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that of transactions as designating (i) an object, (ii) a span of
time, (iii) business data and (iv) an action to take—but another
thing to provide the details. To that task we now turn.
A Taxonomy of Temporal Extent State
Transformations
Because of the complexities of managing temporal data, we
need a way to be sure that we understand how to carry out every
possible
temporal extent state transformation
that could be spe-
cified against one or more asserted version tables. A temporal
extent state transformation is one which, within a given period
of assertion time, adds to or subtracts from the total number of
effective-time clock ticks in which a given object is represented.
We need a taxonomy of temporal extent state transformations.
As we explained in Chapter 2, a taxonomy is not just any hier-
archical arrangement we happen to come up with. It is one
whose components are distinguished on the basis of what they
mean—and not, for example, on the basis of what they contain,
as parts explosion hierarchies are. It is also a hierarchical
arrangement whose components are, based on their meanings,
mutually exclusive and jointly exhaustive. Because good
taxonomies are like this, constructing them is a way to be sure
that we haven't overlooked anything (because of the jointly
exhaustive property) and haven't confused anything with any-
thing else (because of the mutually exclusive property).
We begin with objects and episodes. The target of every tem-
poral transaction is an episode of an object. Semantically, it is
episodes which are created or destroyed, or which are
transformed from one state into another state.
Physically
,of
course, it is individual rows of data which are created and
modified (but never deleted) in an asserted version table. But
what we are concerned with here is semantics, not bits and
bytes, not strings of letters and numerals. From a semantic point
of view,
episodes
are the fundamental managed objects of
Asserted Versioning.
Figure 9.5
shows our taxonomy. Under each leaf node, we
have a graphic representing that transformation. A shaded rect-
angle represents an episode, and a non-shaded rectangle
represents the absence of an episode. A short vertical bar separa-
tes the before-state, on the left-hand side, from the after-state,
on the right-hand side, produced by a transformation.
Each of the nodes in our Allen relationship taxonomy are
referred to, in the text, by surrounding the name of the node with
