[jira] [Created] (CASSANDRA-20528) Support topology-safe changes to Datacenter & Rack for live nodes

["Marcus Eriksson (Jira)" <ji...@apache.org>]

Marcus Eriksson created CASSANDRA-20528:
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             Summary: Support topology-safe changes to Datacenter & Rack for live nodes
                 Key: CASSANDRA-20528
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-20528
             Project: Apache Cassandra
          Issue Type: Improvement
          Components: Transactional Cluster Metadata
            Reporter: Marcus Eriksson
            Assignee: Sam Tunnicliffe


Prior to CEP-21 and CASSANDRA-19488 most snitch implementations would implement their own mechanism for determining the local DC & rack then propagate that information via gossip. 
When queried for the location of a remote endpoint, the gossip state is inspected. Changing the local DC/rack after initial startup is forbidden, but this can be disabled by use of system properties or by disabling the specific startup checks.
In almost all cases, this is a fundamentally unsafe operation and so in trunk there is currently no mechanism to modify a node's DC and rack in ClusterMetadata. The supported way to acheive the same result is to decommission and re-join nodes instead.

It is possible though to add a mechanism to support _safe_ topology changes, meaning only those which do not have a material effect on data placements. Examples include:
* Renaming an empty DC, i.e. nodes exist in the DC but no data is replicated to them
* When using NTS modifications to rack assignments which preserve existing rack diversity e.g. moving adjacent replicas to the same rack is not permitted   
* If using SimpleStrategy exclusively, any change to DC or rack is allowed.



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