The motivation for POSD can be seen in the pages referenced from the
POSD home page. Those pages describe a problem encountered by existing design methods and show how the POSD notation
handles this problem
POSD supports a very primitive model of systems, summarised by the following statements:-
- only one type of object is supported - a system
- systems are composed of component systems (and systems are components of composite systems)
- systems can only interact with each other by means of a shared component system
- the effect of an interaction is to move component systems between interacting systems
The first two provide a static model of system structure and are supported by this tool. The third and fourth
add dynamics to the model but are not as yet supported by the tool.
The tool gives each system a unique identifier (such as "s1", "s2", etc) and allows each system to have
names for its component systems.
The tool supports three graphical styles for displaying a POSD model:-
- as a directed graph - each node in the diagram identifies one system - the system identified at the
bottom of each line is a component of the system identified at the top - the component's name within this system
is given on the line
- as a network diagram for each system - each box in the diagram represents a component system of this
system - each line represents an interaction between the two component systems at its ends
- as a bone diagram for each interaction - the ends of the bone represent the interacting systems
(and their component structures) - the middle of the bone represents the interaction (and its component
structure)
In a diagram for a system you will find tags on the components or composites of the system and can trigger
these tags to view similar diagrams for the components or composites.
The tool maintains a database of system descriptions for each application of the tool. A system description
records identifiers, component structure and names for its system. The tool allows you to move between different
databases (for different applications) and to create new databases (for new applications).
The tool allows you to change system descriptions by including existing components in different composites,
detaching components from composites, renaming components within composites, and creating new components in composites.
You can save the identifier of the current system (the system displayed in a diagram) and can later
recover that remembered system as the current system.
You can provide a type for a system (such as "person", "manage", etc). These typed systems can be used to represent
portals between different "worlds" (eg the software and human worlds, or the levels of a VSM model). You can
switch the display of these special systems on or off (filter).
You can provide annotation or notes for the current system.
The tool provides facilities for tailoring the diagrams by changing color, font size, box shape, etc.
The content of each diagram is extracted from the database by a database search. The scope of this search is controlled
by the search control parameter. This parameter can be modified by the last of the tailoring facilities. Possible
settings of this parameter are described elsewhere. Care is needed in using this capability.
You can print the diagram that is currently displayed. For the time being you will need to minimise margins and
set print-orientation to landscape using the page-setup and print menus if you want the printout to be on one page. We
will be able to simplify this procedure when SVG printing capabilities are made available.