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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