telling code to pull from the database when the data in a table changes

Thomas Seifert thomas-lists at
Mon May 21 14:14:30 UTC 2007

Hi Nathan,

I think thats simply up to the application how this is handled.
E.g. you could just "delete" or "update" the cached variable at the
place where the data in the table is changed, thats
how we handle it in most areas.
Or you use some "version" which increases if the data changed and use
that version-number together with the key to retrieve
data from memcached.


Nathan Nobbe schrieb:
> All,
> I recently began investigating memcached, and im very excited about
> it.  im planning on using it w/ php to implement variables whose
> lifetime can be the duration of the application.
> I understand that storing the results of queries to the database can
> increase efficiency if those variables are checked for the data first
> on consecutive requests for the data they store, but
> i still have a disconnect...
> what i dont understand is when the data profile of a particular table
> changes, and the variable in the memcache that represents that table
> becomes stale, how can the variable be updated efficiently?
> obviously w/ php and a  unix system, a simple cron could be used to
> poll the database periodically to see if there are changes to the data
> profile of a given table.  i was thinking to make this even more
> efficient; triggers could be employed (im using mysql 5+) to update a
> table, call it recent_tables_that_changed, dedicated to signifying
> particular tables in the database have changed.  then the cron'd php
> script could simply check recent_tables_that_changed  to determine
> which, if any, application variables had become stale (do to a change
> in data profile).  the stale variable  could then be updated and the
> record removed from  recent_tables_that_changed.
> though decent, i am personally not a fan of this approach, because
> there would still be a frequent query running against the database
> just to determine what data had changed so that application variables
> in the cache
> would not remain stale for long once theyve become stale. 
> additionally, the mechanisms' responsiveness would be limited to the
> time granularity of cron, which i believe is a minute.  im wondering
> if there is some way to signal the cache only when tables, represented
> by application variables, have changed.  if you dont feel like typing
> out a response, because this is probably a naive question, would
> anyone mind directing me to a resource or two where i could read up on
> the topic myself?
> (sorry for the lengthy posting)
> thanks,
> -Nathan

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