maxIdleBackup, {min,max}IdleSwap query

[Paul DuBois <pa...@snake.net>]

I've been pondering the Tomcat Manager component documentation
(http://jakarta.apache.org/tomcat/tomcat-4.0-doc/config/manager.html)
with regard to storing sessions in JDBC-based backing store.  I am not
clear on the distinction between backing up a session and swapping it
(the behaviors controlled by the maxIdleBackup attribute and the
minIdleSwap and maxIdleSwap attributes of the <Manager> element), and I
don't find anything in the archives.

The relevant section of the documentation reads as follows:

----------
maxIdleBackup
The time interval (in seconds) since the last access to a session before
it is eligible for being persisted to the session store, or -1 to
disable this feature. By default, this feature is disabled.

maxIdleSwap
The time interval (in seconds) since the last access to a session before
it should be persisted to the session store, and passivated out of the
server's memory, or -1 to disable this feature. If this feature is
enabled, the time interval specified here should be equal to or longer
than the value specified for maxIdleBackup. By default, this feature is
disabled.

minIdleSwap
The time interval (in seconds) since the last access to a session before
it will be eligible to be persisted to the session store, and passivated
out of the server's memory, or -1 for this swapping to be available at
any time. If specified, this value should be less than that specified by
maxIdleSwap. By default, this value is set to -1.
----------

Is the distinction simply that for backing up, Tomcat just writes a
copy of the session information to backing store, whereas for swapping,
Tomcat writes it to backing store and forgets it until a client makes a
request that uses the session again?

Also, is there some required relationship between the values of
maxIdleBackup and minIdleSwap?  The documentation states that
maxIdleBackup <= maxIdleSwap and minIdleSwap < maxIdleSwap, but nothing
explicit about maxIdleBackup and minIdleSwap.

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