[LON-CAPA-admin] Adding Access servers

Gerd Kortemeyer korte at lite.msu.edu
Thu Mar 31 08:27:49 EST 2005

Hi Hon-Kie,

Your round-robin might actually be working fine. What's happening is:

When a client computer does a DNS request, the DNS servers sends an IP 
address from the pool. For the next DNS request it gets, it sends the 
next IP, and so on. Round robin.

But: The client then caches the IP number it gets. So within the cache 
duration (a few hours!), even if you hit Shift-Reload, you keep 
connecting to the same IP; no new DNS request is initiated. Of course, 
that's also what you want, since you don't want an IP number switch for 
example in the middle of a running session.

So on an individual client computer, the round-robin looks defunct. But 
between clients, it works fine, because another client will stick with 
another IP.

Our actual host table entry looks like this:

# Addresses and aliases for systems in the Computer Center
#	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}	msu.lon-capa.org 	# {shuffle}

In all of this, don't forget LON-CAPA's internal load balancing! Make 
sure all of your local hosts are in your spare.tab. In this case, the 
servers will load-dependently offload sessions to each other (and 
across institutions, if setup that way!). A combination of round-robin 
and correctly setup spare.tab guarantees best results.

- Gerd.

On Mar 31, 2005, at 8:06 AM, H. K. Ng wrote:

> Mark,
> Do you know how your network staff enters the round-robin 
> registration? If you could get that info, it would helpful to me. When 
> I asked the folks here to do it - it works to some extend - it doesn't 
> actually do a round-robin. It still takes the first entry almost every 
> time. However, if that one is down or not available?, it goes to the 
> next one.
> Thanks,
> -hk
> At 04:19 PM 3/30/2005, you wrote:
>> Mark (and likely Guy),
>> Thanks for the solution below.  I took Mark's implementation to our 
>> network
>> folks, and got this positive response with a caveat:
>> > We can and will support the round-robin scheme for resolving IP
>> > addresses.  I should say I've never done it, so I'm not sure exactly
>> > what all the possible gotchas are, but it's pretty standard 
>> practice and
>> > should work.
>> >
>> > We can't however accomplish this the way you describe.  The feature
>> > (actually it was considered a bug) that allowed an alias to point to
>> > multiple systems ad described in the quote you sent was 
>> removed/fixed in
>> > recent version of BIND.
>> >
>> > The approved method of achieving the same result is to have multiple
>> > hosts that actually have the same hostname.  That is we could have 
>> two
>> > hosts defined with the name "loncapa.mines.edu", each with 
>> different IP
>> > addresses.
>> So this brings me to questions probably for Guy:
>> The hosts.tab file contains hostnames, LON-CAPA names, and IP 
>> addresses.
>> 1) Will LON-CAPA choke if two IP addresses have the hostnames?
>> 2) Should we then give our access servers the same, or different, 
>> names?
>> 3) Or is there some other way to manifest the load-balancing that I 
>> should
>> point our network folks to?
>> Thanks for your help.  I'm learning more about the inner workings of
>> load-balancing than I ever thought I would, but that's a "good thing".
>> Todd
>> > > We have defined an alias, loncapa.phy.ohiou.edu, that has two 
>> references
>> > > in the Domain Name server. DNS hands out the name in a round-robin
>> > > fashion. This does a pretty good job of evening the load. Once 
>> the student
>> > > is logged into a particular server, they stay on that server.
>> --
>> Dr. Todd Ruskell
>> Lecturer, Department of Physics              Office:  Meyer Hall 326
>> Colorado School of Mines                     Phone: 303-384-2080
>> 1523 Illinois Street                         Fax: 303-273-3919
>> Golden, CO 80401
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Gerd Kortemeyer, Ph.D.
Michigan State University

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