This paper reveal the secret of NoSQL. The CAP theorem, the BASE theorem and the Eventual Consistency theorem construct the foundation stone of NoSQL Cassandra is one kind of NoSQL databases, It is used by Twitter, Facebook and some other famous corporations. Taking it for example, this online trading system is based on Cassandra database. I'll design and contrast the relational model and Cassandra-based model of this system, then construct the key space, the column family and do some other configuration. After these jobs done, I'll do some coding to implement this system.