Databases Reference
In-Depth Information
is in fifteen-minute increments, then the time variant metadata of each
row of dimension data must be expressed in fifteen-minute increments.
That would mean every row of dimension data would be able to identify
the moment that row came into effect, and out of effect, in fifteen-minute
increments. Fifteen-minute periodicity would also mean the ETL appli-
cation is reading all Factory Worker values, all Raw Material values, all
Location values, and all Formula values every fifteen minutes. Typically,
the managers of most operational systems do not appreciate that sort of
resource consumption every fifteen minutes, every hour, or any other fre-
quency other than that source system's down time.
Typically, every operational system has its own resource consumption
peaks and valleys. During the resource consumption peaks, an opera-
tional system is busy performing its primary function for the enterprise.
During that time, the managers of an operational system typically prefer
to focus all resources on that primary function. During the resource con-
sumption valleys, an operational system has a higher level of availability
to deliver its data to an Extract function. Chapter 11 discussed the archi-
tectural considerations for a real-time ETL application versus a periodic
batch ETL application. Those considerations, coupled with the resource
availability of operational source systems, would indicate that the Extract
function will happen on a regular periodic frequency that will be synchro-
nized with the resource consumption valleys of the operational source
system. That regular periodic frequency is most often an ETL Cycle that
occurs once every 24 hours (i.e., once daily).
time variant metadata
When the ETL Cycle for a dimension table occurs once per day, the time
variance for that dimension table is at the day level. That means that a row
of data in a dimension table begins on a day and ends on a day, rather than
beginning as of a minute of a day and then ending as of an hour of a day,
or rather than beginning as of a week of a year and then ending as of a
month of a year. Time variance for a dimension table at the minute, hour,
week, and month is irregular and rather absurd. An ETL Cycle of a min-
ute would mean repeating the ETL application and all its functions every
minute. An ETL Cycle of an hour would mean repeating the ETL applica-
tion and all its functions every hour. An ETL Cycle of a week would mean
waiting seven days to repeat the ETL application. Finally, an ETL Cycle
of a month would mean waiting a month to repeat the ETL application.
