Compatibility between OData and STA

At the last TC meeting I gave a little demo of using STA/OData in Excel.

Here are the differences I found so far:		STA	OData 4.0
Self Links	`@iot.selfLink`	`@odata.id`	`@id`
NavLinks	`@iot.navigationLink`	`@odata.navigationLink`	`@navigationLink`
ID property	`@iot.id`	`id` (@ and . confuse clients)	`id` (@ and . confuse clients)
Context		`@odata.context`	`@context`
Flexible Properties	property type ANY	not supported, cast to Edm.String	Edm.Untyped
Time Interval/Value	`phenomenonTime`, `validTime`	not supported, use `phenomenonTime.start` and `phenomenonTime.end`	not supported, use `phenomenonTime.start` and `phenomenonTime.end`
Non existing entities	`404 Not Found`	`204 No Content`	`204 No Content`

Things that STA has that can not easily be mapped to OData:

Type ANY doesn't exist in OData 4.0. Version 4.01 has Edm.Untyped.
No TimeInterval or TimeValue, only Edm.DateTimeOffset. This is an issue for Observation/phenomenonTime, Observation/validTime, Datastream/phenomenonTime and Datastream/resultTime. One solution would be to just use Edm.String, but this doesn't help clients. A better solution is to split the interval into intervalProperty.start and intervalProperty.end in the OData interface.

Our test endpoint, for people that would like to try it out:

STA: https://ogc-demo.k8s.ilt-dmz.iosb.fraunhofer.de/v1.1/
OData 4.0: https://ogc-demo.k8s.ilt-dmz.iosb.fraunhofer.de/ODATA_4.0/
OData 4.01: https://ogc-demo.k8s.ilt-dmz.iosb.fraunhofer.de/ODATA_4.01/

It's the same server instance, so the database in these three endpoints is the same.

Related to #72 Also in the FROST wiki.

Some more progress:

The ANY result type can be forced to Edm.String, since everything can be displayed as a String. This makes Excel content.

Times that are a TimeInterval can be mapped to a custom type with start and end (both mandatory):

"TimeInterval": {
    "$Kind": "ComplexType",
    "@Core.Description": "An ISO time interval.",
    "start": {
        "$Type": "Edm.DateTimeOffset"
    },
    "end": {
        "$Type": "Edm.DateTimeOffset"
    }
}

Times that are a TimeObject (Interval or Instant) can be mapped to a custom type with start and optional end:

"TimeValue": {
    "$Kind": "ComplexType",
    "@Core.Description": "An ISO time instant or time interval.",
    "start": {
        "$Type": "Edm.DateTimeOffset"
    },
    "end": {
        "$Type": "Edm.DateTimeOffset",
        "$Nullable": true
    }
},

opengeospatial / sensorthings

Compatibility between OData and STA #136