1. A method comprising: generating a plurality of session objects in response to client requests, the session objects storing session data defining a state of interaction with each of a respective plurality of clients; defining a hierarchy of session object storage units, the session object storage units configured to store session objects having one or more related characteristics; grouping the session objects within the session object storage units based on the one or more related characteristics of each of the session objects; generating a persistent representation of the hierarchy of session object storage units, the persistent representation of the session object storage units mapping each session storage object unit and each individual session object within each session object storage unit to different persistent data structures within a persistent storage device.