We already have two separate rings. Idea of bidirectional sync is, if one
ring is down, we can still send the traffic to other ring. When original
cluster comes back, it will pick up the data from available cluster. I'm not
sure if it makes sense to have separate rings or combine these two rings
into one.
I am not sure you fully understand how Cassandra is supposed to work -
you do not need two rings to have two complete sets of data that you can
"hot cutover" between.
Cassandra doesn't have support for synchronizing data between two
different rings. The multi-dc support in Cassandra amounts to having a
single ring containing all nodes from all data centers. Cassandra is
told (by configuring the snitch, such as through a property files)
which nodes are in which data center. Using the
NetworkTopologyStrategy, you then make sure to distribute replicas in
DC:s as you see fit.
Using NTS you can configure a single ring into multiple "logical
rings". This is effectively what the property file snitch does in
conjunction with NTS.
I gave a presentation on the NTS internals, and replicating data across
geographically distributed data centers. You can find the slides here
http://files.meetup.com/1794037/NTS_presentation.pdf
Also Edward Capriolio's book "high performance cassandra" has some
recipes for using NTS.
I currently have 4 nodes in two data centers and I use NTS with property
file snitch to write 1 copy of data to each DC (one node per DC) so that
in the event of a total DC failure, we can still get to the data. The
first write is "local" and the replica is asynchronous if you set write
consistency to 1 - so you get fast writes with distribution.
-Eric
