- Title: Aggregation
- OpenAPI specification: TBD
- Conformance Classes:
- Scope: STAC API - Core
- Extension Maturity Classification: Proposal
- Dependencies:
- Owner: @philvarner
The purpose of the Aggregation Extension is to provide an endpoint similar to the Search endpoint
(/search), but which will provide aggregated information on matching Items rather than the
Items themselves. This is highly influenced by the Elasticsearch and OpenSearch aggregation
endpoint, but with a more regular structure for responses.
| Endpoint | Returns | Description |
|---|---|---|
/aggregate |
AggregationCollection | Retrieves an aggregation of the group of Items matching the provided predicates |
/aggregations |
JSON | Retrieves a schema defining the terms which are available for aggregation over. |
/{collectionId}/aggregate |
Retrieves an aggregation of the group of Items matching the provided predicates, restricted to this collection | |
/{collectionId}/aggregations |
JSON | Retrieves a schema defining the terms which are available for aggregation over only this collection. |
The /aggregate endpoint behaves similarly to the /search endpoint, but instead of returning an
ItemCollection of Items, it instead returns aggregated information over the same matching Items
in the form of an AggregationCollection of Aggregation entities.
The /aggregations endpoint returns a schema-like document describing aggregations that can be requested,
similarly to how the queryables endpoint is used in the Filter Extension. These can be passed in
the aggregations parameter to /aggregate to restrict the aggregations performed.
Other STAC API Extensions, notably Query and Filter, can be advertised with https://api.stacspec.org/v0.3.0/aggregation#query and https://api.stacspec.org/v0.3.0/aggregation#filter.
For the links in both landing page (/) and collection (/collections/{collectionId}), it is
required to add a link with the rel value set to aggregate that refers to the relevant
aggregate endpoint in the href property, and a link with the rel value set to aggregations
that refers to the relevant aggregations endpoint in the href property. The method field can
be defined with GET or POST to advertise support for those HTTP methods, or if no method field
is defined, only GET is supported.
The filters for /aggregate are the same as those for /search that are semantically meaningful
(e.g., limit has no meaning when doing aggregations). These filters are passed as query string
parameters or JSON entity fields. For filters that represent a set of values, query parameters
should use comma-separated
string values and JSON entity attributes should use JSON Arrays.
| Parameter | Type | Description |
|---|---|---|
| datetime | string | Single date+time, or a range ('/' seperator), formatted to RFC 3339, section 5.6. Use double dots .. for open date ranges. |
| bbox | [number] | Requested bounding box. Represented using either 2D or 3D geometries. The length of the array must be 2*n where n is the number of dimensions. The array contains all axes of the southwesterly most extent followed by all axes of the northeasterly most extent specified in Longitude/Latitude or Longitude/Latitude/Elevation based on WGS 84. When using 3D geometries, the elevation of the southwesterly most extent is the minimum elevation in meters and the elevation of the northeasterly most extent is the maximum. |
| intersects | GeoJSON Geometry | Searches items by performing intersection between their geometry and provided GeoJSON geometry. All GeoJSON geometry types must be supported. |
| ids | [string] | Array of Item ids to return. All other filter parameters that further restrict the number of search results (except next and limit) are ignored |
| collections | [string] | Array of Collection IDs to include in the search for items. Only Items in one of the provided Collections will be searched |
| aggregations | [string] | A list of aggregations to compute and return |
Only one of either intersects or bbox should be specified. If both are specified, a 400 Bad Request response should be returned.
aggregations: There are no named aggregations that must be implemented. All aggregations which
are available should be advertised in the root rel="aggregations" link. By default, the
/aggregations endpoint must only return the named aggregations specifically requested via
the aggregations parameter.
For the collection subresource, only collections that are specifically advertised by that collection's
/aggregations endpoint should be allowed, and if any others are requested, a 400 Bad Request
exception should be returned. At the root-level /aggregate endpoint, any aggregation may be
requested, even if it does not apply to all items in the result set.
It is recommended to suffix count aggregations with _count and frequency distribution aggregations
with _frequency.
This is a list of recommended aggregations to implement:
- total_count (Single Value of type integer)
- datetime_min (Single Value of datetime)
- datetime_max (Single Value of datetime)
- collection_frequency (Term Count)
- cloud_cover_frequency (Discrete Range)
- datetime_frequency (Datetime Range, automatic interval detection) -- detect a reasonable interval based on the datetime range and distribution of data? Implementation specific.
