Decommission a data center

Steps to properly remove a data center so that no information is lost.

Decommission an HCD data center

Verify that no clients are writing data to any nodes in the data center. The following JMX MBeans provide details on client connections and pending requests:
- Active connections: org.apache.cassandra.metrics/Client/connectedNativeClients and org.apache.cassandra.metrics/Client/connectedThriftClients
- Pending requests: org.apache.cassandra.metrics/ClientRequests/viewPendingMutations or use nodetool tpstats.
Run a full repair with 'nodetool repair --full' to ensure that all data is propagated from the data center being decommissioned.
Shut down the Mission Control Repair Service, if in use.
Change all keyspaces so they no longer reference the data center being decommissioned.
Shut down all nodes in the data center.
From a running node in a data center that is not being decommissioned, run 'nodetool assassinate' for each node in the data center that is being decommissioned:
```
nodetool assassinate <remote_IP_address>
```
If the replication factor (RF) on any keyspace has not been properly updated, then:
1. Note the name of the keyspace that needs to be updated.
2. Remove the data center from the keyspace RF (using 'ALTER KEYSPACE').
3. For a keyspace with a simple strategy RF, run a full repair on the keyspace:
  nodetool repair --full <keyspace_name>
Run nodetool status to ensure that the nodes in the data center are removed.
If the Mission Control Repair Service is disabled, then re-enable it now.

Example

Removing DC3 from the cluster:

Check the status of the cluster:

nodetool status

Status shows that there are three data centers with one node in each:

Datacenter: DC1
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address         Load       Owns    Host ID                               Token                     Rack
UN  10.200.175.11   474.23 KiB  ?       7297d21e-a04e-4bb1-91d9-8149b03fb60a  -9223372036854775808     rack1
Datacenter: DC2
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address         Load       Owns    Host ID                               Token                     Rack
UN  10.200.175.113  518.36 KiB  ?       2ff7d46c-f084-477e-aa53-0f4791c71dbc  -9223372036854775798     rack1
Datacenter: DC3
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address         Load       Owns    Host ID                               Token                     Rack
UN  10.200.175.111  461.56 KiB  ?       ac43e602-ef09-4d0d-a455-3311f444198c  -9223372036854775788     rack1

Run a full repair:
```
nodetool repair --full
```
Using JConsole, check the following JMX Beans to make sure there are no active connections:
- org.apache.cassandra.metrics/Client/connectedNativeClients
- org.apache.cassandra.metrics/Client/connectedThriftClients

Verify that there are no pending write requests on each node that is being removed (The Pending column should read 0 or N/A):

nodetool tpstats

Pool Name                                     Active      Pending (w/Backpressure)   Delayed      Completed...
BackgroundIoStage                                  0                       0 (N/A)       N/A            640...
CompactionExecutor                                 0                       0 (N/A)       N/A           1039...
GossipStage                                        0                       0 (N/A)       N/A           4580...
HintsDispatcher                                    0                       0 (N/A)       N/A              2...

Start cqlsh and decommission DC3 from all keyspace configurations. Repeat for each keyspace that has a RF set for DC3:
```
alter keyspace cycling WITH replication = {'class': 'NetworkTopologyStrategy', 'DC1':1,'DC2':2};
```
Shut down the Mission Control Repair Service, if in use.
Shut down all nodes in the data center.
From a running node in a data center that is not being decommissioned, run 'nodetool assassinate' for each node in the data center being decommissioned, (in this case, DC3):
```
nodetool assassinate <remote_IP_address>
```

In a remaining data center, verify that the DC3 data center has been removed:

nodetool status

Datacenter: DC1
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address         Load       Owns    Host ID                               Token                     Rack
UN  10.200.175.11   503.54 KiB  ?       7297d21e-a04e-4bb1-91d9-8149b03fb60a  -9223372036854775808     rack1
Datacenter: DC2
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address         Load       Owns    Host ID                               Token                     Rack
UN  10.200.175.113  522.47 KiB  ?       2ff7d46c-f084-477e-aa53-0f4791c71dbc  -9223372036854775798     rack1

Decommission a data center

Decommission an HCD data center

Example

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