@abstractmethod<EOL><INDENT>def on_expiration(self, session):<DEDENT>
pass<EOL>
Notification callback that occurs when the corresponding Session has
expired.

Note: this method is almost never called at the exact instant that the
Session expires.  Almost all session management systems, including
Yosai's implementations, lazily validate sessions - either when they
are accessed or during a regular validation interval.  It would be too
resource intensive to monitor every single session instance to know the
exact instant it expires.

If you need to perform time-based logic when a session expires, it
is best to write it based on the session's last_access_time and
NOT the time when this method is called.

:param session: the session that has expired