[jira] [Created] (CASSANDRA-14303) NetworkTopologyStrategy could have a "default replication" option

["Joseph Lynch (JIRA)" <ji...@apache.org>]

Joseph Lynch created CASSANDRA-14303:
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             Summary: 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


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. 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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