ERDs can be considered as graphical representations of data definitions. Information modelling is concerned with the definition of data within the system in terms of its meaning, composition and relationships. One of the methods within Cradle that can be used to represent information modelling is the use of an ERD. ERDs graphically show how different pieces of information are related.
DFDs show the flow of information through the system but play no part in defining the information, apart from being a useful place from which to access data definitions. Flows and stores on DFDs are essentially representations of information that is defined elsewhere.
While the DFD models the active processing of information by the system, the ERD models the static relationships amongst this information that are preserved and maintained by the system. The ERD shows how items of data relate, statically, to each other. ERDs cannot exist in a hierarchy, instead, either a single ERD is produced for the entire system analysis or design (when the ERD is considered to relate to the entire DFD hierarchy), and/or ERDs can be produced as companions to specific DFDs that contain a large quantity of stored data, and contain the processes that create, update, or otherwise maintain the stored data.
ERDs are available in both the Essential and Implementation Domain.
Cradle does not restrict the numbering of ERDs, they can be used in whatever manner is appropriate.
An example ERD is:
The symbols available in ERDs are: