What problems does Cassandra solve?

Cassandra is designed to solve problems associated with operating at a large (web) scale. It was designed under similar principles discussed in Amazon's Dynamo paper,^{[7, p.205]} where in a large, complicated system of interconnected hardware, something is always in a state of failure. Given Cassandra's masterless architecture, it is able to continue to perform operations despite a small (albeit significant) number of hardware failures.

In addition to high availability, Cassandra also provides network partition tolerance. When using a traditional RDBMS, reaching the limits of a particular server's resources can only be solved by vertical scaling or scaling up. Essentially, the database server is augmented with additional memory, CPU cores, or disks in an attempt to meet the growing dataset or operational load. Cassandra, on the other hand, embraces the concept of horizontal scaling or scaling out. That is, instead of adding more hardware resources to a server...