Substitution groups in the message model

Substitution groups are an XML Schema feature which provides a means of substituting one element for another in an XML message. A substitution group is a list of global elements which can appear in place of another global element, called the head element.

A substitution group is defined by setting the substitution group property on one global element (the member element) to point at another global element (the head element). This adds the member element to the substitution group of the head element.

Note: If you are generating dictionaries from your message set, and deploying them to WebSphere Message Broker, you should read the Schema restrictions in the message model topic, and note the section which relates to substitution groups.
Tip: If your messages are never rendered as XML, or if you have a simple message model, you should consider using a complex type or a group with Composition set to Choice, instead of using substitution groups.

Elements

Head elements
A head element is simply an element which can be substituted. When a message is parsed, one of its member elements can appear in place of the head element without causing a validation error.
Abstract elements
An abstract element is a head element which must be substituted. The 'abstract' attribute on the element indicates this. Usually, abstract elements will have other elements in their substitution group - otherwise they are of little use. Wherever an abstract element appears in a message definition, a member of its substitution group must appear instead.

Attributes

The block attribute on elements
The block attribute on an element limits the set of global elements which can substitute for the element. The block attribute can take any subset of the values restriction, extension, substitution, or all.
  • If the block attribute contains restriction, an element which is based on a restriction of the element's type cannot substitute for the element.
  • If the block attribute contains extension, an element which is based on an extension of the element's type cannot substitute for the element.
  • If the block attribute contains substitution, an element which is a member of the element's substitution group cannot substitute for the element.
  • If the block attribute contains all, all of the above limits apply.
The final attribute on elements
The final attribute on an element limits the set of global elements which can be a member of the element's substitution group. The final attribute can take any subset of the values restriction, extension or all.
  • If the final attribute contains restriction, an element which is based on a restriction of the element's type cannot be in the substitution group of the element.
  • If the final attribute contains extension, an element which is based on an extension of the element's type cannot be in the substitution group of the element.
  • If the final attribute contains all, both of the above limits apply.
The block attribute on complex types
The block attribute on a complex type limits the set of other types which can substitute for that type. The block attribute can take values restriction, extension, or all. The meanings for these values are the same as those shown for the block attribute on an element above. An element which is a member of a substitution group can only substitute for the head element if its type is compatible with the block attribute on the type of the head element.
Default block and final attributes
A default for the block and final attributes can be set at the message definition file level. If a default for one or both of these attributes has been set and the relevant block or final attribute has not been set at the object level, the default setting will be used for that object. You can override the default setting at the object level.