[lug] High Availability and Failover options

[Riggs, Rob]

I've never seen a good way to handle shared session-level data in the OS.
It's almost always done at the application level. It will almost certainly
be a non-portable solution. And it would still require some cooperation at
the application level.

The real question to ask yourself is how important is it to maintain the
sessions during a failure? What we found was that system failure on our end,
and the resulting few lost sessions, were trivial compared to the day-to-day
failures people experience all the time on the net. Routes get munged, lines
go down, firewall rules are hosed, modems hang up, etc. All of these common
failures dwarfed the expected session failure rate due to hardware failure.
It's rarely worth it. With something like 6 front-end servers (Dell 2450s),
we had zero hardware failures in over a year.

With that said, it is usually not too difficult to store session information
in a common data store. What is difficult is, in a very active site, making
the common data store robust. It's usually a database. And you have to worry
about real-time database replication and failover. I have yet to see that
done in practice.


-----Original Message-----
From: Andy Jolley [mailto:Jolley at firsttrust.com]
Sent: Friday, January 04, 2002 3:01 PM
To: lug at lug.boulder.co.us
Subject: [lug] High Availability and Failover options


My company is looking for possibilities for a high availability java (JSP,
API, BEANS) server and one of the options is Tomcat on Linux.  We currently
have load balancers in front of another server solution that is frankly
becoming too pricy and it's not living up to the salesmen's promises (big
surprise).  So we're looking for a  new platform.  One of our main sticking
points is ensuring the applciations have session level failover.  We
currently have hardware level failover and load balancing via a hardware IP
redirector.  While we're very happy with our hardware solution, we still
have the issue of sessions being lost when a server goes belly up.

The development staff is working on coding to store the session information
on an external data store, but I was wondering if there was a way to
approach this a little more from the hardware/OS side.
I was looking at Beowulf clusters, but what happens if one of the nodes in
the cluster dies, and consequently what happens to the user sessions on that
node.  I'm hoping the distributed memory model would help with this (unless
the session was stored in the physical memory in the node that died).
Redhat's website also mentions a product called Piranah, but I'm not too
sure what that one is all about, it looks like basic load balancing.
Does anyone have experience with any of this kind of thing..I'm starting to
get pretty comfortable with Linux, but I can't even get Tomcat 3.3 running
at home, so I'm obviously still pretty green with some of this stuff.

Thanks

Andy Jolley
LAN/WAN Administrator
jolley at firsttrust.com 
303-294-5785


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