CODEWITHSUNDEEP
cassandracluster-computingreplicationsharding
Charlie Scene
Charlie Scene

Reputation: 95

cassandra possible node loss

I am fairly new to Cassandra and found this website
https://www.ecyrd.com/cassandracalculator/
Not sure how accurate it is, but i have one misunderstanding. Consider following example:

As a result i get that I can lose 4 nodes without impacting the application. Does anyone know what calculation leads us to this result? Thanks in advance.
P.S. I would like to remark that I am not interested in any other aspect, except "how many nodes I can loose without impacting application". The answer I am looking for is not how consistency works, or anything else, but exclusively what equation stands behind described result for "how many nodes I can loose without impacting application" and why.

Upvotes: 0

Views: 235

Answers (2)

LetsNoSQL
LetsNoSQL

Reputation: 1538

You can tune consistency at query level also based on requirements. SELECT * FROM users WHERE state='ABC' USING CONSISTENCY QUORUM; Yes, CL ONE provides good availability than quorum but quorum will give you more consistent data.

Upvotes: 0

Chris Lohfink
Chris Lohfink

Reputation: 16400

A CL.ONE can always lead to the possibility of data loss. As an example: A replica as the coordinator gets requests, writes locally and sends ack to client. If that system is then hit by a meteor before the data to the other replicas has been sent theres data loss.

If you use local_quorum or quorum then with a RF=5 you can have 2 nodes of the replica set fail without any data loss (excluding cases like not following expected operational practices around repairs). However with CL.ONE your application can still run even if 4 of the 5 replicas died, in some cases availability is more important than the durability and consistency. I would recommend always start with quorum and then only change your consistency if theres an unmet availability of performance requirement.

Upvotes: 1

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