An ontology is a set of declarative knowledge representing
every significant concept within a particular application domain. The significance
of a concept is easily assessed, if meaningful interaction can not occur between
agents without both parties being aware of it, then the concept is significant
and must be modelled. Note that for convenience we use the term 'fact' throughout
ZEUS to describe an individual domain concept.
Prior to attempting this stage you should have already identified
the following:
When the ontology has been loaded and parsed, its contents
will appear in the Ontology Editor's windows. A screenshot of the Ontology
Editor is shown in Figure 2.1.
Note that the fact pane of the Ontology Editor is divided
into two, an upper panel that displays the Fact Hierarchy, and a lower panel
that displays the attributes of the currently selected fact. The panels are
separated by a split-pane that can be moved up or down to enlarge one panel
at the expense of the other.
Figure 2.2: A Screenshot of one of the panels of the Ontology Editor.
Notice how the one of the nodes in the fact hierarchy (Monitor) has been selected,
and its attributes are being shown in the table below.
ONT-3: How to Create a New Fact
The first stage in creating a new fact is to determine its
position within the existing fact hierarchy:
If the
new fact has no parents in the existing hierarchy you will typically choose
the Entity node and click on the "Add New Child Fact" button.
The Entity fact is used to represent concepts that
have physical realisations, like objects and commodities. Where this is not
appropriate the ZEUS fact hierarchy also has a fact called Abstract that
does not possess cardinality and value attributes.
If the
new fact inherits from an existing fact, select the parent node and click on
the "Add New Child Fact" button.
To make
the new fact share the same parent as another, (a useful short cut), select
the existing fact and click the "Add New Peer Fact" button.
A new entry will now appear in the fact tree, double click on it and rename to whatever is appropriate.
ONT-4: How to Create a New Attribute
Ensure that the fact to which the attribute will be added is the selected node of the
fact hierarchy, if not, clicking on the fact selects it. The existing attributes
for the selected fact will then appear in the Attribute Table beneath the Fact
Hierarchy window.
Click on the "New" button in the toolbar above the Attribute Table. This
will create a new entry in the table, double-click on it to rename it as appropriate.
Clicking on the 'Toggle Shown Attributes' will hide or show the attributes inherited
from the fact's parents. This can be useful for determining whether an attribute
has already been declared higher in the fact hierarchy.
ONT-5: How to Set the Type of an Attribute
Types serve to restrict the set of valid values the attribute
can take, and will be one of the basic types, a restriction or another fact.
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| Figure 2.3: A screenshot of the Type Selection window |
Double-click on an entry in the Attribute Table, this will summon a window showing
all a hierarchy of currently known types, as shown in Figure 2.3.
The basic types have been predefined, they are:
String: any combination
of alpha-numeric characters, plus the _ and $ characters only
Integer: whole numbers
in the range -2147483648 to +2147483647, (these are the minimum and maximum
integer values in Java).
Real: all numbers in the range 4.94e-324
to 1.798e308, (these are the minimum and maximum double values in Java).
Boolean: the words true or false
Date: numerically expressed in the European format: dd/mm/yyyy, (i.e. 2 digits for day/ 1 or 2
for month/4 for year). Terms can also be separated by dashes, i.e. dd-mm-yyyy.
ZEUS is millennium compliant.
Time: numerically expressed using the 24-hour clock
format, as either hh:mm:ss, (i.e. 1 or 2 digits for each of hours:minutes:seconds).
For convenience, seconds can be omitted, and hh:mm used instead.
JavaObject: an opaque reference to a runtime Java object
Restrictions are custom types used to further restrict the
values possible for basic types. These are entered separately, see 'How to
Create a Restriction'. All previously entered restrictions are listed under
the "Restrictions" node of the type selection window.
Where attributes need to assume more complex values, their
type can be specified as a particular fact. All previously entered facts are
listed under the "Facts" node of the type selection window.
ONT-6: How to Restrict an Attribute
An alternative to creating a restriction rule is to restrict each attribute individually, this
is achieved by double clicking on the "Restriction" cell of the attribute
entry, which opens a small green-coloured panel through which the restriction
can be typed.
The syntax of restrictions is that of a simplified expression,
full details will be included in a later release.
ONT-7: How to Set Attribute Defaults
To give an attribute a default value, double click on the "Default" cell of
the attribute entry, this opens a small green-coloured panel through which the
default value can be typed.
The syntax of the value is
dependent on the attribute type. Defaults can not be set for attributes whose
type is a restriction or another fact.
ONT-8: How to Create a Restriction
Restrictions are types that have been constrained to a particular
set or range of values. Although it is possible to restrict attributes without
creating a restriction (see 'How to Restrict an Attribute'), restrictions allow
for reuse, and provide meaningful names. Restrictions are entered through the
'Restriction Definitions' pane of the Ontology Editor.
To create
a restriction click on 'Restriction Definitions' tab at the bottom of the Ontology
Editor. This shows the list of known restrictions (initially empty).
Add a new
restriction by clicking on the "New" button on the toolbar, this will
create a new entry in the Restriction Table, double click on it to edit its
name to something more appropriate.
Now set
the type of the restriction by double clicking on its "Type" cell,
this brings up a window with a hierarchy of basic types (see 'How to Set the
Type of an Attribute' for details) and current restrictions. Facts can not
be chosen as a type.
Now double
click on the "Restriction" cell, this opens a small green-coloured
panel through which the default value can be typed. The syntax will be dependent
on the type chosen previously.
The restriction will be syntactically checked upon entry,
invalid entries being displayed in red.
ONT-9: How to Save an Ontology
Ontologies are currently saved in an ASCII file using our
own format; saving ontology in a more standardised format, like XML is under
consideration.
If the
Ontology Editor is not open, click on the "Save" button in the Generator's
Ontology Options toolbar, or select the "Save Ontology" menu option
from the Ontology Menu.
If the Ontology Editor is still open, choosing the "Save" menu option will
save its contents.
If the ontology is unnamed a file requester will appear, and you will be prompted for
a filename to save it under.
If the ontology has already been named it will be saved under that name. To save it
under a different name, choose the "Save Ontology As…" menu option.
Merging Ontologies
This function is not yet supported in the current Ontology Editor.
If the
Ontology Editor is not running, click on the "Edit" button in the
Generator tools' Ontology Options toolbar, or select the "New Ontology"
menu option from the Ontology Menu. This will create a new (empty) ontology
and launch the Ontology Editor. From now onward all facts entered through this
editor will become part of this ontology.
If the
Ontology Editor is already running, select the "New" menu option;
if there is currently an ontology in memory you will be asked whether you want
to save it. Clicking on the "New" button in the Generator tools'
Ontology Options toolbar will clear the current ontology without opening the
Editor.
If the
Ontology Editor is not running, click on the "Load" button in the
Generator tools' Ontology Options toolbar, or select the "Load Ontology"
menu option from the Ontology Menu.