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https://issues.apache.org/jira/browse/CASSANDRA-14303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16713789#comment-16713789
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Jeff Jirsa commented on CASSANDRA-14303:
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There are a bunch of little code style issues with this patch that can be
cleaned up in a ninja commit later, but
[this|https://github.com/apache/cassandra/commit/1f19d5f7a243cc4227da923459f5eb2f66066778#diff-f27f3a1ef1ddb3a6418ecdda750b3bb5R260]
is concerning to me.
Going from simplestrategy|replication_factor=3 to nts|replication_factor=3
silently multiplies the replica count and violates consistency if you have more
than one DC.
(Yes, it's true that going from simple strategy to NTS is almost always going
to violate consistency, but we don't hide that fact behind an auto-expansion of
old arguments).
> Auto-expand replication_factor for NetworkTopologyStrategy
> ----------------------------------------------------------
>
> Key: CASSANDRA-14303
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14303
> Project: Cassandra
> Issue Type: Improvement
> Components: Configuration
> Reporter: Joseph Lynch
> Assignee: Joseph Lynch
> Priority: Minor
> Labels: 4.0-feature-freeze-review-requested
> Fix For: 4.x
>
>
> Right now when creating a keyspace with {{NetworkTopologyStrategy}} the user
> has to manually specify the datacenters they want their data replicated to
> with parameters, e.g.:
> {noformat}
> CREATE KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'dc1': 3, 'dc2': 3}{noformat}
> This is a poor user interface because it requires the creator of the keyspace
> (typically a developer) to know the layout of the Cassandra cluster (which
> may or may not be controlled by them). Also, at least in my experience, folks
> typo the datacenters _all_ the time. To work around this I see a number of
> users creating automation around this where the automation describes the
> Cassandra cluster and automatically expands out to all the dcs that Cassandra
> knows about. Why can't Cassandra just do this for us, re-using the previously
> forbidden {{replication_factor}} option (for backwards compatibility):
> {noformat}
> CREATE KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'replication_factor': 3}{noformat}
> This would automatically replicate this Keyspace to all datacenters that are
> present in the cluster. If you need to _override_ the default you could
> supply a datacenter name, e.g.:
> {noformat}
> > CREATE KEYSPACE test WITH replication = {'class':
> > 'NetworkTopologyStrategy', 'replication_factor': 3, 'dc1': 2}
> > DESCRIBE KEYSPACE test
> CREATE KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'dc1': '2', 'dc2': 3} AND durable_writes = true;
> {noformat}
> On the implementation side I think this may be reasonably straightforward to
> do an auto-expansion at the time of keyspace creation (or alter), where the
> above would automatically expand to list out the datacenters. We could allow
> this to be recomputed whenever an AlterKeyspaceStatement runs so that to add
> datacenters you would just run:
> {noformat}
> ALTER KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'replication_factor': 3}{noformat}
> and this would check that if the dc's in the current schema are different you
> add in the new ones (_for safety reasons we'd never remove non explicitly
> supplied zero dcs when auto-generating dcs_). Removing a datacenter becomes
> an alter that includes an override for the dc you want to remove (or of
> course you can always not use the auto-expansion and just use the old way):
> {noformat}
> // Tell it explicitly not to replicate to dc2
> > ALTER KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> > 'replication_factor': 3, 'dc2': 0}
> > DESCRIBE KEYSPACE test
> CREATE KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'dc1': '3'} AND durable_writes = true;{noformat}
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