Thing Description TTL

A TD may have a (non-normative) time-to-live (TTL) value to enable automatic purging by the directory. The TTL is optional, given in seconds as the value of ttl attribute on the top level of the TD.

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 and uses modified attribute of the TD during first registration and follow-up updates. The directory keeps TDs as long as the current time is larger than modified+TTL. The consumers may rely on TTL to know when this registration becomes obsolete (i.e. modified+TTL), 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.