On 6/09/2010 5:38 PM, kmpm wrote:
On Sep 6, 9:02 am, Mike Dewhirst<mi...@dewhirst.com.au> wrote:
On 6/09/2010 4:19pm, kmpm wrote:
I have a project running in a manufacturing industry that is actually
built upon django.
In this I need to generate a unique serial, batch or lot number
(depending on what you would like to call it) that is a running number
from 0 to whathever for each and every day.
So when every day ticks over at midnight, the serial number starts at
zero again? Does production continue across the time 00:00? Why don't
you use a continuously increment number?
I don't know what the "right" thing to do is. I'm sure there will be a
classic solution. I think I would be writing a singleton function
nextnum() which returns the next number as required and then
lot_no = models.IntegerField(default=nextnum)
I'm not sure what would happen threadwise but a singleton would be a
natural place to deal with thread-locking and restarting the sequence
when the clock ticks over.
lot_no then gets the value prior to the save()
Mike
Yes it ticks over at midnight, and no there is no production at that
time. More or less office hours only.
A singleton would be nice but as you said it's not thread safe and
won't scale nicely if we were to use multiple frontend servers or
anything like that.
Then it would need to be a database trigger. At least in PostgreSQL you
could write it in the Python proc language supplied. It could create a
new temporary table with an incrementing column each night as a source
for the sequence number. That would cover occasional system stoppages
during the day.
I'm not sure how to integrate that in a nice way with django models
except as you alreadhy indicated with unique for date.
There is a project called django-idmapper[1] that I just found that
might solve it by using a model that seems to be stored in shared
memory.
If running on multiple frontends that would need to be something like
memcached but I don't know about thread safety and concurrency there
either.
[1]http://github.com/dcramer/django-idmapper
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