Note that a version can
actually be seen as a kind of logical time.
??? Freshness index by data deviation: For the sake of completeness, we also
want to mention a definition of the freshness index based on data deviation
(also known as arithmetic or numerical coherency condition) (Gallersd?¶rfer &
Nicola, 1995). However, such a freshness index definition is only meaningful
for single numerical data items. Let d(x,c) denote the data value of the replica
x stored at some node c. Then the data deviation freshness index is defined as
f(x,c) := 1 - | (d(x,c) ??’ d(x, c0)) / d(x, c0)|.
For a middleware-based architecture pursuing a component-oriented approach, only
the two first mentioned freshness indexes are feasible. The reason is that the coordinator
follows a ???black-box??? principle when managing the cluster nodes, which
means that it does not have direct access to the content of the nodes. Hence, it can-
OLAP with a Database Cluster 24
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of Idea Group Inc. is prohibited.
not base its freshness definition on concrete data values. Delay freshness offers the
most intuitive and workload-independent way of specifying the freshness needs of
a client (???at most 10 minutes old??? instead of ???at most 1000 updates behind???).
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