[squid-dev] [RFC] reduce MISS on transients collision

[Amos Jeffries]

On 18/07/2016 6:59 a.m., Alex Rousskov wrote:
> On 07/17/2016 05:01 AM, Amos Jeffries wrote:
>> I've just been looking at the Store::Controller::find() implementation
>> and it struck me that if the transients lookup has an error the object
>> will fail to HIT on any existing cache entries.
> 
> If the transients table tells us that the transient object is in a "bad"
> state, then trying to load that same object from a store will fail at
> best or result in a stale/truncated/stuck response at worse.
> 
> 
>> Alex; am I missing something undocumented here ?
> 
> You might be missing something documented: The Transients definition or
> purpose. Transients is not one of the many "if not here than possibly
> there" cache stores. Transients is dedicated to cache entries in
> transient state. If a given entry can be correctly loaded from a regular
> cache store, then, by definition, that entry is not transient [any more]
> and would not be in Transients. Consequently, if the entry is in
> Transients, then it is impossible to load it correctly from a regular store.
> 
> It is possible that a lock contention or a similar SMP race condition
> inside Transients would result in a cache miss instead of reading from a
> Transients-controlled entry, but, bugs notwithstanding, that should not
> happen often.

Aha, thanks.

Amos