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From Brantley Hobbs <sbho...@uga.edu>
Subject Re: Keeping busy site responsive
Date Thu, 28 Jun 2007 14:15:37 GMT
Clinton,

I think it might be worth your trouble to find out what number it is 
that causes the box to "[stagger] to a halt issuing redirects".  I 
started having network saturation on the little switch I was using for 
my test rig before the load started getting bad on the box.  I don't 
have the exact numbers in front of me.

I think Len was referring to the F5 key on your keyboard (it being the 
hotkey for the browser to refresh the page).

One thing to keep in mind with a hardware load balancer to make this 
decision for you is that for it to know if you have an already 
established session it will need to examine the inbound request, AFAIK.  
If you're using SSL, you'll need a load-balancer capable of terminating 
the SSL connection.

I really think it might be worth your while to simply try the filter first.

B.

Parham, Clinton wrote:
> 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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>>   
>>     
>
>
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>   

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