STAC API - Sort Extension Specification

• STAC API - Sort Extension Specification
  • Overview
  • HTTP GET
  • HTTP POST JSON Entity
  • Sortables

Overview

• Title: Sort
• OpenAPI specification:openapi.yaml
• Conformance Classes:
  • STAC API - Item Search binding: https://api.stacspec.org/v1.1.0/item-search#sort
  • STAC API - Features binding: https://api.stacspec.org/v1.1.0/ogcapi-features#sort
  • STAC API - Collection Search binding: https://api.stacspec.org/v1.1.0/collection-search#sort
• Scope: STAC API - Features, STAC API - Item Search, STAC API - Collection Search
• Extension Maturity Classification: Stable
• Dependencies:
  • STAC API - Item Search
  • STAC API - Features
  • STAC API - Collection Search
• Owner: @philvarner

This specification defines a new parameter, sortby, that allows the user to define the fields by which to sort results. Only string, numeric, and datetime attributes of Item (id and collection only) or Item Properties (any attributes) may be used to sort results.

It is not required that implementations support sorting over all attributes, but implementations should either implement Sortables or just return a 400 Bad Request status code when attempting to sort over a field name that does not support sorting. Implementers may choose to require fields in Item Properties to be prefixed with properties. or not, or support use of both the prefixed and non-prefixed name, e.g., properties.datetime or datetime.

Sort behavior may be bound to any of the following endpoints by advertising the relevant conformance class:

• STAC API - Item Search (/search endpoint),
• STAC API - Features (/collections/{collectionId}/items endpoint)
• STAC API - Collection Search (/collections endpoint)

Fields may be sorted in ascending or descending order. The syntax between GET requests and POST requests with a JSON body vary. The sortby value is an array, so multiple sort fields can be defined which will be used to sort the data in the order provided (e.g., first by datetime, then by eo:cloud_cover).

HTTP GET

When calling a relevant endpoint using GET, a single parameter sortby with a comma-separated list of item field names must be provided. The field names may be prefixed with either "+" for ascending, or "-" for descending. If no sign is provided before the field name, it will be assumed to be "+". Note that + is used commonly by URL encoding as a space, so some tools may require escaping this literal + with a URL encoding of %2B.

Examples of sortby parameter:

1. GET /search?sortby=properties.created
2. GET /search?sortby=+properties.created
3. GET /search?sortby=properties.created,-id
4. GET /search?sortby=+properties.created,-id
5. GET /search?sortby=-properties.eo:cloud_cover

Note that examples 1 and 2 are symantically equivalent, as well as examples 3 and 4.

HTTP POST JSON Entity

When calling the relevant endpoint using POST withContent-Type: application/json, this adds an attribute sortby with an object value to the core JSON search request body.

The syntax for the sortby attribute is:

{
"sortby": [
{
"field": "<property_name>",
"direction": "<direction>"
}
]
}

{
"sortby": [
{
"field": "properties.created",
"direction": "asc"
},
{
"field": "properties.eo:cloud_cover",
"direction": "desc"
},
{
"field": "id",
"direction": "desc"
},
{
"field": "collection",
"direction": "desc"
}
]
}

Sortables

Additional endpoints that provide so called "Sortables" support clients that want to discover the list of resource properties with their types and constraints that may be used to sort resources.

These Sortables endpoints return lists of properties (or aliases) that can be used in the sortby parameter. It returns a JSON Schema that defines the properties allowed in sortby. The precise definition of this can be found in the OGC API - Features - Part 5: Schemas.

In particular:

• No property SHALL be of type "object" or "array".
• No property SHALL be a spatial property.
• If additionalProperties is not included or has the default value true, any property name is valid in a sorting expression on the collection that is evaluated by the server and the property reference SHALL evaluate to null, if the property does not exist for a resource.
• If additionalProperties is set to false, property references that are not explicitly declared in the sortables schema SHALL result in a 400 response.

All Sortables endpoints SHALL be referenced with a link with the link relation type http://www.opengis.net/def/rel/ogc/1.0/sortables.

Sortables Endpoint	Endpoint linking to the Sortables Endpoint	Conformance class	Applicable sortby endpoints
GET /sortables	GET /	https://api.stacspec.org/v1.1.0/item-search#sortables	GET /search and POST /search
GET /collections/{collectionId}/sortables	GET /collections/{collectionId}	http://www.opengis.net/spec/ogcapi-features-5/1.0/conf/sortables	GET /collections/{collectionId}/items
GET /... (Endpoint name to be chosen by implementation)	GET /collections	https://api.stacspec.org/v1.1.0/collection-search#sortables	GET /collections

An example for a link to the sortables endpoint could be:

{
"href": "https://stac.example/sortables",
"type": "application/schema+json",
"rel": "http://www.opengis.net/def/rel/ogc/1.0/sortables",
"title": "Sortables"
}

For an example of a sortables endpoint response, please see the openapi.yaml.