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From "Howard W. Smith, Jr." <>
Subject Re: [Seriously OT] Help in diagnosing server unresponsiveness
Date Wed, 06 Feb 2013 01:03:51 GMT
On Tue, Feb 5, 2013 at 6:10 PM, Christopher Schultz
<> wrote:
> Hash: SHA256
> Chris,
> On 2/2/13 11:47 AM, chris derham wrote:
>>> In the Java world, most people would only call it a consider it
>>> a "cluster" if the app servers actually know about each other --
>>> for instance, if you are using session replication. IMO session
>>> replication is a dog, and there are better ways to achieve
>>> similar goals that yield much higher performance.
>> I am curious to hear your thoughts - care to elaborate?
> We've totally hijacked Zoran's thread. After this, let's move to a new
> OT thread (or threads).
> Tomcat's session replication basically blasts everything around to all
> other Tomcat instances (or you can choose one and use BackupManager).
> Every time you modify your session, you get a burst of traffic between
> all your cluster members.
> I dunno if Tomcat bothers updating the last-touched-time on the
> session if that's the only thing that changed during the request, but
> if so it means that there is a burst of replication traffic after
> /every single request/.


> Most web applications I've seen use HttpSession for a variety of
> things, and not all of them need to be replicated. If you want to
> hand-roll some wrapper objects to allow you to stash things into the
> session that are not serializable (or shouldn't be serialized, like a
> "transient" wrapper) you can avoid some of this stuff, but you really
> shouldn't have to do this kind of thing.
> Instead, rely on some other data-sharing mechanism such as a database,
> memcached, etc. You can create a token that you pass to the client
> just like a JSESSIONID cookie and if they fail-over to a different
> node, you can still fetch your stuff when you need it.

interesting... i was reading about a memcached (open source) project
that works really well with tomcat, and i was going to ask about it on
this list. I liked the blog that I saw about this, because 'honestly'
it seemed easier to read and easier to implement than reading tomcat's
'cluster/session-replication documentation page'.  :(

an example or blog is always a great read. :)

> The problem with using HttpSession + session replication is that lazy
> programmers will stash just about anything into the session without
> thinking about it, and it will get replicated all over the place.

Ouch, that hurt again! I think I did a lot of this when I first began
Java EE development, but have learned that it is not necessary any
longer, and honestly, I may still be guilty of stashing stuff into the

> If you use an external data storage mechanism, it's much more obvious
> that something "special" is happening, and (hopefully) your devs will
> take more care with dipping into that data store all the time.
> - -chris
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