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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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