Difference between consistency level and replication factor in cassandra?

Question

I am new to cassandra and I wanted to understand the granular difference between consistency level and replication factor.

Scenario: If I have a replication factor of 2 and consistency level of 3, how the write operation would be performed? When consistency level is set to 3, it means the results will be acknowledged to the client after writing to the 3 nodes. If data is written to 3 nodes, then it gives me a replication factor of 3 and not 2..? Are we sacrificing the replication factor in this case?

Can someone please explain where my understanding is wrong?

Thanks!

Answer 1

Replication factor : How many nodes should hold the data for this keyspace.

Consistency level : How many nodes needs to respond the coordinator node in order for the request to be successful.

So you can't have a consistency level higher than the replication factor simply because you can't expect more nodes to answer to a request than the amount of nodes holding the data.

Here are some references:

• Understand cassandra replication factor versus consistency level
• http://docs.datastax.com/en/cassandra/2.1/cassandra/architecture/architectureDataDistributeReplication_c.html
• http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/dml/dml_config_consistency_c.html

Answer 2

You will get an error: Cannot achieve consistency level THREE.

You can do some further reading here

Answer 3

Consistency levels are of two types, write consistency and read consistency. Consistency levels can be one, two, three or quorum. If it's quorum, atleast half of the nodes should be available for the operation. Otherwise (for one, two, three), the name itself gives you the definition.

Replication factor is the number of copies that you are planning to maintain in the cluster. If the strategy is simple, you will have just one replication factor. If you are having network topology strategy and is using multi dc cluster, then you have to set replication factor for each data centre.

In your scenario, if you have RF as 2 and CL as 3, it will work(I am assuming there are more than 3 nodes in the cluster and atleast one seed node). In this scenario, it will check whether three nodes are up and normal to receive the data and if the CL is met, it will write two copies to two nodes.

For your second question

When consistency level is set to 3, it means the results will be acknowledged to the client after writing to the 3 nodes. If data is written to 3 nodes, then it gives me a replication factor of 3 and not 2..?

As far as I understood cassandra, It will not be acknowledged to cassandra. It just needs the CL to be met and the number of nodes acknowledged about new data will be equal to the RF. So, there is no question in sacrificing RF.