Catch NULL values in messaging protocol

[mkrebs81]

Situation

A messaging (MQTT) device receives data on a subscribed data point.

When the message transformation results in a string value of "null", e.g. when the requested JSON key has no value or actually has null as value, the data point value cannot be updated due to conversion error.

The error message is For input string: "null", indicating that it could not be converted to a float, int or whatever.

Options

What to do about it?

a) keep exception
b) check for NULL value in SGrMessagingDevice and do not execute callback/consumer in case of a NULL value

[mkrebs81]

@ergo-furrer the current state seems to work more or less in the testlab environment.
When you have some time left, please have a look at it.

[ergo-furrer]

@mkrebs81
Indeed, the Problem arises whenever the received value cannot be converted to the data Format expected by the Generic interface (e.g. Numbers or Enums).

Probably it is a good idea to add some response-validation to the SGrMessagingDevice implementation in order to report the cause of the error more accurately such as "invalid message received, received value is not a number...".

Regarding, whether we let the user know when invalid data is received or we just skip the received message:
I think the user should get to know when invalid data is received.

• When reading a value synchronously getVal(): it would be suitable to throw an exception, however i think it is better when getVal() throws and not Value.getFloat32() for example (means some additional validation within the SGrMessagingDevice).

• When reading asynchrounously using subscribe() we should callback with an Either.left() that contains the Exception containing the validation error. It is up to the client-user then to decide how to handle the Error (or ignore it ...)

• Regarding caching the last read value: I think we should not chache the invalid value (currently the invalid value is cached).