Adding a datacenter to a cluster

Add a datacenter to an existing cluster.

1. Complete the following steps to prevent client applications from prematurely connecting to the new datacenter, and to ensure that the consistency level for reads or writes does not query the new datacenter:
   Warning: If client applications are not properly configured, they might connect to the new datacenter before it is online. Incorrect configuration results in connection exceptions, timeouts, and/or inconsistent data.
   1. Configure client applications to use the DCAwareRoundRobinPolicy.
   2. Direct clients to an existing datacenter. Otherwise, clients might try to access the new datacenter, which might not have any data.
   3. If using the QUORUM consistency level, change to LOCAL_QUORUM.
   4. If using the ONE consistency level, set to LOCAL_ONE.

   See the programming instructions for your driver.

2. Configure every keyspace using SimpleStrategy to use the NetworkTopologyStrategy replication strategy, including (but not restricted to) the following keyspaces.

   If SimpleStrategy was used previously, this step is required to configure NetworkTopologyStrategy.

   1. Use ALTER KEYSPACEALTER KEYSPACE to change the keyspace replication strategy to NetworkTopologyStrategy for the following keyspaces.

      ALTER KEYSPACE keyspace_name WITH REPLICATION = 
      {'class' : 'NetworkTopologyStrategy', 'ExistingDC1' : 3};

      • All keyspaces created by users
   2. Use DESCRIBE SCHEMA to check the replication strategy of keyspaces in the cluster. Ensure that any existing keyspaces use the NetworkTopologyStrategy replication strategy.

      DESCRIBE SCHEMA ;

3. In the new datacenter, install Cassandra on each new node. Do not start the service or restart the node.
   Important: Use the same version of DDAC on all nodes in the cluster.
4. Configure properties in cassandra.yaml on each new node, following the configuration of the other nodes in the cluster.
   1. Configure node properties:
      • -seeds: internal_IP_address of each seed node
        Important: Include at least one seed node from each datacenter. DataStax recommends more than one seed node per datacenter, in more than one rack. Do not make all nodes seed nodes.
      • auto_bootstrap: true

        This setting has been removed from the default configuration, but, if present, should be set to true.

      • listen_address: empty

        If not set, Cassandra asks the system for the local address, which is associated with its host name. In some cases, Cassandra does not produce the correct address, which requires specifying the listen_address.

      • endpoint_snitch: snitch

        See endpoint_snitch and snitches.

        Important: Do not use the SimpleSnitch (default). The SimpleSnitch (default) is used only for single-datacenter deployments (or single-zone deployments in public clouds), and does not recognize datacenter or rack information.

        Snitch	Configuration file
        GossipingPropertyFileSnitch	cassandra-rackdc.properties file
        Amazon EC2 single-region snitch
        Amazon EC2 multi-region snitch
        Google Cloud Platform snitch
        PropertyFileSnitch	cassandra-topology.properties file

      • If using a cassandra.yaml file from a previous version, check the Upgrade Guide for removed settings.
   2. Configure node architecture (all nodes in the datacenter must use the same type):

      Virtual node (vnode) allocation algorithm settings

      • Set num_tokens to 8 (recommended).
      • Set allocate_tokens_for_local_replication_factor to the target replication factor for keyspaces in the new datacenter. If the keyspace RF varies, alternate the settings to use all the replication factors.
      • Comment out the initial_token property.

      For more information, refer to Virtual node (vnode) configuration.

      Single-token architecture settings

      • Generate the initial token for each node and set this value for the initial_token property.

        See Adding or replacing single-token nodes for more information.

      • Comment out both num_tokens and allocate_tokens_for_local_replication_factor.
5. In the cassandra-rackdc.properties (GossipingPropertyFileSnitch) or cassandra-topology.properties (PropertyFileSnitch) file, assign datacenter and rack names to the IP addresses of each node, and assign a default datacenter name and rack name for unknown nodes.
   Note: Migration information: The GossipingPropertyFileSnitch always loads cassandra-topology.properties when the file is present. Remove the file from each node on any new cluster, or any cluster migrated from the PropertyFileSnitch.

   # Transactional Node IP=Datacenter:Rack
   110.82.155.0=DC_Transactional:RAC1
   110.82.155.1=DC_Transactional:RAC1
   110.54.125.1=DC_Transactional:RAC2
   
   # default for unknown nodes
   default=DC1:RAC1

   Note: After making any changes in the configuration files, you must the restart the node for the changes to take effect.
6. Make the following changes in the existing datacenters.
   1. On nodes in the existing datacenters, update the -seeds property in cassandra.yaml to include the seed nodes in the new datacenter.
   2. Add the new datacenter definition to the cassandra.yaml properties file for the type of snitch used in the cluster. If changing snitches, see Switching snitches.
7. After you have installed and configured Cassandra on all nodes, start the seed nodes one at a time, and then start the rest of the nodes:
8. Rotate starting Cassandra through the racks until all the nodes are up.
9. After all nodes are running in the cluster and the client applications are datacenter aware, use cqlsh to alter the keyspaces to add the desired replication in the new datacenter.

   ALTER KEYSPACE keyspace_name WITH REPLICATION = 
   {'class' : 'NetworkTopologyStrategy', 'ExistingDC1' : 3, 'NewDC2' : 2};

   Warning: If client applications, are not properly configured, they might connect to the new datacenter before it is online. Incorrect configuration results in connection exceptions, timeouts, and/or inconsistent data.
10. Run nodetool rebuild on each node in the new datacenter, specifying the datacenter to rebuild from. This step replicates the data to the new datacenter in the cluster.

    nodetool rebuild -- datacenter_name

    CAUTION: You must specify an existing datacenter in the command line, or the new nodes will appear to rebuild successfully, but might not contain all anticipated data.

    Requests to the new datacenter with LOCAL_ONE or ONE consistency levels can fail if the existing datacenters are not completely in-sync.

    1. Use nodetool rebuild on one or more nodes at the same time. Run on one node at a time to reduce the impact on the existing cluster.
    2. Alternatively, run the command on multiple nodes simultaneously when the cluster can handle the extra I/O and network pressure.
11. Check that your cluster is up and running:

    installation_location/bin/nodetool status

    Note: If the cluster has problems starting, look for starting DSE troubleshooting and other articles in the Support Knowledge Center.

DC: datacenter1
==============================================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Tokens    Owns    Host ID             Rack
UN 110.82.155.0    21.33 KB    256       33.3%   a9fa31c7-f3c0-...   RAC1
UN 110.82.155.1    21.33 KB    256       33.3%   f5bb416c-db51-...   RAC1
UN 110.54.125.1    21.33 KB    256       16.7%   b836748f-c94f-...   RAC1
datacenter2
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Owns      Host ID               Tokens         Rack
UN 110.54.125.2    28.44 KB    13.0.%    e2451cdf-f070- ...    -922337....    RAC1
UN 110.82.155.2    44.47 KB    16.7%     f9fa427c-a2c5- ...    30745512...    RAC1 
UN 110.82.155.3    54.33 KB    23.6%     b9fc31c7-3bc0- ..-    45674488...    RAC1

datacenter2
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Owns      Host ID               Tokens         Rack
UN 110.54.125.3    15.44 KB    50.2.%    e2451cdf-f070- ...    9243578....    RAC1
UN 110.82.155.4    18.78 KB    49.8.%    e2451cdf-f070- ...    10000          RAC1