Initializing multiple datacenters per workload type

In this scenario, a mixed workload cluster has more than one datacenter for each type of workload. For example, the following ten-node cluster is spans five datacenters, whereas a single datacenter cluster has only one datacenter for each node type.

  • DC1 = 2 DSE Analytics nodes

  • DC2 = 2 Transactional nodes

  • DC3 = 2 DSE Search nodes

  • DC4 = 2 DSE Analytics nodes

  • DC5 = 2 Transactional nodes

The ten-node cluster spans two racks across five datacenters. Applications in each datacenter will use a default consistency level of LOCAL_QUORUM. One node per rack will serve as a seed node.

Node IP address Type Seed

node0

110.82.155.0

Transactional

✓

RAC1

node1

110.82.155.1

Transactional

RAC1

node2

110.54.125.1

Transactional

RAC2

node3

110.55.120.1

Transactional

RAC1

node4

110.54.125.2

Analytics

RAC1

node5

110.54.155.2

Analytics

✓

RAC2

node6

110.82.155.3

Analytics

RAC1

node7

110.55.120.2

Analytics

RAC1

node8

110.54.125.3

Search

RAC1

node9

110.82.155.4

Search

RAC2

Prerequisites

Complete the prerequisite tasks outlined in Initializing a DataStax Enterprise cluster to prepare the environment.

If the new datacenter uses existing nodes from another datacenter or cluster, complete the following steps to ensure that old data will not interfere with the new cluster:

  1. If the nodes are behind a firewall, open the required ports for internal/external communication.

  2. Decommission each node that will be added to the new datacenter.

  3. Clear the data from DataStax Enterprise (DSE) to completely remove application directories.

  4. Install DSE on each node. Do not start the service.

Procedure

  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:

    If client applications, including DSE Search and DSE Analytics, 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. In existing datacenters, if the SimpleStrategy replication strategy is in use, change it to the NetworkTopologyStrategy replication strategy.

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

      ALTER KEYSPACE keyspace_name WITH REPLICATION =
{'class' : 'NetworkTopologyStrategy', 'DC1' : 3};

    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 ;
      CREATE KEYSPACE dse_perf WITH replication =
{'class': 'NetworkTopologyStrategy, 'DC1': '3'}  AND durable_writes = true;
...

CREATE KEYSPACE dse_leases WITH replication =
{'class': 'NetworkTopologyStrategy, 'DC1': '3'}  AND durable_writes = true;
...

CREATE KEYSPACE dsefs WITH replication =
{'class': 'NetworkTopologyStrategy, 'DC1': '3'}  AND durable_writes = true;
...

CREATE KEYSPACE dse_security WITH replication =
{'class': 'NetworkTopologyStrategy, 'DC1': '3'}  AND durable_writes = true;

  3. Install DSE on each node in the new datacenter, install DSE. Do not start the service or restart the node.

    Use the same version of DSE 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.

    If you used Lifecycle Manager to provision the nodes, configuration is performed automatically.

    Use the yaml_diff tool to review and make appropriate changes to the cassandra.yaml and dse.yaml configuration files.

    1. Configure node properties:

    2. Configure node architecture (all nodes in the datacenter must use the same type):

      Virtual node (vnode) allocation algorithm settings

      See Virtual node (vnode) configuration for more details.

      Single-token architecture settings

  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.

    Migration information: The GossipingPropertyFileSnitch always loads cassandra-topology.properties when the file is present. Remove the file from each node on any new datacenter, or any datacenter 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
110.54.125.2=DC_Analytics:RAC1
110.54.155.2=DC_Analytics:RAC2
110.82.155.3=DC_Analytics:RAC1
110.54.125.3=DC_Search:RAC1
110.82.155.4=DC_Search:RAC2

# default for unknown nodes
default=DC1:RAC1

    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 DataStax Enterprise on all nodes, start the nodes sequentially, beginning with the seed nodes. After starting each node, allow a delay of at least the value specified in ring_delay_ms before starting the next node, to prevent a cluster imbalance.

    Before starting a node, ensure that the previous node is up and running by verifying that it has a nodetool status of UN. Failing to do so will result in cluster imbalance that cannot be fixed later. Cluster imbalance can be visualised by running nodetool status $keyspace and by looking at the ownership column. A properly setup cluster will report ownership values similar to each other (±1%). That is, for keyspaces where the RF per DC is equal to allocate_tokens_for_local_replication_factor.

    See allocate_tokens_for_local_replication_factor for more infomation:

  8. Rotate starting DSE 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};

    If client applications, including DSE Search and DSE Analytics, 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>

    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 the new cluster is up and running:

    dsetool status

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

  12. Complete 3 through 11 to add the remaining datacenters to the cluster.

Results

The datacenters in the cluster are now replicating with each other.

DC: Cassandra   Workload: Cassandra  Graph: no
==============================================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Tokens    Owns    Host ID             Rack
UN 110.82.155.0    21.33 KB    256       50.2%   a9fa31c7-f3c0-...   RAC1
UN 110.82.155.1    21.33 KB    256       49.8%   f5bb416c-db51-...   RAC1

DC: Analytics
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Owns      Host ID               Tokens         Rack
UN 110.54.125.2    28.44 KB    50.2.%    e2451cdf-f070- ...    -922337....    RAC1
UN 110.82.155.2    44.47 KB    49.8%     f9fa427c-a2c5- ...    30745512...    RAC2

DC: Solr
=======================
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          RAC2

DC: Cassandra2   Workload: Cassandra  Graph: no
==============================================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Tokens    Owns    Host ID             Rack
UN 110.54.125.1    21.33 KB    256       16.7%   b836748f-c94f-...   RAC2
UN 110.55.120.1    21.33 KB    256       16.7%   b354798g-c94f-...   RAC2

DC: Analytics2
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address         Load        Owns      Host ID               Tokens         Rack
UN 110.82.155.3    54.33 KB    50.2%     b9fc31c7-3bc0- ..-    45674488...    RAC1
UN 110.55.120.2    54.33 KB    49.8%     b8gd45e4-3bc0- ..-    45674488...    RAC2

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