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https://issues.apache.org/jira/browse/CASSANDRA-14404?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16448626#comment-16448626
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Duarte Nunes commented on CASSANDRA-14404:
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Ah, CL is now a function of RF + count(Witnesses).
> 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://www2.cs.uh.edu/~paris/MYPAPERS/Icdcs86.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 temporarily late to respond by sending writes
> to additional replicas if necessary.
> Transient replicas can generally service reads faster because they don't have
> 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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