[plug] Uninterupted service network

[shayne]

That would mean you have to guarantee two servers to work to get 100%
uptime. Using failover means that you reeeealy only have to guarantee
the first but hope the second one works if the first one kaks.

Scenario 1 (ALTERNATING DNS)
Each guarantee a portion of 50% of the whole time, and say provides 80%
uptime each
<-------server1--50%----><--------server2--50%--->100%
<-----UP----------><down><-----UP----------><down>100%
You still have only statistically 80% uptime
Scenario2 (Failover)
<-------server1--80%----------------><-S280%-><dn>100%
Ultimately in fail over scheme you have 20% x 20% downtime,
or 4% (I think? No mathbrain turned on now) downtime!!!

The bastard of course is in the Sync!

On Sat, 2002-10-12 at 21:31, Justin Hall wrote:
> Could you use 'round robin' DNS table? DNS hands out an IP from a pool of 
> addresses for a HOST name... then only 1 of n addresses would have a problem 
> if a router goes down... (so with 2 servers half of accesses would get the 
> bad server?)
> but that is not quite good enough..
> 
> what about you have the 'magic' ip address that the services should respond to 
> XXX.
> Primary has addresss YYY and an Alias to XXX, a secondary server has another 
> address ZZZ.
> The secondary monitors the state of the primary server (through XXX ip 
> address)... primary goes down.. secondary puts up an address alias of XXX...
> Primary comes back up with only the address YYY - sees that XXX is active on 
> the secondary.. so it now monitors the secondary.. the secondary goes down.. 
> it alias' the XXX address - when the secondary comes up it ....
> oh you may need aliases on both interfaces.. as it is the router ;)
> 
> Is that thinking sound or useful? Or am i out of my tree?
> 
> Justin 
> On Sat, 12 Oct 2002 11:54 am, Jeff Williams wrote:
> > I was wanting to setup up a network where:
> >
> > All of the servers (www, mail, sql) and their backups are machines on a
> > 192.168.0.* network.
> > All of the DNS entries for the servers point to the outside address of
> > the router for the 192.168.0.* network, and the router
> > uses NAT to send the data to the appropriate server.
> > In the event of a server down the router routes data to the backup
> > server, util the master comes back up again.
> >
> > Now, I don't have a problem with setting up the above, but I what
> > happens if the router dies? I was wanting any failures to be covered
> > without manual intervention, but I can't think how you can go to a
> > backup router. Is there a solution, or am I heading in the wrong direction?
> >
> > JEff
>