Mixing workloads in a cluster

Creating a virtual datacenter 

When you create a keyspace using CQL, Cassandra creates a virtual datacenter for a cluster, even a one-node cluster, automatically. You assign nodes that run the same type of workload to the same datacenter. The separate, virtual datacenters for different types of nodes segregate workloads that run DSE Search from those nodes that run other workload types. Segregating workloads ensures that only one type of workload is active per datacenter.

Workload segregation 

In a cluster that has BYOH and DSE Hadoop nodes, the DSE Hadoop nodes would have priority with regard to start up. Start up seed nodes in the BYOH datacenter after starting up DSE Hadoop datacenters.

Occasionally, there is a use case for keeping DSE Hadoop and Cassandra nodes in the same datacenter. You do not have to have one or more additional replication factors when these nodes are in the same datacenter.

To deploy a mixed workload cluster, see "Multiple datacenter deployment."

This diagram shows DSE Hadoop analytics, Cassandra, and DSE Search nodes in separate datacenters. In separate datacenters, some DSE nodes handle search while others handle MapReduce, or just act as real-time Cassandra nodes. Cassandra ingests the data, Solr indexes the data, and you run MapReduce against that data in one cluster without performing manual extract, transform, and load (ETL) operations. Cassandra handles the replication and isolation of resources. The DSE Search nodes run HTTP and hold the indexes for the Cassandra table data. If a DSE Search node goes down, the commit log replays the Cassandra inserts, which correspond to Solr inserts, and the node is restored automatically.

Restrictions 

• Do not create the keyspace using SimpleStrategy for production use or for use with mixed workloads.
• Within the same datacenter, do not run Solr workloads on some nodes and other types of workloads on other nodes.
• Do not run DSE Search and DSE Hadoop on the same node in either production or development environments.
• Do not run some nodes in DSE Hadoop mode and some in Spark mode in the same datacenter.

  You can run all the nodes in Spark mode, all the nodes in Hadoop mode or all the nodes in Spark/DSE Hadoop mode.

Recommendations

Run the CQL or Thrift inserts on a DSE Search node in its own datacenter.

NetworkTopologyStrategy is highly recommended for most deployments because it is much easier to expand to multiple datacenters when required by future expansion.

Getting cluster workload information 

SELECT workload FROM system.local;

The output looks something like this:

 workload 
----------
 Analytics

DESCRIBE FULL schema

CREATE TABLE peers (
  peer inet,
  data_center text,
  host_id uuid,
  preferred_ip inet,
  rack text,
  release_version text,
  rpc_address inet,
  schema_version uuid,
  tokens settext,
  workload text,
  PRIMARY KEY ((peer))
) WITH
  . . .;

Replicating data across datacenters 

You set up replication by creating a keyspace. You can change the replication of a keyspace after creating it.