DataStax Enterprise 3.1 Documentation

Configuring replication

This documentation corresponds to an earlier product version. Make sure this document corresponds to your version.

Latest DSE documentation | Earlier DSE documentation

Cassandra performs replication to store multiple copies of data on multiple nodes for reliability and fault tolerance. To configure replication, you need to choose a data partitioner and replica placement strategy. Data partitioning determines how data is placed across the nodes in the cluster. For information about how this works, see Data distribution and replication. Nodes communicate with each other about replication and other things using the gossip protocol. Be sure to configure gossip, as described in About internode communications (gossip).

Virtual nodes

DataStax Enterprise 3.1 and above can use virtual nodes. Virtual nodes simplify many tasks in Cassandra, such as eliminating the need to determine the partition range (calculate and assign tokens), rebalancing the cluster when adding or removing nodes, and replacing dead nodes. For a complete description of virtual nodes and how they work, see About virtual nodes, and the Virtual nodes in Cassandra 1.2 blog.

Using virtual nodes

In the cassandra.yaml, file uncomment num_tokens and leave the initial_token parameter unset. Guidelines for using virtual nodes include:

  • Determining the num_tokens value: The initial recommended value for num_tokens is 256. For more guidance, see Setting up virtual nodes.
  • Mixed architecture: Cassandra supports using virtual node-enabled and non-virtual node data centers. For example, a single cluster could have a vanilla cassandra data center with vnodes enabled and a search data center without vnodes.
  • Migrating: To upgrade existing clusters to virtual nodes, see Enabling virtual nodes on an existing production cluster.


Currently, DataStax recommends using virtual nodes only on data centers running purely Cassandra workloads. You should disable virtual nodes on data centers running either Hadoop or Solr workloads.

Using the single-token-per-node architecture in DSE 3.1 and above

If you don't use virtual nodes, you must make sure that each node is responsible for roughly an equal amount of data. To do this, assign each node an initial-token value and calculate the tokens for each data center as described in Generating tokens located in the Datastax Enterprise 3.0 documentation. You can also use the Murmur3Partitioner and calculate the tokens as described in Cassandra 1.2 Generating tokens.

Partitioner settings

You can use either the Murmur3Partitioner or RandomPartitioner with virtual nodes.

The Murmur3Partitioner (org.apache.cassandra.dht.Murmur3Partitioner) is the default partitioning strategy for new Cassandra clusters (1.2 and above) and the right choice for new clusters in almost all cases. You can only use Murmur3Partitioner for new clusters; you cannot change the partitioner in existing clusters. If you are switching to the 1.2 cassandra.yaml, be sure to change the partitioner setting to match the previous partitioner.

The RandomPartitioner (org.apache.cassandra.dht.RandomPartitioner) was the default partitioner prior to Cassandra 1.2. You can continue to use this partitioner when migrating to virtual nodes.

Snitch settings

A snitch determines which data centers and racks are written to and read from. It informs Cassandra about the network topology so that requests are routed efficiently and allows Cassandra to distribute replicas by grouping machines into data centers and racks. All nodes must have exactly the same snitch configuration.

The following sections describe three commonly-used snitches. All available snitches are described in the Apache Cassandra documentation. The default endpoint_snitch is the DseDelegateSnitch. The default snitch delegated by this snitch is the DseSimpleSnitch (org.apache.cassandra.locator.DseSimpleSnitch). You set the snitch used by the DseDelegateSnitch in the dse.yaml file:

  • Packaged installations: /etc/dse/dse.yaml
  • Tarball installations: <install_location>/resources/dse/conf/dse.yaml


DseSimpleSnitch is used only for DataStax Enterprise (DSE) deployments. This snitch logically configures each type of node in separate data centers to segregate the analytics, real-time, and search workloads. You can use the DseSimpleSnitch for mixed-workload DSE clusters located in one physical data center or for multiple physical data centers. When using multiple data centers, place each type of node (Cassandra, Hadoop, and Solr) in a separate physical data center.

