title: What is the relationship between Spark, Hadoop and Cassandra
body: <p>My understanding was that Spark is an alternative to Hadoop. However, when trying to install Spark, the installation page asks for an existing Hadoop installation. I'm not able to find anything that clarifies that relationship.</p>
  
  <p>Secondly, Spark apparently has good connectivity to Cassandra and Hive. Both have sql style interface. However, Spark has its own sql. Why would one use Cassandra/Hive instead of Spark's native sql? Assuming that this is a brand new project with no existing installation?</p>