daniel,
i gave db_cluster module a try. i configured serial connections and it worked fine: when i stopped mysql server behind the higher priority connection, queries started to work over the lower priority connection.
there is one thing that bothers me. when higher priority connection is not available, i get these kind of error to syslog even when queries succeed over the lower priority connection:
Mar 29 20:24:52 sip /usr/sbin/sip-proxy[18966]: ERROR: db_mysql [km_dbase.c:122]: driver error on query: Can't connect to MySQL server on '192.98.102.13' (115) Mar 29 20:24:52 sip /usr/sbin/sip-proxy[18966]: ERROR: <core> [db_query.c:156]: error while submitting query
it would be nice if those error messages could somehow be suppressed when one of the connections is still operational and the queries actually succeed. someone may namely run such incomplete cluster many hours and during that time syslog will be filled with those (confusing) error messages.
-- juha