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From "Parham, Clinton" <cpar...@biosignia.com>
Subject RE: Keeping busy site responsive
Date Thu, 28 Jun 2007 12:34:27 GMT
Thank you everyone for your input so far.

Brantley: I like the idea but the part about 'staggering to a halt
issuing redirects' is a concern. If this were to happen, then even users
with established sessions would have trouble getting through - right? I
think the same applies to Tracey's suggestion.

Maybe Christopher is on the right track where some sort of load balancer
is the best solution. One that's smart enough to allow established
sessions through and maybe redirect new ones while the server is at peak
load. This would avoid Tomcat from having to deal with traffic it cannot
handle/trying to issue redirects. 

Does anyone have any experience/recommendations setting something like
this up? Len hinted at F5 - not sure we can afford that... Len: which F5
and how much??

-----Original Message-----
From: Brantley Hobbs [mailto:sbhobbs@uga.edu] 
Sent: Wednesday, June 27, 2007 1:37 PM
To: Tomcat Users List
Subject: Re: Keeping busy site responsive

Clinton,

I implemented this as a filter/listener combination.

The listener tracks session initialized events and increments an atomic
Integer (decrementing it at session destroyed), saving it as a context
attribute.

The filter uses the following logic:

1.  If we're less than or equal to the number of allowed sessions, allow
this one through.
2.  If we've exceeded the limit, check to see if this user already has
an established session (using the request's getSession(false) call).  If
they have an established session, allow them through.
3.  If they don't match either of the above, send them a redirect to a
page letting them know to try back later.

This approach seems to work well (at least until the box staggers to a
halt simply issuing redirects).  I've hit a box with numbers that are at
least double what the limit is and the people that have established
sessions don't notice a thing.  Because it's simple filters and
listeners, it should be fairly portable. I've attached the source with
this mail.  There might be a couple of dependancies you can get rid of
(like our log manager class), but it should be pretty easy to drop in.

Hope this helps.

B.

Parham, Clinton wrote:
> Tomcat Experts:
>
> How do I keep my web application responsive for users already half way

> through an enrollment process when traffic volume is high?
>
> Here's the scenario: I have a set of 5 web pages that users must work 
> through to successfully enroll themselves. Assume the server can 
> handle 250 concurrent requests (maxThreads). While traffic volume is 
> under 250, enrollments complete normally. But once volume exceeds 250 
> and saturates the acceptCount/backlog queue, users half way through 
> enrollments cannot complete their enrollment (connections are refused)

> because new users keep bombarding the site.
>
> What would be acceptable is for new users to see a 'site is busy 
> message' while enrollments in progress are completed. As enrollments 
> complete and concurrent threads drop below 250, new users are allowed 
> through.
>
> I have already considered maxActiveSessions but I don't think this 
> will solve the problem. If maxThreads is reached and the 
> acceptCount/backlog queue is exhausted, then the users with active 
> sessions and already partly through enrollment won't be able to get 
> back in to the site to complete their enrollment - right?
>
> Adding more servers to handle the load is not preferred because most 
> of the time they will be underutilized. Enrollments that experience 
> high traffic don't happen that often but when they do, we need to 
> support them.
>
> Thank you for your time.
>
>
>
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