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Design Issues
Page 158
The trigger would check the index of the file to find the item-id associated with the
quarter, country, and HS code defined in the record. If the index value did not exist, then
it would allow the write to the file; if the item-id was the same as the item currently being
written, then it would allow the write (overwriting the existing item); if the item-id was
different from the item currently being written, then it would disallow the write.
7.4
What is the “Best” ID for the Exports File?
So far, we have really considered two ID structures for the exports file. The first uses
three elements of meaningful data spliced together, while the second uses a sequential
number. Those ID’s for equivalent items are shown below:
20123*CN*02
343198
The biggest problem with the first of these is that country codes change periodically. We
don’t have that issue with the second item-id, but it requires some extra steps to allow
lookups to related items; it requires that we index the file using the unique combination
of elements; and we need to guard against creating multiple entries for nominally unique
combinations of elements.
Given that the major issue is with country codes, why don’t we simply replace these in
the first structure with a numeric code? That would make the first item-id:
20123*156*02
This may not totally get us away from changes in country codes, but it is a good step in
the right direction. With this structure, we can’t create multiple records for the same
unique combination. Related item lookups are relatively easy. We don’t need to index the
file (although indexing will still improve performance).
What lessons can we take from this?
➢
You need to think carefully about your item-ids
➢
It is helpful to construct your
make up a unique combination
ID
from multiple small elements which together
◦
This is better than using a single sequential number to embody all uniqueness
◦
Each element in the ID should not be subject to change
Getting Started in OpenQM - Part 1
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