If-Modified-Since

[Geoff Simmons]

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On 2/24/11 11:10 AM, Poul-Henning Kamp wrote:
> In message <4D662DA4.3030108 at uplex.de>, Geoff Simmons writes:
> 
>> On 2/24/11 9:59 AM, Poul-Henning Kamp wrote:
>>> In message <4D6578CE.7070708 at uplex.de>, Geoff Simmons writes:
>>>
>>>> @phk: If we also implicitly set obj.conditional_timeout = 0 when obj.ttl
>>>> is set to 0, wouldn't existing VCL continue to do what people expect? 
>>>
>>> Yes, I thought I had communicated that already:  when ttl is set to <= zero,
>>> grace and conditional_timeout SHALL also be set to zero.
>>
>> You did indeed. So then my question is, is there a disadvantage to
>> allowing an object to be used for the IMS request if ttl and grace have
>> both elapsed, but conditional_timeout has not yet?
> 
> yes, we don't know why it was expired.

To follow up for the list, we continued the discussion on IRC, and
Poul-Henning wants to be sure that if a ban has been requested for an
object, either with one of the purge commands or by setting obj.ttl=0 in
VCL, then it is expired and not used, regardless of the value of
conditional_timeout.

@phk: One more question. %^) We could do this by checking whether
obj.ttl was set to 0, or whether obj.ttl+obj.grace has elapsed. (We're
always passing over any object with a ban.) In other words, we could
reject the object if the expiration was specifically requested, but
otherwise use it if grace has not yet elapsed and there's still time
left for conditional_timeout; or we could reject it if grace has elapsed
for any reason.

In the first case, we would reject the object for IMS if:

o->ttl <= sp->t_req && HSH_Grace(o->grace) <= 0

In the second case, we reject it if:

o->ttl + HSH_Grace(o->grace) <= sp->t_req

In the second case, there's actually no point in having
obj.conditional_timeout > obj.grace; an object is evicted when grace
elapses, no matter what. In that case, we should *not* do something we
were talking about last week:

>   - the cache eviction time for an object (oc->timer_when) becomes
>     o->ttl + max(o->conditional_timeout, HSH_Grace(o->grace))

If eviction is only controlled by ttl and grace, then this should stay
the way it is now (oc->timer_when = o->ttl + HSH_Grace(o->grace)).


Best,
Geoff
- -- 
UPLEX Systemoptimierung
Schwanenwik 24
22087 Hamburg
http://uplex.de/
Mob: +49-176-63690917
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