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https://issues.apache.org/jira/browse/CASSANDRA-14303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16393423#comment-16393423
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Jeremiah Jordan commented on CASSANDRA-14303:
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An issue with having a default replication would be that you *must* set
autobootstrap:false when adding a new DC, otherwise the first nodes added in
the DC would get all the data. Given proper DC creation, it is not required to
do this right now.
You could get around this by setting the replication to "0" before adding any
nodes from the new DC, but them we would need to remove the guardrails around
setting replication for DC's which don't exist. Also having to set the
replication to "0" would back fire the goal of the ticket which seems to be
that you don't have to manage the RF when adding or removing DC's.
> NetworkTopologyStrategy could have a "default replication" option
> -----------------------------------------------------------------
>
> Key: CASSANDRA-14303
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14303
> Project: Cassandra
> Issue Type: Improvement
> Components: Configuration
> Reporter: Joseph Lynch
> Priority: Minor
>
> 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': 0}
> {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 probably never remove none zero
> rf dcs when auto-generating dcs). Removing a datacenter becomes a two step
> process, e.g. if we wanted to remove {{dc1}} we would do:
> {noformat}
> // First tell it not to replicate to dc1
> ALTER KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'replication_factor': 3, 'dc1': 0}
> // Remove all nodes from dc1
> ALTER KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
> 'replication_factor': 3}{noformat}
> I think the only issue with this would be that I think {{EACH_QUORUM}}
> doesn't handle DCs with 0 replicas very well, but I think that is tractable.
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