I run multiple Zope instances on one server. Each Zope instance listens on a different port (localhost:100xx). Historically I’ve just used Apache as a front end which forwards requests to the Zope instance.
Unfortunately there are periods of the year when one site gets a deluge of requests (for example; when hosting a school site, if it snows overnight, all the parents will check the site in the morning at around about 8am).
Zope is not particularly quick on it’s own – Apache’s “ab” reports that a dual core server with plenty of RAM can manage about 7-14 requests per second – which isn’t that many when you consider each page on a Plone site will have a large number of dependencies (css/js/png’s etc).
Varnish is a reverse HTTP proxy – meaning it sits in-front of the real web server, caching content.
So, as I’m using Debian Lenny….
- apt-get install -t lenny-backports varnish
- Edit /etc/varnish/default.vcl
- Edit Apache virtual hosts to route requests through varnish (rather than directly to Zope)
- I didn’t need to change /etc/default/varnish.
In my case there are a number of Zope instances on the same server, but I only wanted to have one instance of varnish running. This is possible – but it requires me to look at the URL requested to determine which Zope instance to route through to.
So, for example, SiteA runs on a Zope instance on localhost:10021/sites/sitea. My original Apache configuration would contain something like :
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^/(.*) http://127.0.0.1:10021/VirtualHostBase/http/www.sitea.com:80/sites/sitea/VirtualHostRoot/$1 [L,P]
</IfModule>
To use varnish, I’ll firstly need to tell Varnish how to recognise requests for sitea (and other sites), so it can forward a cache miss to the right place, and then reconfigure Apache – so it sends requests into varnish and not directly to Zope.
So, firstly, in Varnish’s configuration (/etc/varnish/default.vcl), we need to define the different backend server’s we want varnish to proxy / cache. In my case they’re on the same server –
backend zope1 {
.host = "127.0.0.1";
.port = "10021";
}
backend zope2 {
.host = "127.0.0.1";
.port = "10022";
}
Then, in the 'sub vcl_recv' section, use logic like :
if ( req.url ~ "/sites/sitea/VirtualHostRoot") {
set req.backend = zope1;
}
if ( req.url ~ "/siteb/VirtualHostRoot") {
set req.backend = zope2;
}
With the above in place, I can now just tell Apache to rewrite Sitea to :
RewriteRule ^/(.*) http://127.0.0.1:6081/VirtualHostBase/http/www.sitea.com:80/sites/sitea/VirtualHostRoot/$1 [L,P]
Instead….. and now we’ll find that our site is much quicker 🙂 (This assumes your varnish listens on localhost:6081).
There are a few additional snippets I found – in the vcl_fetch { … } block, I’ve told Varnish to always cache items for 30 seconds, and to also overwrite the default Server header given out by Apache etc, namely :
sub vcl_fetch {
# ..... <snip> <snip>
# force minimum ttl for objects
if (obj.ttl < 30s) {
set obj.ttl = 30s;
}
# ... <snip> <snip>
unset obj.http.Server;
set obj.http.Server = "Apache/2 Varnish";
return (deliver);
}
I'm happy anyway. :)
Use 'varnishlog', 'varnishtop' and 'varnishhist' to monitor varnish.
