The production version of Hadoop and Cassandra combination needs to go into a separate cluster. The first obvious issue is you probably wouldn't want Hadoop to keep pulling Cassandra nodes, hampering Cassandra performance to end users. The general pattern to avoid this is to split the ring into two data centers. Since Cassandra automatically and immediately replicates the changes between data centers, they will be in sync always. What's more, you can assign one of the data centers as a transactional with a higher replication factor and the other as an analytical data center with a replication factor of 1. The analytical data center is the one used by Hadoop without affecting the transactional data center a bit.

Now, you do not really need to have two physically separated data centers to make this configuration work. Remember NetworkTopologyStrategy? (Refer to the NetworkTopologyStrategy section in Chapter 4, Deploying a Cluster.) You can tweak Cassandra thinking there are two data centers by just assigning the nodes that you wanted to use for analytics in a different data center. You may need to use PropertyFileSnitch and specify the details about data centers in a cassandra-toplogy.properties file. So, your keyspace creation looks something like this:

createkeyspacemyKeyspace
withplacement_strategy = 'NetworkTopologyStrategy'
andstrategy_options = {TX_DC : 2, HDP_DC: 1};

The previous statement defines two data centers: TX_DC for transactional purposes and HDP_DC for Hadoop to do analytics. A node in a transactional data center has a snitch configured like this:

# Transaction Data Center
192.168.1.1=TX_DC:RAC1
192.168.1.2=TX_DC:RAC1

192.168.2.1=TX_DC:RAC2

# Analytics Data Center
192.168.1.3=HDP_DC:RAC1

192.168.2.2=HDP_DC:RAC2
192.168.2.3=HDP_DC:RAC2

# For new/unknown/unassigned nodes
default=TX_DC:RAC1

We are mostly done setting up machines. A couple of things to remember: