Data Structure Diagram

Purpose

Data Structure Diagrams (DSDs) can be thought of as graphical representations of DD entries. 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 DSDs. DSDs are a graphical means of respresenting the composition of data.

Example

Here is an example Data Structure Diagram.

Description

The DSD has been introduced into Cradle as a supplement to the functional modelling methodology. In essence, DSDs provide a graphical alternative to the composition specification of a DD entry, or a set of DD entries. A DSD contains data items, and shows the decomposition of data items into lower-level data items. If the DSD contains only a data item and its decomposition then it is an exact graphical substitute for the DD entry of the data item. If the DSD additionally shows the decomposition of the lower-level data items into further data items, then it is a graphical substitute for a set of DD entry composition specifications.

DSDs are not normally used instead of DD entry composition specifications. DSDs find a useful application as a graphical representation of the structure of one or more of a system's data items, as a supplement to the DD. Even when DSDs are in use, the DD should still be regarded as the master source of reference information for a system's data and control definitions.

DSDs visually reflect the composition of a number of DD entries such that the composition of each data item is shown in terms of the connected data items below it. The DSD notation corresponds, in effect, to a graphical version of the DD composition specification BNF.

Diagram Conversions

None.

Characteristics

APPEARS IN MODELS	Essential and Implementation
NUMBERING	Yes but only as a matter of consistency with the other diagram types
HIERARCHICAL	No

Linkage

None.

Symbols

Symbol	Name	Description	Definition	Expansion
	Comment	Makes a note anywhere in the diagram. Are always surrounded by * characters.	None	None
	Boundary point	A connection point for the initial transition to enter the initial state.	None	None
	Data object	An item of data in the system's DD.	DD Entry	None
	Iteration data object	An item of data that appears as an iterative (repeated) component of another, higher level, data item. It appears in the composition specification of the higher level data item within a (...) or n {...} m construction.	DD Entry	None
	Selection data object	An item of data that appears as an optional (selected) component of another, higher level data item. It appears in the composition specification of the higher level data item within a {...|...} construction.	DD Entry	None
	Picture	Allows you to choose the location of a GIF or JPEG image to be displayed as a diagram symbol or to be embedded in an existing diagram symbol.	None	None
	Connection	The means of interconnecting data items.	None	None