Questions related to STAC versions

[m-mohr]

Due to my work on a specification (openEO) that is based on STAC, some questions came up recently:

1. Is STAC (i.e. releases and the field stac_version) following Semantic Versioning? So is any breaking change after 1.0.0 leading to a version 2.0? If not, how is the procedure regarding version numbers and breaking changes?
   Why is this important? I want to future-proof the openEO API and not just allow version 0.9.0, but also also all versions that are non-breaking after 1.0.0. Getting this information allows me to actually validate the stac_version field (e.g. with a pattern such as ^(0\.9|1)\.).
2. Is it fine to deliver any STAC version via any STAC API version? For example, can I implement STAC API 0.9 and it responds with STAC 1.0 or 0.8 items/collections? According to the OpenAPI file it seems allowed: https://github.com/radiantearth/stac-api-spec/blob/master/STAC.yaml#L1311
   Why is this important? How can I base a specification on STAC API that is STAC 1.0 compliant without having a "final" STAC API spec yet?
   • If answered with "yes", how to detect which API version is implemented? This is related to New fields: stac_api_version and stac_api_extensions stac-api-spec#27.
3. To use 1.0 extensions that are not part of 0.9, I think the way to expose these via "stac_extensions" is to not use timestamps, but (see Unique schema $id's (host schemas on stacspec) #835) https://schemas.stacspec.org/v1.0.0-beta.1/extensions/timestamps/schema.json, right? (Edit: yes, seems to be backed by "If the versions of the extension and the item diverge, you can specify the URL of the JSON schema file.")

[cholmes]

1. I'll take on - put in language about sem ver
2. Discussed on call for 6-22-20, decided that the answer is 'yes', a STAC API can respond with different versions. For the betas we don't have a great way to resolve which api version is implemented, though the features api version may help a bit. And we will work to get it more clear for 1.0 of stac api.
3. This is resolved

[m-mohr]

1. PR is added sem ver clarification - #850 #855

2. I guess the way to resolve it is as follows:

   • All STAC APIs prior to 0.9 are bound to the respective STAC versions
   • STAC 0.9 can either be a STAC 0.9 or 1.0 API. It is distinguishable by the stac_api_version field, which we'll include for 1.0. If it's not available, it's a 0.9 API.
   • Everything after STAC API 1.0 has the stac_api_version field.

   Do we need to document that somewhere? I just wrote and pinned STAC API for STAC 1.0 #858 for now. So this issue can be closed after added sem ver clarification - #850 #855 is merged.