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https://issues.apache.org/jira/browse/CASSANDRA-14404?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16599457#comment-16599457
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Ariel Weisberg commented on CASSANDRA-14404:
--------------------------------------------

The initial implementation of Transient Replication and Cheap Quorums was 
committed as 
[f7431b432875e334170ccdb19934d05545d2cebd|https://github.com/apache/cassandra/commit/f7431b432875e334170ccdb19934d05545d2cebd].

> Transient Replication & Cheap Quorums: Decouple storage requirements from 
> consensus group size using incremental repair
> -----------------------------------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-14404
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14404
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Coordination, Core, CQL, Distributed Metadata, Hints, 
> Local Write-Read Paths, Materialized Views, Repair, Secondary Indexes, 
> Testing, Tools
>            Reporter: Ariel Weisberg
>            Assignee: Ariel Weisberg
>            Priority: Major
>             Fix For: 4.0
>
>
> Transient Replication is an implementation of [Witness 
> Replicas|http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.146.3429&rep=rep1&type=pdf]
>  that leverages incremental repair to make full replicas consistent with 
> transient replicas that don't store the entire data set. Witness replicas are 
> used in real world systems such as Megastore and Spanner to increase 
> availability inexpensively without having to commit to more full copies of 
> the database. Transient replicas implement functionality similar to 
> upgradable and temporary replicas from the paper.
> With transient replication the replication factor is increased beyond the 
> desired level of data redundancy by adding replicas that only store data when 
> sufficient full replicas are unavailable to store the data. These replicas 
> are called transient replicas. When incremental repair runs transient 
> replicas stream any data they have received to full replicas and once the 
> data is fully replicated it is dropped at the transient replicas.
> Cheap quorums are a further set of optimizations on the write path to avoid 
> writing to transient replicas unless sufficient full replicas are available 
> as well as optimizations on the read path to prefer reading from transient 
> replicas. When writing at quorum to a table configured to use transient 
> replication the quorum will always prefer available full replicas over 
> transient replicas so that transient replicas don't have to process writes. 
> Rapid write protection (similar to rapid read protection) reduces tail 
> latency when full replicas are slow/unavailable to respond by sending writes 
> to additional replicas if necessary.
> Transient replicas can generally service reads faster because they don't have 
> to do anything beyond bloom filter checks if they have no data. With vnodes 
> and larger size clusters they will not have a large quantity of data even in 
> failure cases where transient replicas start to serve a steady amount of 
> write traffic for some of their transiently replicated ranges.



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