Data Structure Diagram (DSD)

DSDs are a graphical alternative to the composition specifications in data definitions. A DSD shows the decomposition of data items into lower-level items. If the DSD contains only a data item and its decomposition then it is a graphical substitute for the data definition of the data item. If the DSD also shows the decomposition of the lower-level data items into further data items, it is a graphical substitute for a set of data definition composition specifications.

DSDs are not normally used instead of data definition composition specifications. DSDs offer a picture of the structure of a system’s data items, as a supplement to the DD. Even when DSDs are used, the DD is still to be regarded as the master source of information for a system’s data and control definitions.

DSDs show the composition of a data definition as the data items below it. The DSD notation corresponds, in effect, to a graphical version of the DD composition specification BNF.

DSDs are not hierarchical.

DSDs can appear in models in the Essential and Implementation Domains.

Numbering of DSDs

Any form of number can be used for DSDs.

Symbols

An example DSD is:

The symbols available in DSDs are:

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.	Data definition	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.	Data definition	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.	Data definition	None
	Connection	The means of interconnecting data items.	None	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

Graphical Equivalence to Compositions