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From some_cs_student <somecsstud...@gmail.com>
Subject [users@httpd] Switching load between two Tomcat servers without outage WITH sticky sessions...is it possible?
Date Sat, 07 Feb 2009 03:35:28 GMT

Hi,

So the challenge is simple, in the WebSphere world, the plugin self
auto-reloads, and therefore you have the option of having say balancing
between 2 servers and changing the "weight" (amount of traffic) sent to that
server to zero, so no new traffic goes to that server, but any existing
sticky session traffic continues to go to that server....

This works great for deploying an application to one cluster, while keeping
another identical instance running on the old version and then switching the
customers over to to the new customer without an outage.

With Tomcat, we are currently fronting with Apache 2, with mod_jk, I did
some research and you *can* do sticky sessions with mod_jk and you can load
balance, but you cannot dynamically reload the workers.properties.
 Soooo, we can use an Apache graceful restart, will this lose all previous
sticky sessions?

In other words, customers are on server A, we do a graceful Apache restart
and server A has zero load in the balancer, server B has 100% load.
 Customer X was on server A (Tomcat session) and presses submit on his
application form to server A, what will Apache (mod_jk) do? (Attempt to send
it to server B?)

So I found:
http://tomcat.apache.org/connectors-doc/reference/apache.html and
JkMountFile could be used to reload the JkMount information, I could use
this to *change* the worker but what happens to the sticky sessions that
were on the other server before I changed this config?

Finally, I do have the option to go Apache 2.2 and mod_proxy (with
mod_proxy_ajp and a balancer), in any of the scenarios can I achieve my
goal?

mod_proxy didn't appear to easily support dynamically reconfiguring load
balancing, if someone knows how I can dynamically re-configure the load
balancing that would be great.

Apologies if this is in the wrong section of the forum, please point me in
the right direction.

Regards,
Gareth
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