> I thought Hinted Handoff was for downed replica's in the local datacentre. I > didn't realise that it would work with a remote datacenter. If the coordinator will store a hint if it detects a replica is down before the request starts, or that the node did not return within rpc_timeout.
> Likewise for Anti Entropy I thought it only worked for the replicas in the > local datacentre. I yet to find any definitive references which mention that > this works across multiple datacentres. It works on the cluster as a whole paying attention to the replication settings. So if you have replicas in 2 dc's it will repair across them. > Does the local coordinator send the updates asynchronously to the local > replicas and the remote coordinator node? > Yes. All iternode communication is async. > 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? > There is a local queue of messages to send, if the messages are in the queue for more than rpc_timeout they will not be sent. > What happens if the connection to the remote coordinator is down? > It depends on the CL you are using. If you are using CL QUOURM your writes will probably fail, depending on the RF settings. If you are using CL LOCAL_QUOURM they will work so long as there is a local quourm. If you are using EACH_QUOURM they will fail. Hope that helps. ----------------- Aaron Morton Freelance Cassandra Developer New Zealand @aaronmorton http://www.thelastpickle.com ----------------- Aaron Morton Freelance Cassandra Developer New Zealand @aaronmorton http://www.thelastpickle.com On 10/01/2013, at 11:40 AM, Jabbar <aja...@gmail.com> wrote: > Hello Simon, > > I thought Hinted Handoff was for downed replica's in the local datacentre. I > didn't realise that it would work with a remote datacenter. > > Likewise for Anti Entropy I thought it only worked for the replicas in the > local datacentre. I yet to find any definitive references which mention that > this works across multiple datacentres. > > I'll keep looking. Obviously there'll probably be some documents I haven't > read read yet. > > > On 9 January 2013 18:38, Simon Guindon <simon.guin...@jsitelecom.com> wrote: > 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 > > > > > -- > Thanks > > A Jabbar Azam
