TD Probing upon registration

[egekorkan]

Coming from w3c/wot-testing#189 (also related to w3c/wot-thing-description#1244).

A directory should do validation of the TD document according to the spec however nothing is said whether it should also test the Thing behind the TD. It could be that someone registers nonexisting Thing TDs which would confuse the Consumers of these TDs. I don't see this as a mandatory feature but if a directory tests the Things by probing them upon registration, I would trust this directory more as a consumer. When some directories do that, can they somehow mention this in their TDs? As in, "If you choose to register your TDs in this directory, all of their interactions will be tested by this endpoint".

[farshidtz]

This has similar challenges to polling of TDs by a directory or healthchecks; see #164 and #18 (comment).

In particular:

• How to authorize the directory to access those endpoints?
• What about devices that are behind a firewall and not directly accessible from the directory?
• What if the device is sleeping/offline during the registration?

[egekorkan]

These are all valid points. I think that this should be an optional feature anyway and only when the directory can invoke the endpoints successfully, it would put test failed or not. If no communication can be established, no need to enrich the TD, which is the current case. For our use case, the directory is in the same network as the devices (which never sleep) and can easily test all of them. The points above are valid but not necessarily blockers in 100% of the cases.