Databases Reference
In-Depth Information
One thing it might mean is “more available than by using
backups and logfiles”. And the most salient feature of the
advance from backups and logfiles to these other methods of
managing historical data is that backups and logfiles require
the intervention of IT Operations to restore desired data from
off-line media, while history tables, warehouses and data marts
do not. When IT Operations has to get involved, emails and
phone calls fly back and forth. The Operations manager com-
plains that his personnel are already overloaded with the work
of keeping production systems running, and don't have time
for these one-off requests, especially as those requests are being
made more and more frequently.
What is going on is that the job of Operations, as its manage-
ment sees it, is to run the IT production schedule and to com-
plete that scheduled work on time. Anything else is extra.
Anything else is outside what their annual reviews, salary
increases and bonuses are based on.
And so it is frequently necessary to bump the issue up a level,
and for Directors or even VPs within IT to talk to one another.
Finally, when Operations at last agrees to restore a backup and
apply a logfile (and do the clean-up work afterwards, the man-
ager is sure to point out), it is often a few days or a few weeks
after the business use for that data led to the request being made
in the first place. Soon enough, data consumers learn what a
headache it is to get access to backed-up historical data. They
learn how long it takes to get the data, and so learn to do a quick
mental calculation to figure out whether or not the answer they
need is likely to be available quickly enough to check out a
hunch about next year's optimum product mix before produc-
tion schedules are finalized, or support a position they took in
a meeting which someone else has challenged. They learn, in
short, to do without a lot of the data they need, to not even
bother asking for it.
But instead of the comparative objective of making temporal
data “more available” than it is, given some other way of manag-
ing it, let's formulate the absolute objective for availability of
temporal data. It is, simply, for temporal data to be as quickly
and easily accessible as it needs to be. We will call this the
requirement to have seamless, real-time access to what we once
believed, currently believe, or may come to believe is true about
what things of interest to us were like, are like, or may come to
be like in the future.
This requirement has two parts. First, it means access to non-
current states of persistent objects which is just as available to
the data consumer as is access to current states. The temporal
