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From java jalwa <java.ka.ja...@gmail.com>
Subject Re: Confusion regarding the terms "replica" and "replication factor"
Date Wed, 23 May 2012 13:07:37 GMT
Thanks Aaron. That makes things clear.
So I guess the 0 - 2^127 range for tokens corresponds to a cluster
-level top-level ring. and then you add some logic on top of that with
NTS to logically segment that range into sub-rings as per the notion
of data clusters defined in NTS. Whats the advantage of having a
single top-level ring ? intuitively it seems like each replication
group could have a separate ring so that the same tokens can be
assigned to nodes in different DC. If the hierarchy is Cluster ->
DataCenter -> Node, why exactly do we need globally unique node tokens
even though nodes are at the lowest level in the hierarchy.

Thanks again.


On Wed, May 23, 2012 at 3:14 AM, aaron morton <aaron@thelastpickle.com> wrote:
>> Now if a row key hash is mapped to a range owned by a node in DC3,
>> will the Node in DC3 still store the key as determined by the
>> partitioner and then walk the ring and store 2 replicas each in DC1
>> and DC2 ?
> No, only nodes in the DC's specified in the NTS configuration will be replicas.
>
>> Or will the co-ordinator node be aware of the
>> replica placement strategy,
>> and override the partitioner's decision and walk the ring until it
>> first encounters a node in DC1 or DC2 ? and then place the remaining
>> replicas ?
> The NTS considers each DC to have it's own ring. This can make token selection in a multi
DC environment confusing at times. There is something in the DS docs about it.
>
> Cheers
>
> -----------------
> Aaron Morton
> Freelance Developer
> @aaronmorton
> http://www.thelastpickle.com
>
> On 23/05/2012, at 3:16 PM, java jalwa wrote:
>
>> Hi all,
>>              I am a bit confused regarding the terms "replica" and
>> "replication factor". Assume that I am using RandomPartitioner and
>> NetworkTopologyStrategy for replica placement.
>> From what I understand, with a RandomPartitioner, a row key will
>> always be hashed and be stored on the node that owns the range to
>> which the key is mapped.
>> http://www.datastax.com/docs/1.0/cluster_architecture/replication#networktopologystrategy.
>> The example here, talks about having 2 data centers and a replication
>> factor of 4 with 2 replicas in each datacenter, so the strategy is
>> configured as DC1:2 and DC2:2. Now suppose I add another datacenter
>> DC3, and do not change the NetworkTopologyStrategy.
>> Now if a row key hash is mapped to a range owned by a node in DC3,
>> will the Node in DC3 still store the key as determined by the
>> partitioner and then walk the ring and store 2 replicas each in DC1
>> and DC2 ? Will that mean that I will then have 5 replicas in the
>> cluster and not 4 ? Or will the co-ordinator node be aware of the
>> replica placement strategy,
>> and override the partitioner's decision and walk the ring until it
>> first encounters a node in DC1 or DC2 ? and then place the remaining
>> replicas ?
>>
>> Thanks.
>

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