Hey folks,
I wanted to share, and also source your ideas on tag syntax for OPC UA.
I've managed to add missing messages and a bit of PoC logic to emulate
event subscription. It seems to work, now its time to adjust
subscription handling logic.
In principle OPC UA specification defines 3 kinds of notifications [1]:
1) Data Change
2) Event
3) Status Change
These have different calls to submit request and deliver different
information. Before we go deeper, remember that in OPC UA everything is
based on request/response paradigm. There is no push side per say, we
send a publish request to receive notification message.
What our Java driver currently cover is data change, so its more or less
cyclic update / data change. The event notification requires pointing
out what fields of event are interesting (i.e Id, Type, Time, Message),
we then receive them. Still, event notification have to be hooked at
specific NodeId, Milo's examples use Server (ns=0;i=2253) and
EventNotifier (12 / 0x0c) attribute, whereas data change aim Value (13 /
0x0d) attribute. These of you who are familiar with OPC-UA can see some
similarities due to multiple levels of complexity (object vs its property).
Interesting animal in this zoo is status change which can be used to
receive information about bad signal quality (whatever it means).
Out of these 3 notification kinds data and status change are pretty much
1:1 (node id -> value, node id -> status), so each subscribed tag
returns single value. The metadata concept [2] opens possibility to
return more information to caller, including status.
The event notification however is different, because first and foremost,
we subscribe static node id (not sure if it can change), and do not send
node id but browse path which is just a text:
new SimpleAttributeOperand(nodeId,
Arrays.asList(new QualifiedName(0, new PascalString >>("EventId")<<)),
AttributeId.Value.getValue(),
OpcuaProtocolLogic.NULL_STRING
)
Additionally, there might be filter criteria [3] (I didn't try yet),
which can be used to supply more conditions for receiving data.
Coming to the point - what do you think about defining new tag type for
OPC UA to cover event fields? With this we could use this tag to specify
event fields (browse path), and eventually use tag attributes for criteria:
.addEventTagAddress("EventId;REAL")
.addEventTagAddress("ns=0;i=2253;EventId;REAL") // same as above
.addEventTagAddress("Time;DATETIME")
.addEventTagAddress("Message;STRING{like=Fatal error}")
In above example the default subscribe is assumed to be ns=0;i=2253, but
each event tag address would require a path.
WDYT?
Best,
Łukasz
[1] https://reference.opcfoundation.org/Core/Part4/v104/docs/7.20
[2] https://github.com/apache/plc4x/pull/1725
[3] https://reference.opcfoundation.org/Core/Part4/v104/docs/7.4.4
