Github user bdeggleston commented on a diff in the pull request:

    https://github.com/apache/cassandra/pull/224#discussion_r188782098
  
    --- Diff: 
src/java/org/apache/cassandra/cql3/statements/AlterKeyspaceStatement.java ---
    @@ -96,7 +98,35 @@ private void warnIfIncreasingRF(KeyspaceMetadata ksm, 
KeyspaceParams params)
                                                                                
                             StorageService.instance.getTokenMetadata(),
                                                                                
                             DatabaseDescriptor.getEndpointSnitch(),
                                                                                
                             params.replication.options);
    -        if (newStrategy.getReplicationFactor() > 
oldStrategy.getReplicationFactor())
    +
    +        validateTransientReplication(oldStrategy, newStrategy);
    +        warnIfIncreasingRF(oldStrategy, newStrategy);
    +    }
    +
    +    private void validateTransientReplication(AbstractReplicationStrategy 
oldStrategy, AbstractReplicationStrategy newStrategy)
    +    {
    +        if (oldStrategy.getReplicationFactor().trans == 0 && 
newStrategy.getReplicationFactor().trans > 0)
    +        {
    +            Keyspace ks = Keyspace.open(keyspace());
    +            for (ColumnFamilyStore cfs: ks.getColumnFamilyStores())
    +            {
    +                if (cfs.viewManager.hasViews())
    +                {
    +                    throw new ConfigurationException("Cannot use transient 
replication on keyspaces using materialized views");
    +                }
    +
    +                if (cfs.indexManager.hasIndexes())
    +                {
    +                    throw new ConfigurationException("Cannot use transient 
replication on keyspaces using secondary indexes");
    +                }
    +            }
    +
    +        }
    +    }
    --- End diff --
    
    If you're keeping the total number of replicas the same, I don't think 
there would  be any difference between reducing the number of transient 
replicas, and increasing the replication factor on a normally replicated 
keyspace. You'd just decrease transient count one at a time and do a full 
repair between each? If you were reducing both (ie: 5/2 -> 4/1), that shouldn't 
cause any problems at all.


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