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From Jeff Jirsa <jji...@gmail.com>
Subject Re: Consistent hashing vnodes and ring in cassandra
Date Sat, 17 Aug 2019 19:05:03 GMT


> 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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