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DataStax Academy migrated to a new learning management system (LMS) in July 2020. We are also moving to a new Cassandra Certification process so there are changes to exam bookings, voucher system and issuing of certificates.

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question

rashmimisal07_190319 avatar image
rashmimisal07_190319 asked ·

What should the replication factor be to survive 5 nodes down in a 20-node DC with consistency LOCAL_QUORUM?

Nodes-20,5 nodes can be afforded to be down, What can be the

replication factor- ?

replication
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1 Answer

Erick Ramirez avatar image
Erick Ramirez answered ·

As I explained in your other question (#7194), requests with consistency level LOCAL_QUORUM with the recommended replication factor of 3 requires a quorum of 3 replicas -- with quorum being "50% + 1", that means 2 out of 3 replicas are required to satisfy the request.

As I understand it, you've passed the certification for both Administrator Associate and Developer Associate so I think you should be able to work out how many replicas you need for requests to succeed when you have 5 nodes down in a 20-node DC using the formula I provided. Cheers!

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what is the ans?

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Hi @rashmimisal07_190319,

you would need a replication factor of 11 to be able to read and write with a consistency of local quorum while 5 nodes are down at the same time.

In practice I have never seen a replication factor that high.

Here is a neat calculator for these kind of questions:

https://www.ecyrd.com/cassandracalculator/

I hope this helps!

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