Interview Question: What is the difference between Cassandra, Hadoop Big Data, MongoDB, CouchDB?
- Is an entire ecosystem of integrated distributed computing tools, at the core of which are a file system (HDFS) and a programming framework (Map-Reduce).
- Is a NoSQL data store based on a key-value pairing system, where value is then further structured into a columnar like store.
- A NoSQL data store based on key-value pairing system where value is JSON documents. Has its own unique querying language.
Cassandra, Mongo, and Couch are pretty similar in that they are key-value based NoSQL data stores. They each have their advantages and disadvantages. If you're interested in one, you should probably at least have a good understanding of when to use one NoSQL vs another.
Hadoop is a much bigger in scope to learn than the others because it is comprised of many different components, including its own columnar (hBase) and SQL like data storage (Hive) platforms.
As a side note, I wouldn't pay a dime to "learn" these technologies. They're all open source and there's no shortage of examples or documentation available for all of them.