================================================== Efficient polling with the changes-since parameter ================================================== The REST API allows you to poll for the status of certain operations by performing a **GET** on various elements. Rather than re-downloading and re-parsing the full status at each polling interval, your REST client may use the ``changes-since`` parameter to check for changes since a previous request. The ``changes-since`` time is specified as an `ISO 8601 `__ dateTime (2011-01-24T17:08Z). The form for the timestamp is **CCYY-MM-DDThh:mm:ss**. An optional time zone may be written in by appending the form ±hh:mm which describes the timezone as an offset from UTC. When the timezone is not specified (2011-01-24T17:08), the UTC timezone is assumed. If nothing has changed since the ``changes-since`` time, an empty list is returned. If data has changed, only the items changed since the specified time are returned in the response. For example, performing a **GET** against:: https://api.servers.openstack.org/v2.1/servers?changes-since=2015-01-24T17:08Z would list all servers that have changed since Mon, 24 Jan 2015 17:08:00 UTC. To allow clients to keep track of changes, the ``changes-since`` filter displays items that have been *recently* deleted. Both images and servers contain a ``DELETED`` status that indicates that the resource has been removed. Implementations are not required to keep track of deleted resources indefinitely, so sending a ``changes-since`` time in the distant past may miss deletions.