Extending the Epicentre Logical Data Model

Rationale and Overview

The rules defining variations of a POSC complying data model, allowing extensions and subsetting, are intended to allow local variation in the implemented data stores to accommodate innovations required by the marketplace, while minimizing negative effects on the interoperability of applications and the exchange of data.

The rules of alteration of a data store are defined for both extensions and subsets, and apply to the Epicentre data model as defined in EXPRESS.

Extending a POSC data store does not compromise interoperability and portability of applications, since an application complying with Epicentre by construction will be compliant with an extended data store.

A data model can have both extensions and subsets. The rules for extensions and subsets are symmetrical. The subsetting rules define a complying data model which is a valid subset of Epicentre. The extension rules define a complying data model for which Epicentre is a valid subset. The subsetting and extensions rules are such that the removal of a valid extension will always obey the subset rules, and the replacement of a valid subset will always obey the extension rules.

Creating a subset POSC data store causes more difficulties because an application designed to work on the full Epicentre Model or another subset will not necessarily be portable to that data store. For a more in-depth discussion of this issue, the reader is urged to see Subsetting the Epicentre Logical Data Model.

General Rules

The following general rules apply to POSC SIP data stores and to applications:

Rule G1: Any extension made to Epicentre following the rules of extensibility may be dropped without breaking the subsetting rules.
Rule G2: Any subset of Epicentre made according to the subsetting rules may be extended back to the original state without breaking the extensibility rules.
Rule G3: The DAE meta-model must not be altered.
Rule G4: Data types must not be altered or eliminated.
Rule G5: The list of data types may not be extended.

Extension Rules

The following general rule applies to all types of extensions:

Rule E1: Extensions must actually extend the scope of the data being stored in a business sense and may not just be a repetition of the existing Epicentre scope with an altered data model. Two types of extensions are valid:
- Addition of entities: Epicentre can be extended in breadth (i.e., addition of new concepts) through the addition of entities. All entities of Epicentre are direct or indirect subtypes of E&P data; therefore, an addition of an entity will always be an additional subtype to a list of existing subtypes.
- Addition of Attributes: Attributes may be added to existing entities. These attributes may point to an entity or to a data type. They cannot be mandatory unless they are attributes of an extended entity.
Rule E2: If an entity is already specialized and if all its subtypes are of type ONEOF, another entity may be added to the existing list of subtypes.
Rule E3: Optional attributes may be added to an entity.
Rule E4: Mandatory attributes may only be added to entities which are extensions of Epicentre.

Extension of the content of Open and Local reference entities is inherently valid.

Notes:

¹ EXPRESS Language Reference Manual, ISO International Standard 10303-11, Product Data Representation and Exchange - Part 11, TC184/SC4/WG5, March 1994.