[NTLUG:Discuss] Use DNS for redundant geographic sites?

[Neil Aggarwal]

Robert:

I am designing this for an e-commerce site, so I would like to make
the switchover automated instead of manual.

I am going to ask my datacenter if they are willing to set up a
Virtual IP.

Thanks,	
	Neil

--
Neil Aggarwal, (214)986-3533, www.JAMMConsulting.com
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-----Original Message-----
From: discuss-bounces at ntlug.org [mailto:discuss-bounces at ntlug.org] On Behalf
Of ntlug at thorshammer.org
Sent: Wednesday, November 22, 2006 4:32 PM
To: NTLUG Discussion List
Subject: Re: [NTLUG:Discuss] Use DNS for redundant geographic sites?

> My ultimate goal is to create a web site that will
> always be available.
> 
> I have two datacenters in two different cities so 
> I thought I could use a server in each of them to
> give me ultimate redundancy.
> 
> Any suggestions?

`	What kind of redundancy are you looking for? Primary and flip a
switch to backup, both active/load balanced, etc? How long after a failure
is it acceptable to be up and running again: realtime, < 1min, < 5min, <
30min, etc? What kind of data needs to be mirrored and how often does it
change: static content (low updates), cataloged content (freq updates),
transactions (realtime updates)?

	An easy solution is to have your DNS server give out both IPs to
both sites (assuming you need non-realtime sync'ing between locations). Each
location monitors the other and stops giving it out when it detects a
failure. In the event of a failure, there will be a 5 min window where 50%
of your traffic will probably fail (hitting the down node). Your outage
window will be the sum of your polling interval and the DNS cache timeout.
If you poll every 30 seconds and the DNS cache timeout is 5 min, then the
longest outage is 5.5 min.

	Can your two datacenters route each other's IPs (more than likely
yes if they are the same company)? This will provide failover at the IP
level and take DNS out of it. This can lower the failover window depending
on what kind of outage it is (site down, IP unreachable, etc). This won't
correct apache failing (or anything else above layer 4 of the OSI model).

Robert

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