Zend Framework comes with a standard set of storage classes which are ready for you to use. Storage handlers is the intermediary between when the session starts and when the session writes and closes. The default session storage is Zend\Session\Storage\SessionArrayStorage.
In the event that you prefer a different type of storage; to create a new custom storage container, you must implement Zend\Session\Storage\StorageInterface which is mostly in implementing ArrayAccess, Traversable, Serializable and Countable. StorageInterface defines some additional functionality that must be implemented.