I read Bernd's message and in the end I do agree with the imperial
need for a notion of local workers, or more exactly for the notion of
non local workers, i.e. workers used exclusively for requests
pertaining to a session they created.
I wrote a piece of Apache configuration to handle Tomcat node shutdown,
I have the following code in my config file:
<IfDefine TOMCAT_SHUTDOWN>
RewriteEngine On
RewriteRule ^@LB_URI@$ [F]
</IfDefine>
where @LB_URI@ is the URI the hardware load balancer checks to see if
the Apache instance is alive and should be used to dispatch requests.
To shut down a node, I issue Apache a graceful restart signal with the
additional parameter -DTOMCAT_SHUTDOWN, from then on the @LB_URI@ will
return a 403 error code and thus the LB will end up excluding the just
gracefully restarted Apache instance from its pool. But before the
Apache instance is excluded, normal requests dispatched to it will
continue to be answered correctly, provided @LB_URI@ is not a URI used
by the clients.
Once the hardware load balancer excluded the Apache instance, wait
until the Tomcat instance has no more active sessions (which will
happen provided no client reactivates its session over and over, and
provided a mod_jk instance will not dispatch a request not part of a
session to a non local worker). When no active sessions exist on the
Tomcat instance, shut it down. Deploy the new webapp version then
start Tomcat again. You can test the new webapp by connecting to the
associated Apache instance directly (instead of the load balancer's
virtual IP). When the webapp is considered correct, do a graceful
restart of your other Apache instances using the -DTOMCAT_SHUTDOWN
option and do a graceful restart of the first Apache instance without
the -DTOMCAT_SHUTDOWN option. Your new webapp is now deployed on the
first instance and the other instances continue to serve the old
version. You can therefore deploy the new version progressively on all
Tomcat instances.
For this procedure to function properly we see that we do need, as
Bernd pointed out, the notion of local and non local workers, i.e. we
need to be able to have workers kown by a jk instance but which are
used ONLY for requests part of a session they created.
I therefore suggest an additional parameter be added for the load
balancing workers, namely the last level of local workers, i.e. any
worker past this level will be considered non local. The default value
for this parameter would be the last level thus considering all
workers local.
This addition should solve Bernd's problem (and even maybe before
Wednesday ;-).
Mathias.
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