nova/api-guide/source/polling_changes-since_parameter.rst

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.