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Use-case example to show why this is urgently needed: The property source of file (P7482) is intended to show the broad nature of the origin of a file.

In the property proposal discussion, it was agreed that this could be rolled out widely now for the simplest case, namely images we can reliably trust to be a user's own personal creation and own direct upload (eg a user's own photo of a monument they have taken and uploaded in the Wiki Loves Monuments campaign), but that for the moment uses in more complicated cases should be restricted until there has been more community discussion and sign-off, to make sure we know how we want to model such cases, before people start applying the property more widely.

This is reflected in the following constraint specification on P7482 (see revision as of 2019-10-27) :

• property constraint (P2302) = one-of constraint (Q21510859)
  • item of property constraint (P2305) = original creation by uploader (Q66458942) -- list of allowed values : initially this is the only approved value
  • constraint status (P2302) = suggestion constraint (Q62026391) -- show a flag warning if the above constraint is not met on a statement involving this property P7482, rather than the more serious exclamation mark warning
  • constraint clarification (P6607) = "only agreed values should be used, other than on test images being considered in discussion environments" -- explanatory message to show, if a constraint violation is flagged. Localised messages may exist for multiple languages.

It is urgent that warning flagging is put in place soon, before people start seeing P7482 "source of file" statements starting widely to appear images, and begin supplying their own new values, other than what is so-far the only community-approved value, Q66458942 "original creation by uploader".

If SDC is to develop in an orderly way, visible flagging of non-conformant (or not yet generally consented to) statements is really needed.

@Lucas_Werkmeister_WMDE How dependent is the wikibase constraint system on SPARQL ?
Maarten was just suggesting that a functioning SPARQL service is required for any of the constraint checking to operate (and so this ticket would be completely blocked until CDQS is reliably up and running)
Is that correct? Or are any of the simplest of the constraint checks (eg format constraints, one-of constraints, etc) available without SPARQL ?

In T230314#5624722, @Jheald wrote:

@Lucas_Werkmeister_WMDE How dependent is the wikibase constraint system on SPARQL ?
Maarten was just suggesting that a functioning SPARQL service is required for any of the constraint checking to operate (and so this ticket would be completely blocked until CDQS is reliably up and running)

This is not correct at all. Two constraint types, “format” and “distinct values”, require a SPARQL service (and disable themselves if none is configured); two others, “type” and “value type”, can fall back to SPARQL for type checks (though they first follow the statements in PHP, since contacting the query service is expensive) but don’t require it. All other constraint types are unrelated to the query service.