I am new to the distributed NoSQL databases like Hadoop, Cassandra, etc. I have few questions for which I seek an expert advice: Can you list problems/challenges one will generally face when making a shift from the present conventional database like MySQL to these large cluster-based databases? What are the difficulties, if any, when one needs to adapt to a newer version of these open source projects? Can you list out the things which are generally stored/kept in memcached for fast rendering of the page? How can I understand the source code of open-source projects so that I can build on it and maybe give back to the community? Above questions may sound to be idiotic and basic but please it's a request for the experts to answer the above questions in detailed and to best of their abilities.