Are identifiers also required or not?

[Silvanoc]

Neither the identifier slot definition nor the documentation regarding identifiers appear to be clear enough on that slot being required too or not.

There are two options:

1. Making identifier: true imply required: true means that the slot marked as an identifier MUST be provided and MUST provide a unique value.

2. Making identifier: true imply required: false means that the slot marked as an identifier MUST NOT be provided, but if it is provided, it MUST provide a unique value.

I would assume that such a decision was already taken long time ago and it is probably fixed throughout the whole code base. Therefore I consider the opinion of @cmungall very important here.

The result of this discussion has implications on discussion #2472 and #2451 (probably others too).

[Silvanoc]

I personally are for identifier: true implying required: true. Basically due to following reasons:

1. Being more of a software-guy than an ontology-guy, I would appreciate being able to rely on an identifier in the software I create to be able to unambiguously identify elements.
2. I intuitively expect identifiers: true to be required: true.
3. My feeling based on the code I know is that this is the current expectation.
4. I do not see so much value on enabling optional identifiers.

I wonder if unique_keys could be used to specify a slot to be unique, but remaining optional.

[matentzn]

I have not deeply thought about this in great depth but I tend to agree. I guess someone could think of how corner use cases like "this object is identified either by the social security number or the passport id" (in this case neither could be "identifier: true", but as a basic assumption I cannot see a reason your proposal would fall short.