How to handle "multiple output"

[fmigneault]

According to owslib.wps, minOccurs/maxOccurs can only be parsed for inputs, outputs are always a single reference or literal data.

In pywps, the outputs have a reference to min_occurs/max_occurs because of inheritance from base classes. They are also returned in their json property.

For CWL, there is no restriction to have multiple outputs (array of string, file, etc.).
Actually, when using a regex glob: *.<ext> pattern, a list of results is automatically created with all matches.

How can we produce the CWL <-> WPS conversion in these cases?
Also, should we enforce explicit values of minOccurs/maxOccurs :

• minOccurs="1" enforced since there should always be an output
  update: since CWL allows optional outputs (eg: File?), minOccurs=0|1 should be set according to nullable detection
• maxOccurs="1" | "<number>" | "unbounded", with "1" being the default (same reason as min), or one of "<number>" | "unbounded" if specified during deployment

ref #17
relates to opengeospatial/ogcapi-processes#37

To Do

• check any test with commented references to How to handle "multiple output" #25

[tomLandry]

Unsure if that answers your question, but please take a look at metalinks: bird-house/emu#64. First implementation here: bird-house/emu@1d6125f

This is a recommendation from ADES&EMS best practices ER: https://docs.opengeospatial.org/per/18-050r1.html#_multiple_outputs

[fmigneault]

see method using metalink like in birdy:
https://github.com/bird-house/birdy/blob/26798f62c6d1add6d28acd8e283c5f7d05083ffd/birdy/client/converters.py#L99-L115

[fmigneault]

implementation in pywps:
geopython/pywps#298

[tomLandry]

@fmigneault Multiple outputs is an "evaluation aspect" of the OGC EO Pilot project. Thus we need to prioritize this relatively high. See https://portal.opengeospatial.org/files/?artifact_id=90733#_application_execution_multiple_outputs

[fmigneault]

Via pypws :
https://github.com/geopython/pywps/blob/master/docs/metalinkprocess.py

[fmigneault]

Another possible solution using STAC #103

[fmigneault]

For reference, some kind of multiple-output handling is already accomplished for WPS-REST interface when a job output is a JSON file listing an array of URL references :

weaver/weaver/processes/utils.py

Lines 87 to 116 in d2992ab

	def _get_multi_json_references(output):
	# type: (owslib.wps.Output) -> Optional[List[JSON]]
	"""
	Since WPS standard does not allow to return multiple values for a single output,
	a lot of process actually return a json array containing references to these outputs.
	This function goal is to detect this particular format.
	:return: An array of HTTP(S) references if the specified output is effectively a JSON containing that,
	``None`` otherwise.
	"""
	# Check for the json datatype and mimetype
	if output.dataType == WPS_COMPLEX_DATA and output.mimeType == CONTENT_TYPE_APP_JSON:
	# If the json data is referenced read it's content
	if output.reference:
	json_data_str = _read_reference(output.reference)
	# Else get the data directly
	else:
	json_data_str = _get_data(output)
	# Load the actual json dict
	json_data = json.loads(json_data_str)
	if isinstance(json_data, list):
	for data_value in json_data:
	if not is_reference(data_value):
	return None
	return json_data
	return None

[fmigneault]

Follow up on opengeospatial/ogcapi-processes#225
This would enforce that only single value are allowed as output (with default handling).

If an output needs to be a JSON list, it should provide in the process description the schema of that output accordingly.
In such case, the output could be loaded and validated to render directly as list (or more complex format).
Requires full support of #245

[ChaamC]

Optional outputs (minOccurs=0|1) would be useful for the asv project, which produces an output file only if a certain input argument is found. Using the optional output has been tested with cwl and was working well (returns a null value for the parameter if the file is not created.)