Drupal cache_get - why it doesn't always give you what you expect

by John McGeechan

Drupal cache_get - a gotcha


Background

Noticed that cache_get did not seem to obey the cache lifetime, returning 'expired' cache data

Overview

After over-abusing the drupal cache tables, to store any and every bit of custom cached data, using the default 'cache' table, I decided to roll my own
I set up a new cache table - easy enough to do, just copy the structure of the rest of the cache tables. But it didn't - as I erroneously assumed - return zilch if the cache had expired, it simply returned the expired entry

Solution/Conclusion

Turns out the cache_get does not check the expiration date before returning the data. If you have regular caching on your site, it will return the cached data to you and leave you to work out if it is stale or not. Only on a clear cache all, will the data get cleared down.
as it goes, checking is fairly trivial...

Cache set 
// cache data for an hour

cache_set($cid,$yourdataToCache,'cache_custom_cache_table',time() + (60*60));

Cache get 
// check cached data

if(($cachedCopy = cache_get($cid,'cache_custom_cache')) && !empty($cachedCopy->data) && (time() < $cachedCopy->expire )) {
  ...reuse cached data
} else {
  ..rebuild cache
  cache_set($cid,$availableOnOtherSites,'cache_custom_cache',time() + (60*1));
}
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