Information Technology Reference
In-Depth Information
Information actions model the information processing in the system.
Every action of interest for modelling purposes is associated with at least
one object. Actions also have types, which characterize the sets of actions
that share particular properties. An example of an information action is the
request by user
"JoeSmith"
for the repair of the user handset with serial
number
"SN33433/09"
at the PhoneMob service centre in downtown Amster-
dam. The corresponding action type is RepairRequest, whose parameters are
a user handset, a user and a PhoneMob service centre. Note that this is an
abstract action, independent of the more detailed functional specification to
be dealt with in the next chapter.
Actions cause state changes in the objects that participate in them. For
example, after a RepairRequest action the state of the information object User
Handset is initialized to Broken | see gure 3.3. These state changes are
described by means of dynamic schemata,
1
as discussed later.
Apart from these elements, the information language also defines three
kinds of structure (called a schema ); this allows us to organize the infor-
mation specification in terms of the behaviour of the information objects,
described by a set of dynamic schemata ; the constraints that apply to the
objects and their behaviours, described by a set of invariant schemata ;
and a configuration of information objects at some moment in time, described
by a set of static schemata .
The different kinds of schema may apply to the whole system, or they may
apply to particular domains within it.
A dynamic schema specifies how the information evolves as the system
operates, describing the allowable state changes of one or more information
objects. A dynamic schema can, for instance, describe what happens when a
valid request for repairing a handset is added to the system (figure 3.3), or
express the overall behaviour of User objects. A state change involving a set
of objects can be regarded as an interaction between those objects.
In addition to describing state changes, a dynamic schema can also describe
the creation and deletion of information objects. For example, a RepairRequest
action results in the creation of a RepairOrder information object and, option-
ally, of a Loan object if the user is allowed to have a loan handset during the
repair process and there is a free loan handset in the shop.
An invariant schema is a set of predicates constraining one or more
information objects that must always be true, for all valid behaviours of the
system. It can describe the types of the information objects, their relation-
ships, and the constraints on those types and those relationships. The pred-
icate constrains the possible states and state changes of the objects to which
it applies. The behaviour specified in any dynamic schema is subject to the
constraints of the invariant schemata. For instance, an invariant schema can
constrain the values of a given information object, or the occurrence of an
action, such as \Repair orders can only be placed by users whose companies
1
In English, the plural of schema isschemata.
