Linux HA may not be the best choice in your situation. [CentOS]High Availability using 2 sites

[Les Mikesell]

On Thu, 2006-01-05 at 21:22, Tim Edwards wrote:
> I'd already given up on Round Robin or any other kind
> >>of DNS 'solution' before I posted, after reading this: 
> >>http://homepages.tesco.net./~J.deBoynePollard/FGA/dns-round-robin-is-useless.html
> > 
> > 
> > That page seems to be written with the premises that all clients
> > are in the same location, served by the same dns cache which
> > certainly won't be the case on the internet,
> 
> Isn't it the other way around - the article is explaining why Round 
> Robin DNS won't be effective when the clients aren't all in the same 
> location, ie. when they're spread around the net.

No, it describes what happens when a large number of clients are
behind the same local DNS server - and not very realistically
at that.

> > and that browsers
> > don't try anything but the first address in the DNS response
> > which isn't true either
> 
> I can't see where it says that. All it says that there is no provision 
> for ordering of records in DNS and therefore clients will most likely 
> disregard any ordering of records you try to impose.

What really happens is that the authoritative server will rotate
the order of the list on each request, but downstream DNS servers
and the client itself will cache a certain order once received.
With a large number of clients connecting from a large number
of places, the order distribution will be essentially random.
Most client apps will try the first address in the list first
and many won't continue if that fails.  The versions of IE
that I've tested do try additional addresses although that
might not happen the same way behind a web proxy.

> > and that statistically distributing
> > the load among servers isn't useful.
> 
> I can't say where it says that either. The author suggests that people 
> who want load balancing should use SRV records or a real load balancer 
> (things like LVS I assume), instead of trying to do it through RR DNS. 
> This is not saying load balancing isn't useful.

SRV records have their advantages including the ability to specify
preferences and ports for a server.  However, multiple A records
work reasonably well to distribute the connections from a large
number of clients.  With applications like IE that will retry
using the additional addresses, you also get failover.  It isn't
perfect and doesn't work even for all http apps (note the
occasional complaint here about yum not connecting to
the mirror.centos.org repository when only a single site is
down) but it will keep a lot of web browsers happy when
your main site is down.

-- 
  Les Mikesell
    lesmikesell at gmail.com