Mapping specific requests to a single ZEO client instance

As a follow up to Apache Round Robin for ZeoClients:

we needed to send session specific things to a single instance.

An example of why this is necessary is for collective.captcha. If the connections are being round robin-ed you get audio and image files from different instances. That is BAD. It doesn't work.

First make sure varnish doesn't cache the captcha images and audio or your captcha protected form page.
In varnish.vcl add:

# Do NOT cache captcha image, audio, request
  if (   req.url ~ "/@@captcha"
      || req.url ~ "/contact-info"
     ) {
        pipe;
       }

Next we need to map a server to do our captcha stuff.
In zopeservers.map add the line:

CAPTCHA localhost:6970

I will leave it to you to think of how to creatively balance requests to the other instances. Perhaps remove teh one that does some captcha and other special work from ALL, or maybe add the other instances more than once... OK I didn't leave much, but there are probably smarter ways to do it.

Finally we need to add a rule for mapping captcha requests to the correct instance.
To the appropriate apache.conf (http.conf) add:

RewriteRule ^/(@@captcha/(audio|image)|contact-info)   \
   http://${zopeservers:CAPTCHA}/VirtualHostBase/http/%{SERVER_NAME}:80/msrd/VirtualHostRoot/$1 [L,P]

Don't forget to restart apache and varnish!!

Thanks to Chris Shenton for suggesting the solution.

Apache Round Robin for ZeoClients

There is a common approach to running Plone/Zope behind some service that distributes requests across numerous ZeoClients. Here's how I have been doing it using Apache (2.2.x).

                                  ________Zeoclient(6968)
                                 | _______ZeoClient(6969)
client-->varnish(80)-->apache(81)|<_______ZeoClient(6970)
                                 |________ZeoClient(6971)

Everybody knows how to setup varnish, right? So I am only going to show the Apache bits here.

First you need a rewrite map.

#zopeservers.map
ALL     localhost:6968|localhost:6969|localhost:6970|localhost:6917

Of course there can be more or less hosts in the map and they don't have to be all localhost either.

Next you need to enter the right stuff in the apache configuration. As here in a VirtualHost.

<VirtualHost *:81>
...
RewriteMap  zopeservers rnd:/usr/local/apache2/conf/zopeservers.map
RewriteRule ^/(.*) \
    http://${zopeservers:ALL}/VirtualHostBase/http/%{SERVER_NAME}:80/ploneroot/VirtualHostRoot/$1 [L,P]
...
</VirtualHost>

How does Emiril say... BAM!

Not so hard at all. A couple of notes:

1. I don't think apache knows how to detect if a client is down or broken. So it will serve up all the errors it gets.

2. Varnish doesn't cache https, so it needs to be handled wihtout varnish...