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https://issues.apache.org/jira/browse/CASSANDRA-14404?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16599094#comment-16599094
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Constance Eustace commented on CASSANDRA-14404:
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So are (basically) these transient nodes basically serving as centralized
hinted handoff caches rather than having the hinted handoffs cluttering up full
replicas, especially nodes that have no concern for the token range involved? I
understand that hinted handoffs aren't being replaced by this, but is that kind
of the idea?
Are the transient nodes sitting around?
Will the transient nodes have cheaper/lower hardware requirements?
During cluster expansion, does the newly streaming node acquiring data function
as a temporary transient node until it becomes a full replica? Likewise while
shrinking, does a previously full replica function as a transient while it
streams off data?
Can this help vnode expansion with multiple concurrent nodes? Admittedly I'm
not familiar with how much work has gone into fixing cluster expansion with
vnodes, it is my understanding that you typically expand only one node at a
time or in multiples of the datacenter size
> 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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