nova/api-guide/source/paginated_collections.rst

Paginated collections

To reduce load on the service, list operations return a maximum number of items at a time. The maximum number of items returned is determined by the compute provider. To navigate the collection, the ``limit`` and ``marker`` parameters can be set in the URI. For example:

?limit=100&marker=1234

The ``marker`` parameter is the ID of the last item in the previous list. By default, the service sorts items by create time in descending order. When the service cannot identify a create time, it sorts items by ID. The ``limit`` parameter sets the page size. Both parameters are optional. If the client requests a ``limit`` beyond one that is supported by the deployment an overLimit (413) fault may be thrown. A marker with an invalid ID returns a badRequest (400) fault.

For convenience, collections should contain atom next links. They may optionally also contain previous links but the current implementation does not contain previous links. The last page in the list does not contain a link to "next" page. The following examples illustrate three pages in a collection of images. The first page was retrieved through a GET to http://servers.api.openstack.org/v2/1234/servers?limit=1. In these examples, the ``limit`` parameter sets the page size to a single item. Subsequent links honor the initial page size. Thus, a client can follow links to traverse a paginated collection without having to input the ``marker`` parameter.

Example: Servers collection: JSON (first page)

{
   "servers_links":[
      {
         "href":"https://servers.api.openstack.org/v2/1234/servers?limit=1&marker=fc45ace4-3398-447b-8ef9-72a22086d775",
         "rel":"next"
      }
   ],
   "servers":[
      {
         "id":"fc55acf4-3398-447b-8ef9-72a42086d775",
         "links":[
            {
               "href":"https://servers.api.openstack.org/v2/1234/servers/fc45ace4-3398-447b-8ef9-72a22086d775",
               "rel":"self"
            },
            {
               "href":"https://servers.api.openstack.org/v2/1234/servers/fc45ace4-3398-447b-8ef9-72a22086d775",
               "rel":"bookmark"
            }
         ],
         "name":"elasticsearch-0"
      }
   ]
}

In JSON, members in a paginated collection are stored in a JSON array named after the collection. A JSON object may also be used to hold members in cases where using an associative array is more practical. Properties about the collection itself, including links, are contained in an array with the name of the entity an underscore (_) and links. The combination of the objects and arrays that start with the name of the collection and an underscore represent the collection in JSON. The approach allows for extensibility of paginated collections by allowing them to be associated with arbitrary properties. It also allows collections to be embedded in other objects as illustrated below. Here, a subset of metadata items are presented within the image. Clients must keep following the next link to retrieve the full set of metadata.

Example: Paginated metadata: JSON

{
    "server": {
        "id": "52415800-8b69-11e0-9b19-734f6f006e54",
        "name": "Elastic",
        "metadata": {
            "Version": "1.3",
            "ServiceType": "Bronze"
        },
        "metadata_links": [
            {
                "rel": "next",
                "href": "https://servers.api.openstack.org/v2/1234/servers/fc55acf4-3398-447b-8ef9-72a42086d775/meta?marker=ServiceType"
            }
        ],
        "links": [
            {
                "rel": "self",
                "href": "https://servers.api.openstack.org/v2/1234/servers/fc55acf4-3398-447b-8ef9-72a42086d775"
            }
        ]
    }
}