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https://issues.apache.org/jira/browse/CASSANDRA-14404?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16448355#comment-16448355
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Ariel Weisberg commented on CASSANDRA-14404:
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This is not sloppy quorums. Sloppy quorums don't provide strong consistency. We
still enforce strict quorum membership.
>From the Dynamo paper:
{quote}To remedy this it does not enforce strict quorum membership and instead
it uses a “sloppy quorum”; all read and write operations are performed on the
first N healthy nodes from the preference list, which may not always be the
first N nodes encountered while walking the consistent hashing ring.
{quote}
We aren't going to allow you to use transient replication with 2i or MV in
version 1.
> 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
> (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwi834a%E2%80%948HaAhWCneAKHdj8DzAQFggpMAA&url=http%3A%2F%2Fwww2.cs.uh.edu%2F~paris%2FMYPAPERS%2FIcdcs86.pdf&usg=AOvVaw0GfCaaAtdzHiM65du1-qeI)]
> 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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