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From Robert Coli <>
Subject Re: Downtime-Limit for a node in Network-Topology-Replication-Cluster?
Date Tue, 27 Oct 2015 22:49:21 GMT
On Sat, Oct 24, 2015 at 2:02 PM, Vasileios Vlachos <> wrote:

>> All other means of repair are optimizations which require a certain
>> amount of luck to happen to result in consistency.
> Is that true regardless of the CL one uses? So, for example if writing
> QUORUM and reading QUORUM, wouldn't an increased read_repair_chance
> probability be sufficient? If not, is there a case where nodetool repair
> wouldn't be required (given consistency is a requirement)?

The only thing which guarantees consistency[1] is repair.

It's likely true that if the following conditions are met :

1) you read and write with QUORUM or ALL
2) you repair periodically
3) you have not dropped any mutations or had a crashed node since you last
4) you have not discarded any hints which happened to be stored for
whatever reason
5) you have not failed to store any hints due to hint backpressure

That you have a system with the property of consistency.

However no step other than 2) provides a *guarantee* of consistency. And it
only provides that guarantee for data that exists when the repair starts.

In a related concept, no step other than 2) *guarantees* that all data is
repaired within gc_grace_seconds, which is essential to consistency.

[1] Durability and consistency are commingled in Cassandra, you are more
likely to fully achieve the former than the latter in the typical RF=3 case.

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