Session state management in Lighttpd-mongrel cluster

I am using file based session store the file is written to
<Rails_Root>\tmp\sessions

Naturally this setup won't work in production/test in a clustered
environment. For my clustering I am using 1 Lighty and 4 Mongrels, all
these 4 mongrels are running on separate machines.

You're right, it won't work.

The question is how do I synchronize the session data across all 4
mongrels, without which a transparent failover won't be possible.

2 possibilities come to mind:

1) Switch to ActiveRecordStore (DB backed sessions)
2) Share session data via NFS or something similar. I don't know if the filesystem session system uses cluster safe unique filenames, though. I'd *guess* offhand that it doesn't as I'd be surprised if anyone running a cluster would opt for filesystem based session storage.