dns and software, was Re: Reliable Cloud host ?

William Herrin bill at herrin.us
Wed Feb 29 14:18:47 UTC 2012

On Wed, Feb 29, 2012 at 7:57 AM, Joe Greco <jgreco at ns.sol.net> wrote:
>> In message <CAP-guGXK3WQGPLpmnVsnM0xnnU8==4zONK=UWTLkYWuduA6T9Q at mail.gmail.com>,
>>  William Herrin writes:
>> > On Tue, Feb 28, 2012 at 4:06 PM, Mark Andrews <marka at isc.org> wrote:
>> > > DNS TTL works. =A0Applications that don't honour it arn't a indication th=
>> > at
>> > > it doesn't work.
>> >
>> > Mark,
>> >
>> > If three people died and the building burned down then the sprinkler
>> > system didn't work. It may have sprayed water, but it didn't *work*.
>> Not enough evidence to say if it worked or not.  Sprinkler systems
>> are designed to handle particular classes of fire, not every fire.
> It is also worth noting that many fire systems are not intended to
> put out the fire, but to provide warning and then provide an extended
> window for people to exit the affected building through use of sprinklers
> and other measures to slow the spread of the fire.

Hi Joe,

The sprinkler system is designed to delay the fire long enough for
everyone to safely escape. As a secondary objective, it reduces the
fire damage that occurs while waiting for firefighters to arrive and
extinguish the fire. If "three people died" then the system failed.
Perhaps the design was inadequate. Perhaps some age-related issue
prevented the sprinkler heads from melting. Perhaps someone stacked
boxes to the ceiling and it blocked the water. Perhaps the water was
shut off and nobody knew it. Perhaps an initial explosion damaged the
sprinkler system so it could no longer work effectively. Whatever the
exact details, that sprinkler system failed.

Whoever you want to blame, DNS TTL dysfunction at the application
level is the same way. It's a failed system. With the TTL on an A
record set to 60 seconds, you can't change the address attached to the
A record and expect that 60 seconds later no one will continue to
connect to the old address. Nor 600 seconds later nor 6000 seconds
later. The "system" for renumbering a service of which the TTL setting
is a part consistently fails to reliably function in that manner.

Bill Herrin

William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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