Cassandra in Multiple Datacenters Active - Standby configuration

[Oleg Tsvinev]

Hi all,

I have a question that documentation has not clear answer for. I have
the following requirements:

1. Synchronously store data in datacenter DC1 on 2+ nodes
2. Asynchronously replicate the same data to DC2 and store it on 2+
nodes to act as a hot standby

Now, I have configured keyspaces with o.a.c.l.NetworkTopologyStrategy
with strategy_options=[{DC1:2, DC2:2}] and use LOCAL_QUORUM
consistency level, following documentation here:
http://www.datastax.com/docs/0.8/operations/datacenter

Now, how do I assign initial tokens? If I have, say 6 nodes total, 3
in DC1 and 3 in DC2, and create a ring as if all 6 nodes share the
total 2^128 space equally.
Now say node N1:DC2 has key K and is in remote datacenter (for an app
in DC1). Wouldn't Cassandra always forward K to the DC2 node N1 thus
turning asynchronous writes into synchronous ones? Performance impact
will be huge as the latency between DC1 and DC2 is significant.

I hope there's an answer and I'm just missing something. My case falls
under Disaster Recovery in
http://www.datastax.com/docs/0.8/operations/datacenter but I don't see
how Cassandra will support my use case.

I appreciate any help on this.

Thank you,
Oleg

[Oleg Tsvinev]

Hi all,

I followed instructions here:
http://wiki.apache.org/cassandra/Operations#Token_selection
to create a Cassandra cluster spanning two datacenters. Now I see that
nodes belonging to DC2 datacenter own 0% of the ring. I would expect
them to own 50%.

Does anyone have an idea what's going on here?

root@casper02:~# nodetool -h 10.4.64.63 ring
Address DC Rack Status State Load
Owns Token

85070591730234615865843651857942052865
10.4.64.63 DC1 RAC1 Up Normal 36.19 MB
50.00% 0
10.4.65.55 DC2 RAC1 Up Normal 36.21 MB
0.00% 1
10.4.65.73 DC1 RAC1 Up Normal 530.12 KB
50.00% 85070591730234615865843651857942052864
10.4.64.166 DC2 RAC1 Up Normal 525.68 KB
0.00% 85070591730234615865843651857942052865

Thank you,
Oleg

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