When to use Cassandra

Some of the silent features of Cassandra are :

• Written in: Java
• Main point: Best of BigTable and Dynamo
• License: Apache 
• A highly scalable second-generation distributed database. 
• Protocol: Custom, binary (Thrift)
• Tunable trade-offs for distribution and replication (N, R, W)
• Querying by column, range of keys
• BigTable-like features: columns, column families
• Has secondary indices
• Writes are much faster than reads (!)
• Map/reduce possible with Apache Hadoop
• All nodes are similar, as opposed to Hadoop/HBase

Best used: 
 When you write more than you read (logging). If every component of the system must be in Java. ("No one gets fired for choosing Apache's stuff.") 
Cassandra's storage engine provides constant-time writes no matter how big your data set grows. If we are using MongoDB then writes are more problematic in MongoDB, partly because of the b-tree based storage engine.
Use Case:
  Banking, financial industry (though not necessarily for financial transactions, but these industries are much bigger than that.) Writes are faster than reads, so one natural niche is real time data analysis.