[Seaside-dev] supporting vm routes

[Philippe Marschall]

Hi

If you're deploying Seaside on modern hardware and not using GemStone
you'll have to cook up some way of implementing sticky sessions and
load balancing. That's quite uncool. mod_jk supported what we need for
years but is limited to AJP. However, as it turns out
mod_proxy_balancer in Apache 2.2 does what we need [1], [2], [3], [4]
and works with HTTP (and AJP and FTP).

Here's what it could reverse engineer how it works. Each Smalltalk
image gets a so called route. The route is appended to the session id
after a '.'. Session ids in URLs and cookies are both supported and
can have a different, configurable keys (_s in our case). On each
incoming request Apache picks the right image according to this route.
If no route is present a "random" image is chosen according to
lbmethod.

Setting up a cluster with four images could look something like this:

<Proxy balancer://mycluster>
	Order deny,allow
	Allow from all

	BalancerMember http://127.0.0.1:8080/ loadfactor=1 route=33 min=1
max=50 keepalive=On
	BalancerMember http://127.0.0.1:8080/ loadfactor=1 route=34 min=1
max=50 keepalive=On
	BalancerMember http://127.0.0.1:8080/ loadfactor=1 route=35 min=1
max=50 keepalive=On
	BalancerMember http://127.0.0.1:8080/ loadfactor=1 route=36 min=1
max=50 keepalive=On

</Proxy>
ProxyPreserveHost On
ProxyPass /seaside balancer://mycluster/seaside STICKYSESSION=_s|_s
nofailover=On
ProxyPassReverse /seaside balancer://mycluster/seaside

To support this in Seaside we need to change the session key
generation. We need to be able to append a suffix that is configured
in the application. Julian you've spent a lot of time on sessions and
cache and stuff. What do you think is the best way to implement this?
A WAClusteredApplication that has a WARouteCache?

How do other people feel about this? Is this useful? Should this be
part of Seaside out of the box or rather be an addon like
Seaside-Cluster?

I know deploying Apache 2.2 can be a problem  in some shops ("there is
nothing new in Apache 2.2 that you need"). However I happen to know a
bug in Apache 2 and 1.3 that is fixed only in Apache 2.2 and not back
ported. Telling your admin you suffer from this tends to work ;-)

 [1] http://httpd.apache.org/docs/2.2/mod/mod_proxy_balancer.html
 [2] http://httpd.apache.org/docs/2.2/mod/mod_proxy.html
 [3] http://laststation.net/2008/03/18/ajp13-made-easy-with-mod_proxy/
 [4] http://www.markround.com/archives/33-Apache-mod_proxy-balancing-with-PHP-sticky-sessions.html

Cheers
Philippe