Ultimate Answer for difference between Storage Node and Compute Node

Compute Node: This is the computer or machine where your actual business logic will be executed.

Storage Node: This is the computer or machine where your file system reside to store the processing data. In most of the cases compute node and storage node would be the same machine

What are the restrictions for Key and Value Class:

The key and value classes have to be serialized by the framework. To make them serializable Hadoop provides a Writable interface. As you know from the java itself that the key of the Map should be comparable, hence the key has to implement one more interface WritableComparable.