A particular type of cloud software, NoSQL systems such as Cassandra and MongoDB gained currency and displaced relational database systems for very large tasks in the cloud. Cassandra took over the heavy lifting from MySQL at Twitter, and became the primary information sorting system at Facebook and Digg, as well. NoSQL doesn't meet the stringent transaction-completion test set for relational databases, so it isn't used for that purpose. But for sorting and maintaining large amounts of information quickly on a large server cluster, it excels. And when it comes to tweeting, wall postings, or online gaming, that's exactly what's needed today.