What are the alternatives to a schema subset?
Alternatives to schema subsets include the use of restriction, modularizing the full JXDD schema, or just copying over only the elements and definitions one needs.
The XML Schema restriction mechanism allows users to take a type and restrict away the elements they don't need and to modify the occurrence restrictions of other elements. While this seems like an acceptable approach, it presents some problems that may not be obvious. The first problem is that if users create a schema subset by restricting the full JXDD schema, the full JXDD schema would still be imported. No benefits would be gained in loading or validation time. The second problem is that restrictions cannot be enforced. To create a restriction, a new local type would be created based on the original JXDD type. Elements could be dropped or their number of occurrences reduced. The local schema would still have to import the full JXDD schema to do this, but using fast validation tools would make this possible. The real problem is in usage. Elements defined to be of the original JXDD type would be able to use the local restricted type in the XML instance through type substitution; however, there is no way to enforce this type substitution to occur. It would still be entirely possible, and in fact easier, for the original unrestricted JXDD type to be used. Validation would not recognize that the local restricted type should be used instead of the original JXDD type. The only way to work around this would be to create a new local element of the new locally restricted type. Validation would then enforce that the local type be used, but the element would have no connection to the JXDD and would not be understood by others to whom the schema is sent. This loses much of the benefit gained by using the JXDD - understandability. The use of restriction is not prohibited, but it offers much less in terms of performance benefits and validation support than schema subsets provide. Furthermore, in most cases restriction is not a sufficient alternative to schema subsets.
Another alternative would be to modularize the full JXDD schema into different components, as has been suggested. The problem with this is that there is no set of lines over which modularization would work and provide the benefits desired. If the full JXDD schema was divided into smaller components, the smaller components would still need to import each other because they are all interrelated. A person module would need reference to a location module, a contact information module, an organization module, a miscellaneous module for common types, and some subset of an activity module for the person subtypes. Little performance gains are made and complexity is increased. A seemingly simpler alternative to building schema subsets would be for users to copy over only those element and type definitions that they need from the full JXDD schema into their own document schema. This approach has problems as well. If users copy over JXDD components into their document schema without putting them into the Justice namespace, then other users would not be able to recognize that those components come from the JXDD. The namespace is what identifies the common source of the components; without this, recognition is lost.
Instead, if users copy over JXDD components into their own document schema and put them under the Justice namespace, then this tie to the JXDD is in name only. Usually, the structural definitions of JXDD components that are used in a local schema are imported from a definition schema in the Justice namespace. The full JXDD schema is one such definition schema - it is an official definition of the JXDD elements and types. The JXDD schema subset is another. A local copy of JXDD components in a document schema is just that - a local copy. There is no official structural definition schema against which to validate and ensure that the components appear as they should. The local document only validates against itself. There is no guarantee that components are actually from the JXDD; at best all you have is a claim. In either case, local copies with or without Justice namespace references, it would not be possible to reference and identify appropriate components as valid Justice elements and types.