Here's a good document on how hinted handoff works http://www.datastax.com/dev/blog/modern-hinted-handoff
I believe if I understand that document correctly that a hinted handoff will get created if the replica is down in the other data center. Also since Cassandra is self-healing, reads will cause read repairs to correct any inconsistent data. Also Cassandra has an anti-entropy mechanism that actively updates replicas to the newest version using a Merkle tree. Here's some text on Anti-entropy http://wiki.apache.org/cassandra/AntiEntropy From: Jabbar [mailto:aja...@gmail.com] Sent: January-08-13 5:34 PM To: user@cassandra.apache.org Subject: remote datacentre consistency I'm a bit confused about how a two datacentre apache cassandra cluster keeps the data consistent. >From what I understand a client application in datacentre1 contacts a >coordinator node which sends the data to the local replicas and it also sends >the updates to the remote coordinator in the remote data centre. Does the local coordinator send the updates asynchronously to the local replicas and the remote coordinator node? What happens if the bandwidth is severely restricted to the remote datacentre? Do the updates for the remote coordinator keep getting buffered up in the local coordinator? What happens if the connection to the remote coordinator is down? Would hinted hand off be used to recover from this scenario? What options are there to synchronise the remote datacentre if the connectivity comes back after a couple of days? -- Thanks A Jabbar Azam
