The introduction of collections to CQL addresses some of the concerns that frequently arose regarding Cassandra's primitive data model. They add richer capabilities that give developers more flexibility when modeling certain types of data.
Cassandra supports three collection types: sets, lists, and maps. In this section, we will examine each of these and take a look at how they're stored under the hood. But first, it's important to understand some basic rules regarding collections:
The size of each item in a collection must not be more than 64 KB
A maximum of 64,000 items may be stored in a single collection
Querying a collection always returns the entire collection
Collections are best used for relatively small, bounded datasets
With those rules in mind, we can examine each type of collection in detail, starting with sets.