This object describes a STAC AggregationCollection, which is the analog of an ItemCollection for the /aggregate operation.
| Field Name | Type | Description |
|---|---|---|
| type | string | REQUIRED Always "AggregationCollection". |
| aggregations | [Aggregation] | REQUIRED A possibly-empty array of Aggregations. |
| Field Name | Type | Description |
|---|---|---|
| name | string | REQUIRED The unique indentifier of the aggregation. |
| data_type | string | REQUIRED The data type of the aggregation |
| buckets | [Bucket] | If the aggregation bucketizes Items, they are defined here. |
| overflow | integer | The count of Items that were not categorized into any of the buckets defined by the buckets field |
| value | string or number or datetime | For a Single Value aggregation, a JSON-type representation of the result value. |
One of either buckets or value is required.
name An identifier for the aggregation result. Should be identical to the value passed to the aggregations query parameter.
data_type numeric, string, datetime, interval_year, interval_month, interval_week interval_day, interval_hour, interval_minute, interval_second
buckets If the aggregation is a Term Count, Datetime Range, or Discrete Range, these are the "buckets" into which each matching Item is categorized.
overflow Some implemenation data stores may have limitations on the aggregation queries that can be performed on them. For example, Elasticsearch limits the number of buckets for a query to 10,000 for performance reasons. Overflow indicates that there were Items matched by the query that are not accounted for in the count of any of the response buckets.
value For Single Value aggregations, this is a representation of the result value as the equivalent JSON type. If the type of the value being aggregated over is a datetime, this is an RFC 3339 datetime, e.g., "2020-08-12T19:06:09Z".
| Field Name | Type | Aggregation Types | Description |
|---|---|---|---|
| key | string | all | |
| data_type | string | all | |
| frequency | integer | all | |
| from | numeric | all | |
| to | numeric | all |
This section has not been updated to match the previous definitions.
effectively a single Term Count Bucket lifted up one level
todo (diff for String, Numeric, and Datetime)
Example:
{
"type": "AggregationCollection",
"aggregations": [
{
"key": "datetime_min",
"value": "2000-02-16T00:00:00.000Z",
"value_as_type": 1.506592E+11
}
]
}- enumeration count multi bucket one per unique value
Example:
{
"type": "AggregationCollection",
"aggregations": [
{
"key": "collections",
"buckets": [
{
"key": "sentinel-2-l2a",
"value": "12649072",
"value_as_type": 12649072
},
{
"key": "landsat8-c2-l2",
"value" : "1071997",
"value_as_type": 1071997
}
],
"overflow": 23414
}
]
}Fields:
- key (string)
- key_as_type ()
- from (optional, missing indicates an open interval) inclusive
- to (optional, missing indicates an open interval) exclusive
- value (integer)
Example:
{
"type": "AggregationCollection",
"aggregations": [
{
"key": "cloud_cover",
"buckets": [
{
"key": "*-5.0",
"to": 5,
"value" : "8644819",
"value_as_type" : 8644819
},
{
"key": "5.0-10.0",
"from": 5,
"to": 10,
"value" : "5644819",
"value_as_type" : 5644819
},
{
"key": "10.0-*",
"from": 10,
"value" : "7644819",
"value_as_type" : 7644819
}
]
}
]
}Fields: datetimes are RFC 3339 string values
- key (string)
- key_as_type (datetime in milliseconds?)
- value (integer) (ES: doc_count)
Example:
{
"type": "AggregationCollection",
"aggregations": [
{
"key": "datetime_yearly",
"buckets": [
{
"key": "2000-01-01T00:00:00.000Z",
"key_as_type": 946684800000,
"to": 5,
"value" : "8644819",
"value_as_type" : 8644819
},
{
"key": "2001-01-01T00:00:00.000Z",
"key_as_type": 978307200000,
"from": 5,
"to": 10,
"value" : "5644819",
"value_as_type" : 5644819
},
{
"key": "2002-01-01T00:00:00.000Z",
"key_as_type": 1009843200000,
"from": 10,
"value" : "7644819",
"value_as_type" : 7644819
}
],
"interval": "year",
"overflow": 98373
}
]
}