Hadoop support

Cassandra support for integrating Hadoop with Cassandra.

Cassandra support for integrating Hadoop with Cassandra includes:
  • MapReduce
  • Apache Pig
You can use Cassandra 2.1 with Hadoop 2.x or 1.x with some restrictions.
  • Isolate Cassandra and Hadoop nodes in separate datacenters.
  • Before starting the datacenters of Cassandra/Hadoop nodes, disable virtual nodes (vnodes).
To disable virtual nodes:
  1. In the cassandra.yaml file, set num_tokens to 1.
  2. Uncomment the initial_token property and set it to 1 or to the value of a generated token for a multi-node cluster.
  3. Start the cluster for the first time.

    Do not disable or enable vnodes on an existing cluster.

Setup and configuration, described in the Apache docs, involves overlaying a Hadoop cluster on Cassandra nodes, configuring a separate server for the Hadoop NameNode/JobTracker, and installing a Hadoop TaskTracker and Data Node on each Cassandra node. The nodes in the Cassandra datacenter can draw from data in the HDFS Data Node as well as from Cassandra. The Job Tracker/Resource Manager (JT/RM) receives MapReduce input from the client application. The JT/RM sends a MapReduce job request to the Task Trackers/Node Managers (TT/NM) and optional clients, MapReduce and Pig. The data is written to Cassandra and results sent back to the client.

The Apache docs also cover how to get configuration and integration support.

Input and Output Formats

Hadoop jobs can receive data from CQL tables and indexes and you can load data into Cassandra from a Hadoop job. Cassandra 2.1 supports the following formats for these tasks:

  • CQL partition input format: ColumnFamilyInputFormat class
  • BulkOutputFormat class

Cassandra 2.1.1 and later supports the CqlOutputFormat, which is the CQL-compatible version of the BulkOutputFormat class. The CQLOutputFormat acts as a Hadoop-specific OutputFormat. Reduce tasks can store keys (and corresponding bound variable values) as CQL rows (and respective columns) in a given CQL table.

Cassandra 2.1.1 supports using the CQLOutputFormat with Apache Pig.

Running the wordcount example

Wordcount example JARs are located in the examples directory of the Cassandra source code installation. There are CQL and legacy examples in the hadoop_cql3_word_count and hadoop_word_count subdirectories, respectively. Follow instructions in the readme files.

Isolating Hadoop and Cassandra workloads

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 running Hadoop from those running Cassandra. Segregating workloads ensures that only one type of workload is active per datacenter. Separating nodes running a sequential data load, from nodes running any other type of workload, such as Cassandra real-time OLTP queries is a best practice.

The location of the cassandra.yaml file depends on the type of installation:
Package installations /etc/cassandra/cassandra.yaml
Tarball installations install_location/resources/cassandra/conf/cassandra.yaml