There is some way to subscribe to Orion (e.g. Car entity), without receive the old entities?
Example: Orion has -> Car A, Car B.
I do onchange subscription and Orion sends me at same time: Car A notification and Car B notification.
We need the following:
Orion has -> Car A, Car B.
I do onchange subscription but don't receive nothing at this moment. If in a future Orion receives Car B, or changes some attribute of Car A or Car B, then send the notification.
Is that possible?
The behaviour is explained in the the user manual:
You may wonder why accumulator-server.py is getting this message if you don't actually do any update. This is because the Orion Context Broker considers the transition from "non existing subscription" to "subscribed" as a change.
We understand that for some uses cases this is not convenient. However, behaving in the opossite way ruins another uses cases which need to know the "inicial state" before starting getting notifications corresponding to actual changes. The best solution to make everybody happy is to make this configurable, so each client can chose what it prefers. This feature is currently in our roadmap (see this issue in github.com).
While this gets implemented in Orion, in your case maybe a possible workaround is just ignore the first received nofitication belonging to a subscription (you can identify the subscription to which one notification belongs by the subscriptionId field in the notification payload). All the following notifications beloning to that subscription will correspond to actual changes.
EDIT: the posibility of avoiding initial notification has been finally implemented at Orion. Details are at this section of the documentation. It is now in the master branch (so if you use fiware/orion:latest docker you will get it) and will be include in next Orion version (2.2.0).