opc ua - OPC UA TranslateBrowsePathsToNodeIds: How stable/volatile are resolved NodeIDs

Question

there is a OPC UA service TranslateBrowsePathsToNodeIds.

How volatile or stable are the NodeIDs results returned by the service --- Is it possible that NodeIDs resolved by the service alter while server runtime?

In our Case we are running our client against a server, which enumerates NodeIDs differently on every server restart. So we decide to resolve IDs by BrowsePath once after each (re)connect to the server. So far so good. But now there is a Problem when the server is restartet and after the first client reconnect no paths can't resolved.. it seems the server is not ready for the job (...).

I am wondering how often I have to update the resolved Node IDs (all or just the onces which was not resolved correctly?).

We are reading data by polling in this case.

Answer 1

I don't know your server, but it seems, you do address an OPC UA Server with a very dynamic behaviour, But I think, it's within the OPC UA spec.

It depends on the OPC UA objects, this server offers and what a client should or wants do with them.

Is it possible that NodeIDs resolved by the service alter while server runtime?

Yes!

Many servers do offer a static node set from the device configuration. In this case, the client will always get the same node ids from TranslateBrowsePathsToNodeIds, the client may use the node ids directly without translation (but has always to resolve the namespace index after connecting to the server). But obviously it's not your kind of server.

Another use case are dynamic objects, with a restricted lifetime, coming and going on any time they want to come an go. Imagine a warehouse in which goods are received and delivered. So a good will be representent with an individual OPC UA object instance, created on receive and removed on delivery. As long the good exists in the warehouse, you may browse to the object, translate its browse pathes to node ids and use them (reading, writing, monitoring). Of course, when the good delivers, the UA object will no longer be needed and could be removed by the server. The node ids removed now could be used later in an other context for another object instance.

Alternatively, the server could offer an interface to create or remove objects with a client.

The question is, how the server deals with the creation or removal of it's objects, how does it announce the changes? There should be a kind of configuration change event you want to monitor with your client. When your client receives an event, it may decide which actions to do, e.g. rebrowse the object and translate the browse pathes. Read the manual!

By the way: TranslateBrowsPathsToNodeIds is widly used to adress subnodes of object instances with a known object type. Once you know the node id for the entire object, you easily get the node ids for the subnodes with their browse pathes.