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[Bug 61891] infinitive loop of AH01323: Purge of LDAP cache failed


https://bz.apache.org/bugzilla/show_bug.cgi?id=61891

--- Comment #2 from Hendrik Harms <hendrik.harms@xxxxxxxxx> ---
A small example to make the issue more clear:

Imagine we have a cache that could store 20 items -> maxentries=20
and the TTL for the items is 10 seconds.
markentries = maxentries*3/4 = 15
We have 11 power users sending a request every 5 seconds
We have 20 lazy users sending a request once a minute (average 1 per 5 seconds)

We start with an empty cache at 0 sec
at  5 sec 12 items a taken (11 power + 1 lazy)
at 20 sec 15 items a taken (11 power + 4 lazy)
  -> markentries reatched -> set marktime = 20
at 45 sec 20 items a taken (11 power + 9 lazy)
at 50 sec a new lazy entry should be added into the cache but cache is full
  -> a purge starts
    The purge could only delete the 4 lazy entries older than marktime
    cause the power entries are already renewed. (TTL=10)
    So we left with 16 entries in our cache.  
    16 is greater than 15 ! So we never set a new marktime and
    all following purges could not delete any item. -> bang -> AH01323

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