[Zope] Static fail-over

[Chris McDonough]

> A fail over system is where the service "fails over" to the backup
> when a problem is detected.  It does not balance anything, least of all
> load.

I think the point Sean was trying to make was that you may need to arrange
for failover for the load balancer itself.  At least you would if you think
it's important to provide redundancy in whatever load balancing setup you
implement.

> I'm highly interested in any real-world, in-production load
> balanced or fail over systems for Zope (esp. using Open Source software).

I have moderate experience with LVS + Mon in combination with Squid and
Zope.  A single Squid handles HTTP requests from clients on the Internet.
It talks to a TCP port on the load balancer.  LVS as the load balancer
provides the balancing service between a number of Zopes.  Mon provides the
capability to remove failed Zopes from the load balancing rotation via
polling often against a method that returns a known response.

An extension of this configuration which I've not implemented yet (but will
need to very soon) will be to put a separate load balancer in front of a
number of ICP-connected Squids, each configured with the virtual IP on the
LVS box as its http_accel port.  I need to do this to be able to scale cache
services and provide redundancy in cache services instead of having a single
Squid frontending the whole shooting match.

> Is there a way to use URL Rewriting rules in Apache (with
> mod_rewrite) to test if a particular box was alive, and only if so, direct
> traffic there?  Maybe have it look if a particular file exists (or some
> such)?

If you find out, please let me know, that sounds very useful!

> (Also note that Apache will do much of what Squid will do using
> mod_proxy.)

mod_proxy isn't very well documented and doesn't do ICP (which is pretty
handy for scaling the cache).  But it's fine for small setups.  I imagine
you could even share its cache directory over NFS if you wanted some
failover capability without losing all that was cached to disk.

- C