Initializing a single datacenter per workload type

In this scenario, a mixed workload cluster has only one datacenter for each type of workload. For example, an eight-node cluster with the following nodes would use three datacenters, one for each workload type:

  • DC1 = 3 DSE Analytics nodes

  • DC2 = 3 Transactional nodes

  • DC3 = 2 DSE Search nodes

In contrast, a multiple datacenter cluster has more than one datacenter for each type of workload.

The eight-node cluster spans two racks across three datacenters. Applications in each datacenter use a default consistency level of LOCAL_QUORUM. One node per rack serves as a seed node.

Node, IP address, type, seed, and rack
Node IP address Type Seed Rack

node0

110.82.155.0

Transactional

✓

RAC1

node1

110.82.155.1

Transactional

RAC1

node2

110.54.125.1

Transactional

RAC2

node3

110.54.125.2

Analytics

RAC1

node4

110.54.155.2

Analytics

✓

RAC2

node5

110.82.155.3

Analytics

RAC1

node6

110.54.125.3

Search

RAC1

node7

110.82.155.4

Search

RAC2
Where is the cassandra.yaml file?

The location of the cassandra.yaml file depends on the type of installation:

Installation Type Location

Package installations + Installer-Services installations

/etc/dse/cassandra/cassandra.yaml

Tarball installations + Installer-No Services installations

<installation_location>/resources/cassandra/conf/cassandra.yaml
Where is the cassandra-topology.properties file?

The location of the cassandra-topology.properties file depends on the type of installation:

Installation Type Location

Package installations + Installer-Services installations

/etc/dse/cassandra/cassandra-topology.properties

Tarball installations + Installer-No Services installations

<installation_location>/resources/cassandra/conf/cassandra-topology.properties
Where is the dse.yaml file?

The location of the dse.yaml file depends on the type of installation:

Installation Type Location

Package installations + Installer-Services installations

/etc/dse/dse.yaml

Tarball installations + Installer-No Services installations

<installation_location>/resources/dse/conf/dse.yaml
Where is the cassandra-rackdc.properties file?

The location of the cassandra-rackdc.properties depends on the type of installation:

Installation Type Location

Package installations + Installer-Services installations

/etc/dse/cassandra/cassandra-rackdc.properties

Tarball installations + Installer-No Services

<installation_location>/resources/cassandra/conf/cassandra-rackdc.properties

Prerequisites

To prepare the environment, complete the prerequisite tasks outlined in Initializing a DataStax Enterprise cluster.

If the new datacenter uses existing nodes from another datacenter or cluster, complete the following steps to ensure that old data does 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 added to the new datacenter.

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

  4. Install DSE on each node.

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

      ALTER KEYSPACE keyspace_name WITH REPLICATION =
{'class' : 'NetworkTopologyStrategy', 'ExistingDC1' : 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. In the new datacenter, install DSE on each new node. 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.

    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

      DataStax recommends not using vnodes with DSE Search. However, if you decide to use vnodes with DSE Search, do not use more than 8 vnodes and ensure that allocate_tokens_for_local_replication_factor option in cassandra.yaml is correctly configured for your environment.

      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.

  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 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
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 DSE on all nodes, start the seed nodes one at a time, and then start the rest of the nodes:

  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 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:

    dsetool status

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

  12. Complete step 3 through step 11 to add the third datacenter (DC3) 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       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-...   RAC2

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    13.0.%    e2451cdf-f070- ...    -922337....    RAC1
UN 110.82.155.2    44.47 KB    16.7%     f9fa427c-a2c5- ...    30745512...    RAC2
UN 110.82.155.3    54.33 KB    23.6%     b9fc31c7-3bc0- ..-    45674488...    RAC1

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

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