[core] draft-groves-core-dynlink-02

[Michael Koster <[e-mail]>]

Hi Christian,

I have been thinking about the use case that we exposed in OCF recently, integrating pubsub and other cloud services into a system of existing server-based sensors.  Currently there is no well defined way to get a sensor device to push changes to a broker or to get an actuator device to subscribe to a topic on a broker. Likewise, there is on way to instruct the broker to observe a resource on a sensor, or to instruct a broker to update an actuator when a topic is published to on the broker.

A binding table, on the devices or on the broker, or both, is a good solution for this and enables the system-wide state update relationships to be maintained in a visible, manageable way.

I have a couple of comments on the current draft.

1. Why don't we use a resource type for the binding table? Its methods and responses conform to a generic link list interface, but the content and side effects are specialized to bindings. I thought we earlier had used rt=core.bnd as a resource type to identify the binding table.

2. We could make the example explicitly name the href of the sensor "switch" and the actuator "light" to help illustrate the directional pattern of observe the link target and update the link context:

   <coap://sensor.example.com/s/switch>;rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"

3. I think we will need to register the new link attribute names with IANA: "pmin", "pmax", "lth", "gth", "stp", "bind"

4. Speaking of attributes, I don't think "bind" is strictly needed to identify whether to push or pull; that is implied by whether the anchor and href are local or remote. I can conceive of a case where the binding both observes and pushes:

   <coap://example.com/pubsub/switch>;rel="boundto";anchor="coap://example.com/pubsub/light";pmin="10";pmax="60"

We may instead want to identify what operations are used for both source (link target/href) and destination (link context/anchor), perhaps change it to the explicit: src=obs; dst=push with default being observe used on the source and update (push) used on the target. 

Best regards,

Michael