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https://issues.apache.org/jira/browse/CASSANDRA-14303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16393438#comment-16393438
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Joseph Lynch commented on CASSANDRA-14303:
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Yes, that edge case as well as others (gossip inconsistency mostly) is why I
propose only evaluating the DCs at the time of a CREATE or ALTER statement
execution. The operator would still have to go run:
{noformat}
ALTER KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
'replication_factor': 3}{noformat}
manually after adding a new datacenter to trigger the re-generation of the dcs.
It's also worth noting that as proposed if you then described the keyspace you
would get all the dcs that it is actually replicated to:
{noformat}
cqlsh> DESCRIBE KEYSPACE test
CREATE KEYSPACE test WITH replication = {'class': 'NetworkTopologyStrategy',
'us-west-1': '3', 'us-east-1': 3} AND durable_writes = true;{noformat}
> 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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