Multiple data center deployment per workload type

A deployment scenario with a mixed workload cluster has more than one data center for each type of node.

In this scenario, a mixed workload cluster has more than one data center for each type of node. For example, if the cluster has 4 Hadoop nodes, 4 Cassandra nodes, and 2 Solr nodes, the cluster could have 5 data centers: 2 data centers for Hadoop nodes, 2 data centers for Cassandra nodes, and 1 data center for the Solr nodes. A single data-center cluster has only one data center for each type of node.

In Cassandra, a data center can be a physical data center or virtual data center. Different workloads should use separate data centers, either physical or virtual. Using separate data centers prevents Cassandra transactions from being impacted by other workloads and keeps requests close to each other for lower latency. Replication is set by data center amd depending on the replication factor, data can be written to multiple data centers. However, data centers should never span physical locations. Uses for multiple data center deployments include:
  • Isolating replicas from external infrastructure failures, such as networking between data centers and power outages.
  • Distributing data replication across multiple, geographically dispersed nodes.
  • Between different locations in a physical data center.
  • Between public cloud providers and on-premise managed data centers.
  • Use separate physical or virtual data centers to prevent the slow down of a real-time analytics jobs by other analytics jobs on live data.
For more information about replication:

Prerequisites

To configure a multi-node cluster with multiple data centers:

  • A good understanding of how Cassandra works. Be sure to read at least Understanding the architecture, Data Replication, and Cassandra's rack feature.
  • DataStax Enterprise is installed on each node.
  • Choose a name for the cluster.
  • For a mixed-workload cluster, determine the purpose of each node.
  • Determine the snitch and replication strategy. The GossipingPropertyFileSnitch and NetworkTopologyStrategy are recommended for production environments.
  • Get the IP address of each node.
  • Determine which nodes are seed nodes. Do not make all nodes seed nodes. Please read Internode communications (gossip).
  • Develop a naming convention for each data center and rack, for example: DC1, DC2 or 100, 200 and RAC1, RAC2 or R101, R102.
  • Other possible configuration settings are described in the cassandra.yaml configuration file and property files such as cassandra-rackdc.properties.
  • Set virtual nodes correctly for the type of data center. DataStax recommends using virtual nodes only on data centers running Cassandra real-time workloads. See Virtual nodes.

Procedure

This configuration example describes installing a 6 node cluster spanning 2 data centers. The default consistency level is QUORUM.

  1. Suppose you install DataStax Enterprise on these nodes:
    • node0 10.168.66.41 (seed1)
    • node1 10.176.43.66
    • node2 10.168.247.41
    • node3 10.176.170.59 (seed2)
    • node4 10.169.61.170
    • node5 10.169.30.138
  2. If the nodes are behind a firewall, open the required ports for internal/external communication. See Configuring firewall port access.
  3. If DataStax Enterprise is running, stop the nodes and clear the data:
    • Installer-Services and Package installations: 
      $ sudo service dse stop
$ sudo rm -rf /var/lib/cassandra/* ## Clears the data from the  default directories
    • Installer-No Services and Tarball installations:

      From the install directory:

      $ sudo bin/dse cassandra-stop
$ sudo rm -rf /var/lib/cassandra/* ## Clears the data from the default directories
      Note: If you are clearing data from an AMI installation for restart, you need to preserve the log files.
  4. Set the properties in the cassandra.yaml file for each node, located in:
    • Installer-Services and Package installations: /etc/dse/cassandra/cassandra.yaml
    • Installer-No Services and Tarball installations: install_location/resources/cassandra/conf/cassandra.yaml
    Important: After making any changes in the cassandra.yaml file, you must restart the node for the changes to take effect.

    Properties to set:

    Note: If the nodes in the cluster are identical in terms of disk layout, shared libraries, and so on, you can use the same copy of the cassandra.yaml file on all of them.
    • num_tokens: 256 for Cassandra nodes
    • num_tokens: 1 for Hadoop and Solr nodes
    • -seeds: internal_IP_address of each seed node
    • listen_address: empty

      If not set, Cassandra asks the system for the local address, the one associated with its host name. In some cases Cassandra doesn't produce the correct address and you must specify the listen_address.

    • auto_bootstrap: false

      Add the bootstrap setting only when initializing a fresh cluster with no data.

    • endpoint_snitch: snitch

      For more information, see endpoint_snitch and About Snitches.

    • If you are using a cassandra.yaml from a previous version, remove the following options, as they are no longer supported by DataStax Enterprise:
      ## Replication strategy to use for the auth keyspace.
auth_replication_strategy: org.apache.cassandra.locator.SimpleStrategy

auth_replication_options:
    replication_factor: 1

    Example:

    You must include at least one seed node from each data center. It is a best practice to have more than one seed node per data center.

    cluster_name: 'MyDemoCluster'
num_tokens: 256
seed_provider:
  - class_name: org.apache.cassandra.locator.SimpleSeedProvider
    parameters:
         - seeds: "10.168.66.41,10.176.170.59"
listen_address:
  5. In the cassandra-topology.properties or cassandra-rackdc.properties file, use your naming convention to assign data center and rack names to the IP addresses of each node, and assign a default data center name and rack name for unknown nodes.
    • Installer-Services and Package installations: /etc/dse/cassandra
    • Installer-No Services and Tarball installations: install_location/resources/cassandra/conf

    Example:

    # Cassandra Node IP=Data Center:Rack
10.168.66.41=DC1:RAC1
10.176.43.66=DC2:RAC1
10.168.247.41=DC1:RAC1
10.176.170.59=DC2:RAC1
10.169.61.170=DC1:RAC1
10.169.30.138=DC2:RAC1

# default for unknown nodes
default=DC1:RAC1
  6. After you have installed and configured DataStax Enterprise on all nodes, start the seed nodes one at a time, and then start the rest of the nodes:
    Note: If the node has restarted because of automatic restart, you must stop the node and clear the data directories, as described above.
  7. Check that your cluster is up and running:
    • Installer-Services and Package installations: $ nodetool status
    • Installer-No Services and Tarball installations: $ install_location/bin/nodetool status

Results

Datacenter: DC1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address          Load        Tokens    Owns    Host ID             Rack
UN 10.168.66.41     45.96 KB    256       27.4%   c885aac7-f2c0-...   RAC1
UN 10.168.247.41    66.34 KB    256       36.6%   fa31416c-db22-...   RAC1
UN 10.169.61.170    55.72 KB    256       33.0%   f488367f-c14f-...   RAC1
Datacenter: DC2
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address          Load        Tokens    Owns    Host ID             Rack
UN 10.176.43.66     45.96 KB    256       27.4%   f9fa31c7-f3c0-...   RAC1
UN 10.176.170.59    66.34 KB    256       36.6%   a5bb526c-db51-...   RAC1
UN 10.169.30.138    55.72 KB    256       33.0%   b836478f-c49f-...   RAC1

What's next