[jira] [Comment Edited] (CASSANDRA-13225) Best Consistency Level

["Jeremiah Jordan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-13225?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15868797#comment-15868797 ] 

Jeremiah Jordan edited comment on CASSANDRA-13225 at 2/15/17 11:32 PM:
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If your app works correctly when this happens and the 3rd node comes up again:

bq. BEST_QUORUM would succeed as 2 replicas are up and would return success when 2 replicas get the write

Then you should just use QUORUM.  You are getting no advantage from "BEST_QUORUM".  It is not like the coordinator doesn't write the data to all 3 nodes when they are all up.

If what you are worried about is overloading nodes and want to get all 3 acks back to slow down the writes, then you might want to take a look at using a back_pressure_strategy, this is exactly the case that was implemented for, reducing the number of dropped mutations when all nodes are up and there is a constant stream of writes coming in.


was (Author: jjordan):
If you app works correctly when this happens and the 3rd node comes up again:

bq. BEST_QUORUM would succeed as 2 replicas are up and would return success when 2 replicas get the write

Then you should just use QUORUM.  You are getting no advantage from "BEST_QUORUM".  Its not like we don't write the data to all 3 nodes when they are all up.

If what you are worried about is overloading your nodes and want to get all 3 acks back to slow down your writes, then you might want to take a look at using a back_pressure_strategy.

> Best Consistency Level
> ----------------------
>
>                 Key: CASSANDRA-13225
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13225
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Connor Warrington
>            Priority: Minor
>
> When writing data into a cluster there are a few consistency levels to choose from. When choosing the consistency level to write with you are making a tradeoff between consistency and failover availability. If you choose consistency level ALL then all replicas have to be up and when a write succeeds all replicas received the write. If you choose consistency level QUORUM then a quorum number of replicas have to be up and when a write succeeds at quorum number of replicas received the write. The tradeoff comes in when there are more then quorum nodes available for the write. We would like a write to succeed only when all replicas that are up have received the write. Hence the suggestion of best as a consistency level. This would be available for the existing consistency levels. The main idea behind this feature request is that we are okay with a replica going down (fault tolerance) but when the cluster is in a good state we don't mind waiting for all nodes to get the write. This would enable the writer to operate at speed of the slowest node instead of potentially getting into a state where that slow node gets even further behind. This would also enable back pressure to be better propagated through the system as the slowest node likely has back pressure which is trying to tell the client about but if we don't wait for that node the writer loses that information.
> Example scenarios:
> If we have replication factor of 3: 
> ALL consistency means 3 replicas have to be up and 3 replicas have to successfully get the write. 
> QUORUM consistency means 2 replicas have to be up and 2 replicas have to successfully get the write. 
> BEST_QUORUM consistency means 2 replicas have be up and all up replicas have to successfully get the write.
> If 3 replicas are up with replication factor of 3: 
> ALL would succeed as all 3 replicas are up and would return success when all 3 replicas get the write 
> QUORUM would succeed as all 3 replicas are up and would return success when 2 replicas get the write 
> BEST_QUORUM would succeed as all 3 replicas are up and would return success when all 3 replicas get the write
> If 2 replicas are up with replication factor of 3: 
> ALL would fail as only 2 replicas are up 
> QUORUM would succeed as 2 replicas are up and would return success when 2 replicas get the write 
> BEST_QUORUM would succeed as 2 replicas are up and would return success when 2 replicas get the write



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