What happens when a client tries to create a topic that already exists?

[mjkoster]

Should we return 4.09 Conflict for the case when the client tries to create a topic that already exists?

Indicates that the request could not be processed because of conflict in the request, such as an edit conflict between multiple simultaneous updates.

The draft currently specifies 4.03

The request was valid, but the server is refusing action. The user might not have the necessary permissions for a resource, or may need an account of some sort.

[akeranen]

LGTM

[mjkoster]

Will change to 4.09

[akeranen]

4.03 is same as HTTP 403 per 7252:

The HTTP 403 Forbidden client error status response code indicates that the server understood the request but refuses to authorize it.

4.09 is registered by 8132: https://tools.ietf.org/html/rfc8132#section-6
https://tools.ietf.org/html/rfc8132#section-3.4

Conflicting state: If the modification specified by a PATCH or
iPATCH request causes the resource to enter an inconsistent state
that the server cannot resolve, the server can return the 4.09
(Conflict) CoAP response.

[jaimejim]

This case is now handled here: https://core-wg.github.io/coap-pubsub/ps-rev/draft-ietf-core-coap-pubsub.html#name-updating-the-topic-resource