
        <xs:element ref="qualification"/>
      </xs:sequence>
    </xs:restriction>
  </xs:complexContent>
</xs:complexType>

The first purpose of block attributes in element definitions is to control the type substitution through xsi:type, which controls substitutions in the instance documents such as the following (if the person element is defined as having a type personType):

<person xsi:type="authorType" id="CMS"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <name>
    Charles M Schulz
  </name>
  <born>
    1922-11-26
  </born>
  <dead>
    2000-02-12
  </dead>
  <book>
    Being a Dog Is a Full-Time Job
  </book>
</person>
               
<person xsi:type="characterType" id="Snoopy"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <name>
    Snoopy
  </name>
  <born>
    1950-10-04
  </born>
  <qualification>
    extroverted beagle
  </qualification>
</person>

TIP: This example shows that the impact of both derivation types on the applications is very different. A type substitution by restriction has virtually no impact on the applications, since the content model matching the restricted type must also match the original type. Alternatively, a type substitution by extension allows content models not allowed by the original type (such as the addition of the book element in the previous example), and the risk of breaking applications that do not expect these additions is higher. Thus, a conservative attitude might be to include extension in the default values of block in your schemas.

That being said, we've seen only one side of the block attribute. It serves a second, different, yet not independent, purpose and also restricts the usage of substitution groups. To illustrate this purpose, let's define three different element members of a substitution group whose head is person. There is also a fourth element member, which is just a synonym for the element person and has the same type:

<xs:element name="person" type="personType"/>
              
<xs:element name="author" type="authorType"
  substitutionGroup="person"/>
              
<xs:element name="character" type="characterType"
  substitutionGroup="person"/>
              
<xs:element name="human" type="personType"
  substitutionGroup="person"/>

After defining this substitution group and without blocking anything, we allow not only block and the type substitutions seen above, but also element substitutions, such as:

<author id="CMS">
  <name>
    Charles M Schulz
  </name>
  <born>
    1922-11-26
  </born>
  <dead>
    2000-02-12
  </dead>
  <book>
    Being a Dog Is a Full-Time Job
  </book>
</author>
              
<character id="PP">
  <name>
    Peppermint Patty
  </name>
  <born>
    1966-08-22
  </born>
  <qualification>
    bold, brash and tomboyish
  </qualification>
</character>
<human id="CMS.">
  <name>
    Charles M Schulz
  </name>
  <born>
    1922-11-26
  </born>
  <dead>
    2000-02-12
  </dead>
</human>

The first substitution is by an element of the substitution group whose type is derived by extension from the head. It can be blocked by specifying a block attribute including the value extension. The second one is a substitution by an element of the substitution group whose type is derived by restriction from the head. It can be blocked by specifying a block attribute including the value restriction. Those two are similar to the two substitutions given as examples for the type substitutions, with the difference that the element name is now used to differentiate the type instead of the xsi:type attribute. The third substitution is new, since the type is the same as the type of the head, and it is blocked only by specifying a block attribute that includes the value substitution. Note that including substitution in a block attribute blocks any element substitution, while stating that not all the combinations are possible to express. To block everything, you would define author as:

<xs:element name="author" type="authorType" block="#all"/>

12.2.2.2. Final elements

12.2.2.3. Abstract elements

12.2.3.1. Blocking complex types

Figure 12-1. Block attribute on elements and complex type definitions

For each restriction and extension, the substitutions can first be switched off for both element and type substitutions by the block attribute of the element definition, and then the type substitutions can be blocked by the block attribute of the complex type definition. The value substitution in the block attribute of the element definition acts as a global switch to block all element substitutions, including a third line, which allows element substitutions with the same type and cannot be blocked separately.

12.2.3.2. Final complex types

Unlike the block attribute which is linked with its counterpart in the element definition, the final attribute in a complex type definition applies only to the complex type itself and, like the abstract attribute in the element definition, it acts at the level of the schema itself and has no impact on the instance documents.

12.2.3.3. Abstract complex types

We need to insist that abstract by itself doesn't mean that the complex type cannot be used in a content model. However, if it is used, it must be substituted through xsi:type in the instance documents. To define a complex type that will not be usable in a content model, we need to use both the final and block attributes; the complex type's only use is as a base type for derivations.