Reference vs. Extension Schema

The NIEM Naming and Design Rules version 5.0 defines two classes of conformant XML Schema documents: reference schema documents (REF) and extension schema documents (EXT). The following outlines the differences in NDR rules between REF and EXT schema documents.

• Background
• Conformance targets
• Extension and restriction
• Block and final
• Nillable
• Choice
• Wildcards
• Augmentation points
• External adapters

Background

Beginning with NDR 3.0, NIEM XML schemas must declare themselves as either a reference schema document or an extension schema document through the use of the schema-level conformance targets attribute.

• Reference schema document (REF): A schema document that is intended to provide the authoritative definitions of broadly reusable schema components. NIEM release schemas are REFs because they are intended for wide reuse.

• Extension schema document (EXT): A schema document that is intended to provide definitions of schema components that are intended for reuse within a more narrow scope than those defined by a reference schema document. IEPD extension schemas are commonly EXTs so they can meet specific exchange requirements as needed, such as precise cardinality.

Conformance targets

REF and EXT schema documents have different conformance target identifiers. All REF and EXT schema documents must have a conformance target identifier, which tells readers of the schema what class of schema document it is. The conformance target identifiers are different for REF and EXT. These two rules identify the conformance target for each type of schema document.

• Rule 4-5, Schema claims reference schema conformance target (REF)
• Rule 4-6, Schema claims extension conformance target (EXT)

Extension and restriction

REF schema documents must use xs:extension to derive complex types. EXT schema documents are allowed to use xs:restriction for complex types. These rules require xs:extension for REF schemas. EXT schema documents are allowed to use xs:extension and xs:restriction.

• Rule 9-30, Complex content uses extension (REF)
• Rule 9-33, Simple content uses extension (REF)

These rules ensure that REF and EXT have xs:sequence as the top-level model compositors; they are different rules because REF requires xs:extension, while EXT also allows xs:restriction

• Rule 9-62, xs:sequence must be child of xs:extension (REF)
• Rule 9-63, xs:sequence must be child of xs:extension or xs:restriction (EXT)

Block and final

REF schema documents are not allowed to use block and final, which restrict extension and reuse of elements and complex types. EXT schemas are allowed to use these. These restrictions are placed on elements, complex types and on the schema, where defaults are set.

• Rule 9-34, No complex type disallowed substitutions (REF)
• Rule 9-35, No complex type disallowed derivation (REF)
• Rule 9-43, No element disallowed substitutions (REF)
• Rule 9-44, No element disallowed derivation (REF)
• Rule 9-86, No disallowed substitutions (REF)
• Rule 9-87, No disallowed derivations (REF)

Nillable

REF elements must be nillable.

• Rule 9-47, Element declaration is nillable (REF)

Choice

EXT schemas are allowed to use xs:choice. REF schemas are not allowed to use xs:choice. (Neither type of schema can use xs:all).

• Rule 9-64, No xs:choice (REF)

Since REF schemas are not allowed to use xs:choice, these rules are applied only to EXT schemas.

• Rule 9-60, Model group does not affect meaning (EXT)
• Rule 9-65, xs:choice must be child of xs:sequence (EXT)

These rules limit cardinality on xs:choice, applicable only to EXT schema documents.

• Rule 9-68, Choice has minimum cardinality 1 (EXT)
• Rule 9-69, Choice has maximum cardinality 1 (EXT)

Wildcards

REF schema documents cannot use wildcards.

• Rule 9-70, No use of xs:any (REF)
• Rule 9-71, No use of xs:anyAttribute (REF)

Augmentation points

REF schema documents must include augmentations on complex types.

• Rule 10-23, Augmentable type has augmentation point element (REF)
• Rule 10-29, Augmentation point element use is optional (REF)
• Rule 10-30, Augmentation point element use is unbounded (REF)

External adapters

REF schemas are not allowed to use attributes from external (not NIEM-conformant) schemas. EXT schemas are allowed to reference external attributes.

• Rule 10-13, External attribute use only in external adapter type (REF)

Prior to NDR 5.0, the following rule applied only to REF. It has since been adjusted to apply to EXT as well, and is no longer a difference between the two targets.

• Rule 10-70, External adapter type indicator annotates complex type (REF)

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