Default values for query parameters

[cportele]

Currently the default values for query parameters on a feature collection (bbox, datetime or any additional filter parameter) is always to select all features.

In some cases, a different behavior would be more appropriate. An example identified in the Testbed-14 Complex Feature Handling work were collections where the features are versions of a real-world entity, each valid for a given time interval - without overlaps between the time intervals. In this case, it would be more natural, if by default the value of datetime would be "now" so one fetches the current snapshot/situation.

In other types of datasets a default bbox or default values for filter parameters on string values might be useful, too.

This could be addressed in at least two ways:

• Define default values in the OpenAPI definition of the parameters plus in the collection metadata. If no default values are provided the current behavior would apply (all features are selected).
• Another approach could be to not use default values, but add sufficient information in the collection metadata so that clients can always add explicit query parameters for the type of collection. That is, in the example above, clients would always add an explicit datetime parameter, if they want to fetch only features for a certain snapshot of the dataset.

[cportele]

20-MAY-2019 Conference call: We probably do not want to preclude that implementations can use default values. We just need to check that we do not prohibit that an API definition includes default values for the parameters. Clemens to check.