Cassandra Cluster 1.1.10

1
votes

I am new to Cassandra and at work I have a 4 node cluster. nodetool gossipinfo tells me that there are one datacentre, 2 racks and 2 nodes in each rack. Replication factor is defined as 2. nodetool ring tell me that each node has 50% ownership. There are 2 seed nodes in our config. Each rack has 1 seed node.

Does this mean that for each rack, there is one seed node and its replicated node. If that is the case then why is datasize not the same for seed node and its replicated node.

what happens if one node goes down. Will it have any impact on the data availability of the cluster.

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cassandra

1 Answers

1
votes

Seeds

Seeds nodes are only special in the way that new nodes that join the cluster contact the seed nodes to find out about other nodes and the topology of the ring. But in Cassandra, all nodes are the same, i.e. there are no master or slave, no primary or secondary node. Because of this, you can elect any (or all) node as the seed.

Since seeds only relate to gossip information, it does not have anything to do with replicated data.

Size

In relation to data size, each node will never be exactly the same since each partition/row size is never the same. If you look at the nodetool cfstats output, you will see that there is a big range between minimum and maximum sizes.

Availability

If the reads are done with a consistency level CL=ONE, then if a node is down the other replica will continue to serve requests. But if reads are done with a higher consistency, then reads will fail since it needs 2 nodes to be available, i.e. CL=LOCAL_QUORUM requires [ RF/2 + 1 ] nodes to respond.

EDIT: Response to:

Shouldn't each node own 25%?

Ownership

In Cassandra, data is not "distributed" across ALL nodes in ALL DCs. In fact, a DC is a copy of another DC depending on the replication factor.

To illustrate, consider the following keyspace definition:

CREATE KEYSPACE "myKS"
    WITH REPLICATION = {
        'class' : 'NetworkTopologyStrategy', 
        'DC1' : 2,
        'DC2' : 2};

Based on this definition, it means that the myKS keyspace has 2 replicas in DC1 and 2 replicas in DC2. Since each of your data centres only have 2 nodes, this effectively means that each DC is a copy of each other.

Following from that, since the tokens are split between 2 nodes, each node owns half of the data which is 50%. So in DC1, each node owns 50% and in DC2 (which is a copy of DC1) each node also owns 50%.