Server events

[EliasVerstappe]

Hi,

I am using this library for a university project.
Currently, I am developing my project in Docker, which works great.
I have one container where a server is running, and another container with a client.
The client changes a value in the server with the '.set_value()' method.
I was wondering if it's possible to have the server act on a data change in its address space.
(Client sets value in server, server notices that a value has been set and performs some king of action.)

Here I have a diagram:

One solution is to have an extra client on the servers host subscribe to that node in the server, and perform the desired action in the datachange_notification SubHandler function. I was just wondering if the server could do this without needing an extra client. It seems kind of redundant to make a client on host 1, just to perform an action when the value of a variable node in the server changes.

The thing I need is some kind of event handler on the server, that triggers when a given node's value changes.

I have seen the 'server_events' example but it's not clear to me what it does. Information about this topic is sparse on the internet. Maybe I just want to do something that is not possible.

Does anyone have any info / tips for me?

Thanks in advance,
Kind regards

[swamper123]

Hey Elias,

the line you may looking for is this one:

python-opcua/opcua/server/address_space.py

Line 687 in 0d595e5

def add_datachange_callback(self, nodeid, attr, callback):

With these you can add a callback, in case something changes, to your Node (even for other attributes, but value is the common one). You can even add more than one callback. You may have to use partial ´, depending on what kind of action you want to do.

Keep in mind, that these functions shouldn't consume much time. Like in SubHandler, they will otherwise block the server.
Keep also in mind, that this is not OPC UA intended, it is just a way to do "crazy" stuff in python.
Keep also also in mind, that opcua-asyncio is the actual version, since this repo is in maintainance mode (there was some stuff fixes there, what isn't fixed here).

Cheers!

[EliasVerstappe]

Thank you very much for the feedback @swamper123 !

[zerox1212]

You don't need to create a client. The server has nearly all the same methods that a client has because much of it is shared between them. The server can use the methods in the node class or create subscriptions on its own.

[EliasVerstappe]

Interesting, will look into it, thanks!