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saitaladev_142721 avatar image
saitaladev_142721 asked ·

In a network partition where parts of the cluster are disconnected, can the largest group of nodes still satisfy queries?

As part of the Cassandra developer training from Datastax, I had the below question "In a full network partition, that is, parts of the cluster are completely disconnected from the whole, only the largest group of nodes can still satisfy queries."

I gave the answer as "YES" here. Because even though we break the Cassandra cluster, the largest group can still reset itself to satisfy the consistency level and serve the request.

But I see my answer was wrong. Can any one please explain me why ?

cassandra
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@saitaladev_142721 Which course was it and which section? It would be great if you could update your original question with the details. Cheers!

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Erick Ramirez avatar image
Erick Ramirez answered ·

@saitaladev_142721 The question is a little bit more nuanced than it seems. Cassandra does not use a "majority wins" principle to determine if a request can be satisfied.

The "largest group of nodes" does not guarantee that:

  • they belong to the same data centre
  • there is enough replicas to satisfy the required consistency level

Having enough replicas in the same data centre is relevant because for consistency level such as LOCAL_QUORUM, the application only cares about replicas in the local DC and there's a chance the contactable nodes won't necessarily be local and could be from remote DCs. Cheers!

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