David Fisher wrote:
An interesting discussion. Since I am about to configure such a load
balancer and we prefer to use DNS, understanding this type of detail is
critical.
The OP said that the reason that the DNS did not resolve was that the
machine had been moved off the network. That may have been an event out
of the control of the sysadmin for the web service. Suppose someone
takes a server away and then a week later apache and mod_jk are
restarted via a cron job in the middle of the night? Suddenly the web
service is down.
I think you misunderstand the issue.
If a server (hosting a Tomcat corresponding to a worker) is simply down,
or not there, that will not stop mod_jk starting up.
What stops it starting up, is that /the DNS lookup for this machine's IP
address does not work/.
As long as the DNS system is working, it will provide an IP address, and
mod_jk will be happy and start up.
Later, when mod_jk will try to access that worker (always by IP), it
will notice that it is down, and just mark it so and check it from time
to time until it is up again.
The point of Rainer was that if in your configuration you use DNS names
instead of IP addresses, then it means that you consider DNS a pretty
important part of your application/setup.
Thus if at startup the DNS system is not working for whatever reason, it
will prevent mod_jk to start up properly.
But only then.
To my knowledge, the only case where the DNS would fail to provide an IP
address of a correctly-written FQDN name, is if you have some
configuration where your hosts register themselves under some variable
IP address when they startup. But that would be a strange setup for
servers, no ?
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