Remove the is_updating variable. The HTTP request are synchronous, so it is no needed

http://docs.python.org/2/library/socketserver.html
These four classes process requests synchronously; each request must be completed before the next request can be started. 

I think the var opts.being_updated it is not needed, because only one process will be executing the code at the same time.

Also, if the MySQL server shutdown between connection and query, opts.being_updated will remain True forever, and the dameon will answer always "CACHED: Percona XtraDB Cluster Node is synced".