Change how cardinalities are specified in the Metaschema

[david-waltermire]

User Story:

As an OSCAL model developer, I need to be able to define cardinalities in the Metaschema for assembly and field elements.

Goals:

The current Metaschema allows for the cardinality of assembly and field elements to be defined using two capabilities. The first is by making the assembly and field plural and the second is by using the @required="yes" attribute. These two features result in the following:

	Singular	Plural
required="yes"	1-1	1-many
required="no"	0-1	0-many

This approach differs from how cardinalities are defined in the XML and JSON schema languages.

To be more consistant the following changes need to be made:

• Minimum occurance needs to be specified using @min-occurs allowing a non-negative number value to be provided. This will replace the @required attribute. The default value for @min-occurs if not provided will be "0".

  For example: @min-occurs="0" or @min-occurs="1".

• Maximum occurance needs to be specified using @max-occurs allowing a positive number value or the constant value unbounded to be provided. This will allow for a set upper bound using a positive number, or any number of occurances using unbounded. This will replace the use of assemblies and flags. This approach adds the ability to set an actual upper bound, which is not currently supported in the Metaschema. The default value for @max-occurs if not provided will be "1".

  For example: @max-occurs="1", @max-occurs="5", or @max-occurs="unbounded".

• Generated XML and JSON schema need to be updated to support these new constructs.

  XML Schema uses the same constructs.

  JSON Schema uses similar constructs as follows:

  {
    "type": "array",
    "minItems": 2,
    "maxItems": 3
  }
  

  This must also work with the singlton/array optomization we are using in the generated schemas.

  • When @max-occurs is 1, the resulting schema must assume that only a singlton value will be provided. Otherwise, the schema will allow an array or a singleton.
  • When @min-occurs is 0 or 1, the resulting schema must allow for either a singleton or an array to be provided. If the @min-occurs is greater than 1, then the schema will allow only an array.

Dependencies:

None.

Acceptance Criteria

• All generated XML and JSON schema are updated to reflect these changes. Content instances are checked by the CI/CD workflow.
• XML and JSON documentation generated by the CI/CD workflow correctly represents the correct cardinalities.
• The Metaschema definitions are updated in a way that result in no changes to associated content.
• Documentation on the use of Metaschema is updated to fully document use of the @min-occurs and max-occurs features.
• A single PR is approved that contains the above changes.

[iMichaela]

7/11/2019

This issue and the related ones (noted in #435 ) for the metaschema need to be reviewed together and unitested. However, more unit tests need to be created for the other features.

[david-waltermire]

This will be closed once PR #464 is pulled.

[david-waltermire]

This has been completed in PR #464.