def poll_invalidations():
    """Poll the storage for external changes.

        Returns either a sequence of OIDs that have changed, or None.  When a
        sequence is returned, the corresponding objects should be removed
        from the ZODB in-memory cache.  When None is returned, the storage is
        indicating that so much time has elapsed since the last poll that it
        is no longer possible to enumerate all of the changed OIDs, since the
        previous transaction seen by the connection has already been packed.
        In that case, the ZODB in-memory cache should be cleared.
        """
