Cassandra vnodes replicas

Question

Setting up the context: Cassandra currently implements vnodes. 256 by default which is tweakable in the cassandra.yaml file Vnodes as I understand are token-ranges/hash-ranges. Eg. (x...y], where y is the token number of the vnode. Each physical node in Cassandra is assigned random 256 tokens, and each of those tokens are the boundary value of a hash/token range. The tokens assigned are within the range of 2^-63 to 2^63-1 (the range of hash numbers which murmur3 has partitioner may generate). So far so good.

Question: 1. Is it that a token range(vnode) is a fixed range. Once set, this token range will be copied to other Cassandra nodes to satisfy the replication factor like a token range(vnode) being a fundamental chunk of data(tokens) which goes around together. Only in case of bootstrap of a new node in the cluster, this token range(vnode) might break apart to be assigned to other node.

2. Riding on the last proposition, (say the last proposition is true). Then a vnode must only contain tokens which belong a given keyspace. Because each keyspace(container of column family/tables) has a defined replication strategy and replication factor. And it is highly likely that replication factor of keyspaces in a Cassandra cluster will vary. Consider an example. "system_schema" keyspace has a RF of 1 whereas I created a keyspace "test_ks" with RF 3. If a row of system_schema keyspace has a token number 2(say) and a row of my test_ks has token number 5(say). these 2 tokens can't be placed in the same token range(vnode). If a vnode is consistent chunk of token ranges, say token 2 and 5 belong to vnode with token number 10. so vnode 10 has to be placed on 3 different physical nodes to satisfy the RF =3 for test_ks, but we are unnecessary placing token 2 on 3 different nodes whose RF is supposed to be 1.

Is this proposition correct that, a vnode is only dedicated to a given keyspace? which boils down to out of 256 tokens on a physical node... 20(say) vnodes currently belong to "system" keyspace, 80 vnodes(say) belong to test_ks.

3. Again riding on the above proposition, this means that each node should have the info of keyspace-wise vnodes currently available in the cluster. That way when a new write comes in for a Keyspace the co-ordinator node would locate all vnodes in the cluster for that keyspace and assign the new row a token number which falls within the token range of those keyspaces. That being the case can I know how many vnodes currently belong to a keyspace in the entire cluster/ or on a given node.

Please do correct me if I'm wrong. I have been following the below blogs and videos to get an understanding of this concept:

https://www.scribd.com/document/253239514/Virtual-Nodes-Strategies-for-Apache-Cassandra

https://www.youtube.com/watch?v=GddZ3pXiDys&t=11s

Thanks in advance

Answer 1

There is no fixed token-range, the tokens are just generated randomly. This is one of the reasons that vnodes were implemented - the idea being that if there are more tokens it is more likely that the resulting token-ranges will be more evenly distributed across nodes.

Token generation was recently improved in 3.0, allowing Cassandra to place new tokens a little more intelligently (see CASSANDRA-7032). You can also manually configure tokens (see initial_token), although it can become tricky to keep things balanced when it comes time to expand the cluster unless you plan on doubling the number of nodes.

The total number of tokens in a cluster is the number of nodes in the cluster multiplied by the number of vnodes per node.

In regards to placement of replicas, the first copy of a partition is placed in the node that owns that partition's token. The additional n copies are placed sequentially on the next n nodes in the ring that are in the same data centre. There is no relationship between tokens and keyspaces.

When a new write comes into a coordinator node, the coordinator node determines which node owns the partition by hashing the partition key. Note that for better performance this can actually be done by the driver instead if you use TokenAwarePolicy. The coordinator sends the write to the node that owns the partition, and if the data needs to be replicated the coordinator node also writes the replicas to the next two nodes sequentially in the token-space.

For example, suppose that we have 3 nodes which each have one token: node1: 10, node2: 20 & node3: 30. If we write a record whose partition key hashes to 22, to a keyspace with RF3, then the first copy goes to node2, the second goes to node3 and the third goes to node1. Note that each replica is equally valid - there is nothing special about the "first" replica other than that it happens to be stored on the "first" replica node.

Vnodes do not change this process, they just split up each node's token ranges by allowing each node to have more than one token. For example, if our cluster now has 2 vnodes for each node, it might instead look like this: node1: 10, 25, node2: 20, 3 & node3: 30, 21. Now our write that hashed to 22 goes to node3 (because it owns the range from 21-24), and the copies go to node1 and node2.