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From Inquistive allen <inquial...@gmail.com>
Subject Re: Consistent hashing vnodes and ring in cassandra
Date Sat, 17 Aug 2019 19:21:56 GMT
Hello Jeff,

Thanks for the response.
I just got it right. One Last thing, when a read request comes in to the
coordinator node, the partition key is hashed and a node is located where
corresponding data is previously stored. How does the coordinator node
locate the replica nodes for this row.
The first copy of this was written based on hash number, but the replica
copies were written based on replication strategy.
Will a hash of any partition key list out all nodes where data is present .

Thanks


On Sun, 18 Aug, 2019, 12:35 AM Jeff Jirsa, <jjirsa@gmail.com> wrote:

>
>
> > On Aug 17, 2019, at 10:53 AM, Inquistive allen <inquiallen@gmail.com>
> wrote:
> >
> > I am a newbie in cassandra. I have asked this question on various
> platforms but never got a satisfying answer.
> > Hence thought of bringing up the topic here. Sorry for this might be a
> simple question.
> >
> > 1. I studied the paper on consistent hashing (which is being implemented
> in Cassandra)
> > 2. Cassandra has the concept of Vnodes. The vnodes( As I understand a
> Vnode is a collection of Hashes) , are the basic blocks of replication in
> cassandra. It is the vnodes which are replicated across the cluster. Please
> do correct me I'm wrong
>
> Vnodes JUST mean each host has more than one token
>
> > 3. Suppose I have a Keyspace A with replication factor 3 and Keyspace B
> with replication factor 2.
> > 4. Is it that a Vnode is a collection of hashes of data from various
> Keyspaces.
> > 5. In that case, Keyspace with varying replication factors , replicating
> them to other nodes would be a problem
> > 6. Now from the consistent hashing paper, I get a feeling  that , ach
> Keyspace has a different ring. Also the name "KEYSPACE", points to a ring
> of keys in the ring.
> >    So is it that each keyspace has a different ring. If it is so,
> everything else like replicating vnodes among nodes in the cluster would
> fall in place.
> >    Each Keyspace has a different ring ---> each Vnode has data of
> various tables from a given keyspace----> hence copies equal to RF is only
> made in the cluster.
> >
> > I know I am missing something. This way of understanding thing might be
> wrong.
> > Kindly help me understand the same. As this would help me visualise
> repair, bootstrap, adding cluster, streaming operations in a much better
> way.
> >
>
> The easiest way to visualize most cassandra operations is to draw the
> tokens in a circle. Vnodes means extra tokens
>
> Replica sets are adjacent tokens. You steam from any node in the replica
> set in the common replacement case, or the losing replica in the expansion
> case
>
>
>
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