P013 OPC UA grouping value changes makes losing some values

[dvribeira]

Hi,

After upgrading to P013, if we have several messages for the same DPE but with different values sent to the WinCC OA OPC UA server very fast (within same millisecond), only the first one was being recorded in PARA (and only one dpConnect triggered). We checked with wireshark and the OPC UA server does receive all the messages and accepts all of them.

Since we had seen in the patch notes the new "maxVcMessageSize" option for this new grouping value changes, we assumed 1 would mean de facto no grouping (BTW, what 0 means is not documented, one would guess no limit?). So we tried setting this option to 1 and then the behaviour is the expected one, we receive all the values.

I think this is not working as designed, so I thought about letting you know. If it is actually working as designed, then I am afraid I am missing something.

Kind regards,
Daniel

[leoknipp]

The config entry you have mentioned is used for the message direction WinCC OPC UA server --> Event Manager.
Did you receive the values changes (in same the milli second) from a OPC UA client or was the value change simulated in WinCC OA?

Best Regards
Leopold Knipp
Senior Support Specialist

[dvribeira]

The values were sent form an OPC UA client, not simulated.

[leoknipp]

If the OPC UA server receives data with the same time stamp it is grouped into one message.

This is a known behavior in WinCC OA (by design) that you get only the first value change if the same dp element + config + attribute is send in the same message.

Is this a real scenario that your OPC UA client sends the same item within the same milli second?

Best Regards
Leopold Knipp
Senior Support Specialist

[dvribeira]

Yes it is a real scenario, that's why we spotted it: some clients might occasionally send 2 or 3 values for the same item very fast (state changes on initialization). The discarding of values is indeed by design Event Manager behaviour, but due to the new grouping done by the OPC UA Server, it potentially affects working OPC UA setups that as said it were working fine before without the grouping mechanism introduced with P013. Ourselves we're happy just putting the value of maxVcMessageSize to 1 (default is 200) so it behaves the same way as before the patch, so no big deal for us. Unfortunately this means setups like ours will not profit from this new feature at least for now (we were already planning to introduce a throttle mechanism into these clients), but since we were not suffering from performance issues either, no harm done.

I hope I might have nevertheless provided some useful feedback to you.

Kind regards,
Daniel

[leoknipp]

It is very uncommon that the same item is updated several times within the same millisecond in a OPC UA client.
Therefore I do not see a problem for most of the applications using the OPC UA server.

Best Regards
Leopold Knipp
Senior Support Specialist