Section 3.1.1.3. DDL versions

Ideally, compatibility between the data dictionaries originating from specific subdisciplines would be ensured by the adoption of the same attribute sets for data items. However, at this point in the evolution of the CIF standard, two slightly different attribute sets have become established. These are expressed in two versions of the dictionary definition language, DDL1 and DDL2 (detailed in Chapters 2.5 and 2.6 , respectively). The differences arise because some subdisciplines benefit from a strict data model that is not appropriate in other areas. The core data items in crystallography must of course be accessible across the field, and so there are two formulations of the dictionary of core items, one in each DDL version. The existence of two formulations can make full information interchange across all areas of crystallography difficult, so work is under way to bring about a convergence of the two current representations (Hall et al., 2002). It is particularly important for future interchange between crystallography and other related disciplines that a full understanding be reached of the best way to include different data structure models within a common interchange format.

In this chapter, there will be some discussion of the differences in practice between the DDL versions DDL1 and DDL2, as these will strongly influence the choice of formalism for a dictionary relevant to a subdiscipline not yet represented.