synchronising failover web servers

[Aristedes Maniatis]

We have two web servers configured in an identical manner. Both 
machines have an identical web server (Apache) setup with identical 
copies of the html directories and config files. Each machine is 
capable of handling the load alone, but the redundancy allows us to 
upgrade one at a time, or have a second machine available as failover 
in case of hardware failure. We simply swap the IP addresses of the 
interfaces, or apply DNAT at the upstream router.

I know there are more automatic failover options available, but this is 
sufficient for our needs.

Both machines are currently FreeBSD 4.9. There is a separate secure 
gigabit network linking the servers together which carries no internet 
traffic.

The problems we have right now are:

1. how to sync the html directories
2. how to sync config files (eg httpd.conf, contents of /usr/local/etc, 
and so on)

I have been experimenting with various options. For (2), I have been 
thinking of creating a cvs repository to which we commit all changes 
and then creating cvs checkout scripts on the deployment machines. That 
way rollback is easy, we can comment changes, and testing on a third 
testing box is easy. However, cvs is not well suited to files scattered 
all over a file system. It expects to deal with a single folder full of 
files and folders.

For (1), users have the ability to upload files directly to the server 
via ftp, so cvs is not appropriate. I've looked at rsync, but found it 
to not be ideal for a two way synchronisation - it is better suited for 
a mirror type situation where one machine is master. In our setup, it 
is possible, through DNS changes that either machine is the master at 
any point in time.

What solutions have others used for these situations? We are at the 
small end of the scale, with dozens of sites rather than hundreds, so a 
simple solution is what we are looking for. Our sites are all complex 
database driven beasts, but I only need to solve this particular part 
of the problem now.


Cheers
Ari Maniatis


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