It would be nice to have replication functionality to Derby; many potential Derby users seem to want this. The attached functional specification lists some initial thoughts for how this feature may work.
Dag Wanvik had a look at this functionality some months ago. He wrote a proof of concept patch that enables replication by copying (using file system copy) and redoing the existing Derby transaction log to the slave (unfortunately, I can not find the mail thread now).
DERBY-2852 contains a patch that enables replication by sending dedicated logical log records to the slave through a network connection and redoing these.
Replication has been requested and discussed previously in multiple threads, including these: