CORS Pre-flight OPTIONS request

[dschep]

Hi! When I initially added SPARQL support to Ultra, I used POST requests sending the query as the body and a Content-Type: application/sparql-query header. This works great with the Wikidata Query Service and QLever.

This is a valid means of calling SPARQL per the protocol: https://www.w3.org/TR/sparql11-protocol/#query-operation

Sophox also works with this invocation method when using curl, however it doesn't work in the browser because Sophox doesn't support CORS Preflight requests.

[dschep]

I dug in a bit more, and the issue is that /sparql returns 404 for an OPTIONS request, which CORS requests from other domains must use for a preflight request.

[dschep]

from overpass-ultra.us (or realistically, any domain other than sophox.org)

vs from sophox.org, which being on the same domain, does not make a preflight OPTIONS request.

[dschep]

I figured out a solution to Sophox not supporting CORS Preflight requests: Don't send one 😅

namely, use a GET request or a form encoded POST request with Content-Type: application/x-www-form-urlencoded