Thing Description TTL

A TD may have a (non-normative) ttl (time-to-live) value to enable automatic purging by the directory. The ttl is optional and given in seconds.

The ttl is the lifespan of the TD registration after which the directory removes the entry in the absence of any keepalive request.

Internally, the directory sets modified attribute of the TD during first registration and follow-up updates. The directory keeps TDs as long as the current time is smaller than expiry=modified+ttl. The consumers may rely on the expiry to know when this registration becomes obsolete, after which they have to discover the TD again to get the up-to-date metadata.

The keepalive request should be sent by the registrator via PUT (full TD) or PATCH (partial TD / no body) request over the RESTful API. The PATCH support is WIP.