Why can't cassandra survive the loss of no nodes without data loss. with replication factor 2

Question

Hi I was trying out different configuration using the site https://www.ecyrd.com/cassandracalculator/

But I could not understand the following results show for configuration

Cluster size  3
Replication Factor  2
Write Level 1   
Read Level 1

You can survive the loss of no nodes without data loss.

For reference I have seen the question Cassandra loss of a node

But it still does not help to understand why Write level 1 will with replication 2 would make my cassandra cluster not survive the loss of no node without data loss?

A write request goes to all replica nodes and the even if 1 responds back , it is a success, so assuming 1 node is down, all write request will go to the other replica node and return success. It will be eventually consistent.

Can someone help me understand with an example.

Answer 1

I guess what the calculator is working with is the worst case scenario.

You can survive the loss of one node if your data is available redundantly on two out of three nodes. The thing with write level ONE is, that there is no guarantee that the data is actually present on two nodes right after your write was acknowledged.

Let's assume the coordinator of your write is one of the nodes holding a copy of the record you are writing. With write level ONE you are telling the cluster to acknowledge your write as soon as the write was committed to one of the two nodes that should hold the data. The coordinator might do that before even attempting to contact the other node (to boost latency percieved by the client). If in that moment, right after acknowledging the write but before attempting to contact the second node the coordinator node goes down and cannot be brought back, then you lost that write and the data with it.