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Re: 20000510: LDM product keys

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  • Subject: Re: 20000510: LDM product keys
  • Date: Wed, 10 May 2000 14:25:10 -0600
> >From: address@hidden
> >Subject: LDM product keys
> >Organization: NOAA/NCDC
> >Keywords: 200005101645.e4AGji417069 LDM product headers
>
> How are the product keys determined in LDM?
>
> Basically I want to recieve a product, file it, then re-insert it with a
> new name into the ldm.pq.
>
> For example:
> NOAAPORT.NWSTG.TEXT.SAUS70.101521.0001972081.2000131152205121.*
>
> would be filed as:
> DISPLAY.NWSTG.TEXT.SAUS70.101521.0001972081.2000131152205121.*
>
> and re-inserted into the ldm.pq with the new filename.
>
> However, when I pqinsert it, it tells me the product is already in the
> queue.  With a new name I assumed this would a new unique key.
>
> Alan.
>
> ------- End of Forwarded Message

Hi Alan,

In the LDM product keys are determined by calculating a 'checksum' on the
product.  Basically, every bite of the product is examined and from them a
numeric value is calculated that almost uniquely describes the product.
(Some people refer to it as a 'fingerprint'.)

The checksum is calculated on the product alone, and not on the header
information that is prepended to the product.  So, if you're changing
values in the header, that won't be reflected in the checksum, and the LDM
will continue to insist that the product is already in the queue, which,
indeed, it is.

If you want the product to be reinserted into the queue, you will need to
remove the original product or modify the original product before
reinsertion.  It would be possible to remove the original product using
pqexpire and giving it a feed type and a highly specific pattern, but that
would be costly performance-wise as I think pqexpire will check every
product in the queue.  Modifying the product may or may not be feasible,
certainly less so if it's a binary product.

Why are you trying to reinsert the product under a different header?  If
you're simply trying to store the product on disk under a different name,
that could be achieved by making the right entry in the pqact.conf file.

Anne


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Anne Wilson                     UCAR Unidata Program
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                                  Boulder, CO  80307
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