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:[EMAIL PROTECTED]
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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