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Subject: Re: Confusion regarding the terms "replica" and "replication factor"
From: aaron morton <aaron@thelastpickle.com>
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Date: Thu, 24 May 2012 21:39:27 +1200
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To: user@cassandra.apache.org

This is partly historical. NTS (as it is now) has not always existed and =
was not always the default. In days gone by used to be a fella could run =
a mighty fine key-value store using just a Simple Replication Strategy.=20=


A different way to visualise it is a single ring with a Z axis for the =
DC's. When you look at the ring from the top you can see all the nodes. =
When you look at it from the side you can see the nodes are on levels =
that correspond to their DC. Simple Strategy looks at the ring from the =
top. NTS works through the layers of the ring.=20

> 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.
Nodes having a DC is a feature of *some* snitches and utilised by the =
*some* of the replication strategies (and by the messaging system for =
network efficiency). For background, mapping from row tokens to nodes is =
based on http://en.wikipedia.org/wiki/Consistent_hashing

Hope that helps.
-----------------
Aaron Morton
Freelance Developer
@aaronmorton
http://www.thelastpickle.com

On 24/05/2012, at 1:07 AM, java jalwa wrote:

> 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.
>=20
> Thanks again.
>=20
>=20
> 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.
>>=20
>>> 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.
>>=20
>> Cheers
>>=20
>> -----------------
>> Aaron Morton
>> Freelance Developer
>> @aaronmorton
>> http://www.thelastpickle.com
>>=20
>> On 23/05/2012, at 3:16 PM, java jalwa wrote:
>>=20
>>> 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.
>>> =46rom 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#networkt=
opologystrategy.
>>> 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 ?
>>>=20
>>> Thanks.
>>=20