Data Change when server is experiencing issues - surprising result

[david-orn]

I am wondering if the following is a bug or a feature:

My objective is to asynchronously monitor for data changes on the server. I am subscribing to a list of nodes using the following method. where Node is a valid asyncua.Node object:

subscription = await node.create_subscription(250, handler)

What happened is that the server in question was having some issues. I confirmed this by trying to read directly from the node using

value = await node.read_value() 

Which resulted in an error:

asyncua.ua.uaerrors._auto.BadNoCommunication: Communication with the data source is defined, but not established, and there is no last known value available.(BadNoCommunication)

The issue I had was that the subscription method didn't throw a warning or inform that there was an issue with the node. What happened is that within the datachange_notification the node was empty value=0, VariantType=0.

It took me a while to understand this behavior. There are two issues

1. Why isn't the BadNoCommunication error raised as a warning (on client side) when subscribing
2. Why does the datachange_notification get triggered on create_subscription even if there is no change in the data

Many thanks

[oroulet]

Are you sure the error code was not set on the data value you received in subscription?

[david-orn]

Good question. I didn't validate the error code in the data value.
It's not immediately obvious that additional validation is required on the data value. I'll add that to help capture the issue.