@abstractmethod<EOL><INDENT>def update(self, session):<DEDENT>
pass<EOL>
Updates (persists) data from a previously created Session instance in
the EIS identified by session.session_id.  This effectively propagates
the data in the argument to the EIS record previously saved.

:param session: the Session to update
:raises ValueError: if no existing EIS session record
                     exists with the identifier of session.session_id