[Seaside] Load Balancing Multiple Seaside Images with mod_proxy_balancer [was: Server Identification in Session Keys]

[Philippe Marschall]

Hi

There are several ways to load balance Seaside images, one way is
Apache 2.2 and mod_proxy_balancer [1]. Compared to other solutions
it's relatively easy to set up, doesn't modify the response and is
free of cost. It only needs one small additional Seaside package with
two classes. Additionally mod_proxy_balancer has some advanced
features like a manager application, configurable scheduler algorithm
and load factor.

When load balancing multiple Seaside images care must be taken that
all requests that require a particular session are processed by the
same image because that session is only available in this image
(unless you're on GemStone). This has to work with and without session
cookies. This is referred to as sticky sessions because a session
sticks to its image. mod_proxy_balancer does this by associating each
image with and id called route or jvmRoute. Seaside has to append this
route to the session id. mod_proxy_balancer reads the route from the
request and proxies to the right image. This has the advantage that
mod_proxy_balancer does not have to keep track of all the sessions and
does not have to modify the response. Additionally since the mapping
is defined statically in the the Apache configuration it survives
server restarts.

First you need to define your cluster of images. We'll use two images,
the first one on port 8080 with the route "first" the second on port
9090 with route "second". You can of course chose other routes as long
as they're unique.

<Proxy balancer://mycluster>
    BalancerMember http://127.0.0.1:8080  route=first
    BalancerMember http://127.0.0.1:9090  route=second
</Proxy>

Then we'll need to define the actual proxy configuration, this is very
similar to a single Seaside image behind an Apache.

ProxyPass / balancer://mycluster/ stickysession=_s|_s nofailover=On
ProxyPassReverse / http://127.0.0.1:8080/
ProxyPassReverse / http://127.0.0.1:9090/

Note that we configure _s to be the session id for the the URL and the cookie.

Finally we can optionally add the balancer manager:

ProxyPass /balancer-manager !
<Location /balancer-manager>
    SetHandler balancer-manager
</Location>

Putting everything together it looks something like this (we also
configures server-status):

<VirtualHost *>

        ProxyRequests Off
        ProxyStatus On
        ProxyPreserveHost On
        ProxyPass /balancer-manager !
        ProxyPass /server-status !
        ProxyPass / balancer://mycluster/ STICKYSESSION=_s|_s
        ProxyPassReverse / http://127.0.0.1:8080/
        ProxyPassReverse / http://127.0.0.1:9090/

        <Proxy balancer://mycluster>
                BalancerMember http://127.0.0.1:8080  route=first
                BalancerMember http://127.0.0.1:9090  route=second
        </Proxy>

        <Location /balancer-manager>
                SetHandler balancer-manager

                Order Deny,Allow
                Deny from all
                Allow from 127.0.0.1
                Allow from localhost
        </Location>

        <Location /server-status>
                SetHandler server-status

                Order Deny,Allow
                Deny from all
                Allow from 127.0.0.1
                Allow from localhost
        </Location>

</VirtualHost>

Now all that is left is configuring Seaside. First you need to load
Seaside-Cluster from [2] (only works with Seaside 3.0). Then you need
to configure each image individually with the correct route. It's
important that this matches your Apache configuration from above. The
easiest way to do this is

WAAdmin makeAllClusteredWith: 'first'

in the image on port 8080 and

WAAdmin makeAllClusteredWith: 'second'

in the image on port 9090.

That's it, your sticky session cluster ready to go. You can find the
manager at http://127.0.0.1/balancer-manager and the server status on
http://127.0.0.1/server-status


 [1] http://httpd.apache.org/docs/2.2/mod/mod_proxy_balancer.html
 [2] http://www.squeaksource.com/ajp/

Cheers
Philippe