When defining your keyspace, use Analytics, Cassandra, or Solr for your data center names.


For a single data center (or single node) cluster, the SimpleSnitch is usually sufficient. However, if you plan to expand your cluster at a later time to multiple racks and data centers, it is easier if you use a rack and data center aware snitch from the start, such as the RackInferringSnitch.


The PropertyFileSnitch allows you to define your data center and rack names to be whatever you want. Using this snitch requires that you define network details for each node in the cluster in the configuration file.

  • Packaged installations: /etc/dse/cassandra/
  • Tarball installations: <install_location>/resources/cassandra/conf/

Every node in the cluster should be described in this file, and specified exactly the same on every node in the cluster.

For example, suppose you had non-uniform IPs and two physical data centers with two racks in each, and a third logical data center for replicating analytics data, you would specify them as follows:

# Data Center One

# Data Center Two

# Analytics Replication Group

# default for unknown nodes

Make sure the data center names defined in the /etc/dse/cassandra/ file correlates to what you name your data centers in your keyspace definitions.

Choosing keyspace replication options

When you create a keyspace, you must define the replica placement strategy class and the number of replicas you want. DataStax recommends choosing NetworkTopologyStrategy for single and multiple data center clusters. This strategy is as easy to use as the SimpleStrategy and allows for expansion to multiple data centers in the future. It is much easier to configure the most flexible replication strategy up front, than to reconfigure replication after you have already loaded data into your cluster.

NetworkTopologyStrategy takes as options the number of replicas you want per data center. Even for single data center (or single node) clusters, you can use this replica placement strategy and just define the number of replicas for one data center. For example:

  WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'us-east' : 6};

Or for a multiple data center cluster:

  WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'dc1' : 3, 'dc2' : 3};

When creating the keyspace, what you name your data centers depends on the snitch you have chosen for your cluster. The data center names must correlate to the snitch you are using in order for replicas to be placed in the correct location.

As a general rule, the number of replicas should not exceed the number of nodes in a replication group. However, it is possible to increase the number of replicas, and then add the desired number of nodes afterwards. When the replication factor exceeds the number of nodes, writes will be rejected, but reads will still be served as long as the desired consistency level can be met.

In DataStax Enterprise 3.0.1 and later, the default consistency level has changed from ONE to QUORUM for reads and writes to resolve a problem finding a CassandraFS block when using consistency level ONE on a Hadoop node.

Changing replication settings

The default replication of 1 for keyspaces is suitable only for development and testing of a single node. For production environments, it is important to change the replication of keyspaces from 1 to a higher number. To avoid operations problems, changing the replication of these system keyspaces is especially important:

  • HiveMetaStore, cfs, and cfs_archive keyspaces

    If the node is an Analytics node that uses Hive, increase the HiveMetaStore and cfs keyspace replication factors to 2 or higher to be resilient to single-node failures. If you use cfs_archive, increase it accordingly.

  • dse_system keyspace

    On an Analytics/Hadoop node, this keyspace contains information about the location of the job tracker. If only a single node contains the job tracker replica, other nodes cannot find the job tracker when that node is unavailable for some reason.

To change the replication these keyspaces

  1. Check the name of the data center of the node.

    • Packaged installs: nodetool status
    • Binary installs: <install_location>/bin/nodetool status

    The output tells you the name of the data center for the node, for example, datacenter1.

  2. Change the replication of the cfs and cfs_archive keyspaces from 1 to 3, for example:

      WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'dc1' : 3};
    ALTER KEYSPACE cfs_archive
      WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'dc1' : 3};

    How high you increase the replication depends on the number of nodes in the cluster, as discussed in the previous section.

  3. If you use Hive, update the HiveMetaStore keyspace to increase the replication from 1 to 3, for example.

  4. Alter the dse_system keyspace to increase the replication from 1 to 3, for example.

  5. If the keyspaces you changed contain any data, run nodetool repair to avoid having missing data problems or data unavailable exceptions.