Content Delivery Networks

Tue Aug 14 09:24:19 UTC 2007

"Chris L. Morrow" <christopher.morrow at verizonbusiness.com> writes:

> that sets a lower-bar on TTL in the nscd cache -
>
> (from the manpage for nscd.con)
>
>      positive-time-to-live cachename value
>            Sets the time-to-live for positive entries (successful
>            queries) in the specified cache.   value is in integer
>            seconds.  Larger values increase cache hit  rates  and
>            reduce mean response times, but increase problems with
>            cache coherence.  Note that sites that  push  (update)
>            NIS   maps  nightly  can  set  the  value  to  be  the
>            equivalent  of 12 hours or more with very good perfor-
>            mance implications.
>
>
> This is still a client issue as, hopefully, the cache-resolvers don't
> funnel their business through nscd save when applications on them need
> lookups... (things like ping/telnet/traceroute/blah)

nscd may represent a problem if the application in question is a
http-proxy without it's own resolver.  There's also a number of
more-or-less broken http-proxies doing their own resolver caching
regardless of actual TTL.

Such applications represent a problem wrt any DNS-based load balancing,
including CDNs, since they can serve a large number of end-users,
redirecting them to the "wrong" address long after the TTL should have
expired.

Bjørn