Databases Reference
In-Depth Information
2.1.1 The Memory Function
The memory function is needed by the other two functions. Its objective is
to maintain internally a representation of the state of the domain. Moreover,
the representation must be correct and complete [4].
The memory function can be performed in two execution modes: on
request or autonomously. In the first mode, the system memorizes the state
because a user explicitly tells the system the state and orders (normally
implicitly) the system to memorize it. For example, a system knows custom-
ers addresses because any time a customer changes addresses, a user tells the
IS the new address and expects the system to remember it. The IS has no
way to know customers addresses except by a user explicitly telling it that
information.
In the second mode, autonomously, the system memorizes the state of
the domain without an explicit request from a user. This mode has two vari-
ants. In the first variant, the system is able to observe autonomously the state,
for example, a system that periodically reads a device that gives the tempera-
ture of a building. In that case, the system can maintain a representation of
the temperature because it gets it directly from the environment. The second
variant is related to the active function and will be described later.
The memory function is considered to be passive, in the sense that it
does not perform anything that directly affects users or the domain. How-
ever, it is required by the other functions and constrains what they can
perform.
2.1.2
The Informative Function
With the informative function, the system provides users with information
about the state of the domain. Often, the state of the domain is observable
directly in the domain, and at the same time it is represented in the IS. For
example, the quantity of a given product in the shelves of a retail store may
be observed when necessary, and at the same time it can be represented in the
IS. In those cases there is a redundancy, but it is a desired one, because it may
be easier to ask the system than to observe the domain.
In other cases, the state is represented only in the IS, and it is not possi-
ble (or difficult) to observe it directly in the domain. For example, in a retail
store it is not possible to observe how many units of some product have been
sold up to a given moment. As another example, in the banking domain,
consider balances of accounts. The balance of an account at a given instant
cannot be known by observation of the owner of the account or the bank
office where the account was created. The only place where balances are
