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From Assaf Arkin <>
Subject Re: [TOMCAT.NEXT] Proposal Sources Checked In
Date Sun, 09 Jan 2000 04:28:25 GMT
> Could we this persistence as a piece of the pie for load-blancing/failover
> support?

That's what we have in mind :-)

> I know that JServ does load balancing, and that is awesome.  I just think
> that we should take it one more step and try for failover support as well.
> I am sure that there are many other issues with this, but I am sure that you
> guys have/are thinking about it?

I'm giving this issue some thought, especially since I'm looking at two
types of solution.

In one solution everything in the session comes from the database. For
example, when you view your bank account information all the important
session information is up there in the database. Server goes down,
server comes back, session recreated, reloads itself, and you're back in

In other cases, the session holds information that is not in the
database, e.g. when you fill a multi-page form before committing it. In
this case server goes down, server comes up, session returns to original
state, user gets Web rage.

If the session can be persisted at the end of every servlet response,
you can guarantee that on server restart users will be able to continue
where they left off. It does raise a few problems, though:

1. Performance -- serialization is rather expensive

2. Overlapping -- if you use the two approaches you want Tomcat to do
fail over only for those part of your session that do not come from the

3. ACIDity -- a session cannot be recovered if the server failed in the
middle of executing the Servlet, unless the Servlet is specifically
aware of that

4. Setup -- should fail over rely on persistence in a single server, a
hot stand by, or simply redirecting to another server in the cluster?


> I know that this is probably a long way down the road, but I for one think
> that it would make Tomcat a world-class engine.
> Scott Sanders
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