Apache Cassandra is a highly scalable NoSQL database. This video is a nice 30 min introduction on how it works and why it’s useful to know.
One of the beauties of free/open source is that it powers much of the world and much of the largest websites in the world. Facebook created the project and released the code because they know how valuable it is. One of the benefits of releasing Cassandra’s code as free/open source and making it free as in price is that developers can freely download the code and learn how to use Cassandra. While there are still no formal classes in college or university that teach Cassandra, your company can still find developers out there who have learned it because they spent their spare time learning how to use it. That’s not possible with proprietary packages that charge high license fees (student pricing doesn’t apply to full-time software developers who want to learn in their spare time).
Free/open source software offers more opportunity for education which increases the number of developers out there who can be hired by your organization.