It works, because clustering means session replication. Any changes to
sessions are replicated to the other cluster members, so any member can
server any session, even if no member failed.
But: with stickyness (different jvmRoutes and a session sticky load
balancer) you only rely on the correctness of the replication when a
node actually fails. Since replication takes a little time and is very
difficult to implement, that's the better strategy.
Regards,
Rainer
Kristian Rink wrote:
Folks;
yesterday I dealt with the tomcat clustering feature for the first
time, getting things basically set up and working in quite a short time
(thanks to the documentation which is rather good), and even made my
application distributable / the session attributes serializable really
fast. However, one thing is massively confusing me:
I followed the straightforward approach, reading through [1] where
there is a configuration file in a section named "Simple Engine Cluster
Configuration for all hosts" - without thinking twice, I installed a
configuration similar to that on both of the nodes of my cluster
(including the _same_ jvmRoute parameter on both machines) and it
worked - no matter which one of the nodes went down, my session
remained alive and well.
However, spending some more time trying to study the details and
options of clustering, I came across [2] and learnt that the jvmRoute
parameter should be a per-host not a per-cluster configuration.
Thinking twice about this and its implications on session replication,
this seems reasonable, and looking at the log of mod_jk while running
both tomcats with the same jvmRoute parameter, indeed I see requests
belonging to the same session processed by different nodes (which, if I
got the servlet specifications right, shouldn't be). So: Why does this
setup, having jvmRoute the same in all cluster nodes, work? Just an
undesired side-effect of a broken configuration? Luck? Is there
something I didn't yet get about the cluster configuration?
Thanks in ad
[1] http://tomcat.apache.org/tomcat-5.5-doc/cluster-howto.html
[2] http://tomcat.apache.org/tomcat-5.0-doc/balancer-howto.html
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