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Before removing (decommission) the data-center from the cluster, it is essential to verify that the data-center that is about to be removed does not hold any unique data.
Verify that there are no client writes to nodes in the data-center that is to be decommissioned.
Check the cluster status using the nodetool status command to understand the cluster deployment.
Datacenter: US-DC
Status=Up/Down
State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 192.168.1.201 112.82 KB 256 32.7% 8d5ed9f4-7764-4dbd-bad8-43fddce94b7c B1
UN 192.168.1.202 91.11 KB 256 32.9% 125ed9f4-7777-1dbn-lac8-23fddce9123e B1
UN 192.168.1.203 124.42 KB 256 32.6% 675ed9f4-6564-6dbd-can8-43fddce952gy B1
Datacenter: ASIA-DC
Status=Up/Down
State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 50.191.1.204 112.82 KB 256 32.7% 4d5ed9f4-7764-4ded-dad8-63fdace94b7c B1
UN 50.191.1.205 91.11 KB 256 32.9% 145id9f4-7777-1dvn-nac8-83fdzce917r4 B1
UN 50.191.1.206 124.42 KB 256 32.6% 777ed9f4-6564-6dsd-can8-13fdxce999gy B1
Datacenter: EUROPE-DC
Status=Up/Down
State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 172.91.202.31 112.82 KB 256 32.7% 1d5ed8f4-7764-4dbd-rad8-44fddce94b7v B1
UN 172.91.202.32 91.11 KB 256 32.9% 525ed7g4-7437-1dbn-mac8-53fddce9123c B1
UN 172.91.202.33 124.42 KB 256 32.6% 975edbm4-6564-63bd-san8-73fddce952ga B1
Run the nodetool repair -pr command on each node in the data-center that is going to be decommissioned. This will verify that all the data is in sync between the decommissioned data-center and the other data-centers in the cluster.
For example:
If the ASIA-DC cluster is to be removed, then, run the nodetool repair -pr command on all the nodes in the ASIA-DC
ALTER every cluster KEYSPACE, so that the keyspaces will no longer replicate data to the decommissioned data-center.
For example:
cqlsh> DESCRIBE <KEYSPACE_NAME>
cqlsh> CREATE KEYSPACE <KEYSPACE_NAME> WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', '<DC_NAME1>' : 3, '<DC_NAME2>' : 3, '<DC_NAME3>' : 3};
cqlsh> ALTER KEYSPACE <KEYSPACE_NAME> WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', '<DC_NAME1>' : 3, '<DC_NAME2>' : 3};
For example:
There are three data-centers: US-DC, ASIA-DC and EUROPE-DC, the ASIA-DC is to be removed. The current KEYSPACE is:
cqlsh> DESCRIBE nba
cqlsh> CREATE KEYSPACE nba WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'US-DC' : 3, 'ASIA-DC' : 2, 'EUROPE-DC' : 3};
Use the ALTER KEYSPACE to change the KEYSPACE and remove the decommissioned data-center.
cqlsh> ALTER KEYSPACE nba WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'US-DC' : 3, 'EUROPE-DC' : 3};
Run nodetool decommission on every node in the data center that is to be removed. Refer to Remove a Node from a ScyllaDB Cluster - Down Scale for further information.
For example:
If ASIA-DC is to be removed, then, execute the nodetool decommission command on all the nodes in this data-center.
Verify that the data-center was successfully removed by using the nodetool status command.
For example:
Datacenter: US-DC
Status=Up/Down
State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 192.168.1.201 112.82 KB 256 32.7% 8d5ed9f4-7764-4dbd-bad8-43fddce94b7c B1
UN 192.168.1.202 91.11 KB 256 32.9% 125ed9f4-7777-1dbn-mac8-43fddce9123e B1
UN 192.168.1.203 124.42 KB 256 32.6% 675ed9f4-6564-6dbd-can8-43fddce952gy B1
Datacenter: EUROPE-DC
Status=Up/Down
State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 172.91.202.31 112.82 KB 256 32.7% 1d5ed8f4-7764-4dbd-rad8-44fddce94b7v B1
UN 172.91.202.32 91.11 KB 256 32.9% 525ed7g4-7437-1dbn-mac8-53fddce9123c B1
UN 172.91.202.33 124.42 KB 256 32.6% 975edbm4-6564-63bd-san8-73fddce952ga B1