def invalidate(self, comparison: Comparison[Entity, Entity]) -> None:
        """
        Invalidate paths in a zone. See https://api.cloudflare.com
        /#zone-purge-individual-files-by-url-and-cache-tags

        :param comparison: The comparison whose changes to invalidate.
        :raises requests.exceptions.RequestException: On request failure.
        :raises RuntimeError: If the request succeeded but could not be carried
                              out.
        """

        @backoff.on_exception(backoff.expo,
                              requests.exceptions.RequestException,
                              max_tries=5,
                              giveup=lambda e:
                              400 <= e.response.status_code < 500)
        def _request(chunk: List[str]) -> requests.Response:
            """
            Send a purge cache request to Cloudflare. This method will
            automatically retry with a back-off in case of server-side error.

            :param chunk: The list of paths to purge. These should not have a
                          leading slash, and will be combined with the prefix
                          to form a URL.
            :return: Cloudflare's response to our successful request.
            :raises requests.exceptions.RequestException: If the request fails
                                                          on the 5th attempt.
            """
            response = self._session.delete(
                f'{self._API_BASE}/client/v4/zones/{self._zone}/purge_cache',
                headers={
                    'X-Auth-Email': self._email,
                    'X-Auth-Key': self._key
                },
                json={
                    'files': [self._prefix + path for path in chunk]
                })
            response.raise_for_status()
            return response

        paths = itertools.chain(comparison.deleted(), comparison.modified())
        for chunk_ in util.chunk(paths, self._MAX_INVALIDATIONS_PER_REQUEST):
            chunk_ = list(chunk_)
            if not chunk_:
                # nothing to do
                return
            logger.info('Invalidating %d paths (%s)', len(chunk_),
                        ', '.join(chunk_))
            response_ = _request(chunk_)
            logger.debug('Cloudflare invalidation response [%d]: %s',
                         response_.status_code,
                         response_.text)
            json_ = response_.json()
            if not json_['success']:
                # this would be strange - the API returned a success response
                # code, but success was not "true"
                # TODO more appropriate exception, with handling upstream
                raise RuntimeError('Cloudflare reported failure')
            logger.info('Created invalidation %s', json_['result']['id'])