Is asychronous DNS lookup worth keeping at all?

[Hal Murray]

esr at snark.thyrsus.com said:
> Is asychronous DNS lookup worth keeping at all?  It's costly in code
> complexity and dependencies.  I've been thinking about how it affects
> performance and I'm now doubting that it is effective  in modern conditions.

I think that is premature.

I can't put my finger on a good example, but the use-case space is huge.  I 
think there is too high a chance that we haven't considered an important case 
yet.

There is a lot of interest in getting servers restarted quickly.  Telling all 
those users they can't use any non-local server names seems unwise.

I'd like to be able to do things like have the pool code occasionally verify 
that the servers it is using are still in the pool.  The idea is to help 
people remove their servers from the pool.  (The pool DNS side doesn't 
support that check yet.)

I think the code complexity will improve a lot when Joel rewrites that area.  
The wart that may not be obvious is the interaction with locking everything 
into memory.  In some environments, the end-of-thread code loads another 
library module which may not fit.

How evil are the dependencies?  How many environments don't have pthreads?

I'd be happy to do the DNS lookups from the main thread if the environment 
doesn't support pthreads.  Or at least do it that way until we discover an 
interesting use case.


> Nowadays, 99.9% of all NTP installations are configured to do a single pool
> name lookup.  ...

That's incorrect on two counts.

Most installations should be using the pool mechanism, but the distros aren't 
shipping that yet.  They are shipping an old version of ntp classic (with 
locally applied security fixes) where the pool code didn't quite work.  They 
use the pool via server directives in ntp.conf.  You see things like:
  server 0.fedora.pool.ntp.org iburst
  server 1.fedora.pool.ntp.org iburst
  server 2.fedora.pool.ntp.org iburst
  server 3.fedora.pool.ntp.org iburst

The pool code is likely to do more than one DNS lookup.  It may take more 
than one DNS lookup (with a TTL timeout in between) to get enough servers.  
After startup, if a server stops responding, the pool code drops it and uses 
DNS to find a replacement.


-- 
These are my opinions.  I hate spam.