Is OPC UA Read and Write operations atomic? - opc-ua

Question

In other words, is it possible for a server or a client to receive partial data before the transmission is finished?
I am sure this information is written somewhere in the extensive documentation of the OPC Foundation, but I think this is essential.
I am using the "atomic" phrase as used for database writes. When a programmer updates a table, the update is always atomic in the sense that it is either done or not done. We rely on the DB software for guaranteeing against the operation being only partially successful.
In the case of a PLC acting as an OPC UA server; when the client writes say 1 KByte data, are we sure that the program running on the PLC, on any instant read a part of this data while it is still being communicated and written? Since we are dealing with a very fast reading entity (the PLC) on the other side of the communication, is it possible that the PLC gets the first 100 bytes before the rest is received?

Answer 1

In the case of a PLC acting as an OPC UA server; when the client writes say 1 KByte data, are we sure that the program running on the PLC, on any instant read a part of this data while it is still being communicated and written?
You can be certain that any server will not be processing any read or write until the request has been fully transmitted by the client. There is no facility in OPC UA for dealing with streaming/partial requests from clients.
That said, how the server handles that 1KB of data it just received as a write is not covered by spec. There is no guarantee that it is written atomically to whatever the backing/underlying datasource may be (in memory, shared memory, a file, another device on the network, etc...).
I do believe that most OPC UA servers built in to PLCs are probably doing the right thing to ensure atomicity but there is nothing in the spec that guarantees it and no way to be sure other than contacting that vendor or consulting documentation.

Answer 2

Beckhoff OPC-UA Server communicates with the Beckhoff PLC through the ADS protocol.
The default Max Size of the consistent data sent with the help of the ads router is 16 kByte although it can be changed if needed.
This is important to understand, because the OPC-UA Server is not part of the PLC runtime environment.
When an OPC-UA client writes a node of an OPC-UA Server, the OPC-UA Server sends this data to the PLC.
In the Twincat development environment you have the option to declare a special attribute for structured types:
{attribute 'OPC.UA.DA.StructuredType' := '1'}
This tells the Beckhoff OPC-UA Server to send the data concerning that specific data structure in a consistent way to the PLC when it receives it from a client.
The Beckhoff OPC-UA documentation states:
"StructuredTypes allow you to read or write structures without interpreting each byte, because the UA Server
returns the information type of each element of the structure. Based on complex functions in modern
OPC UA SDKs, OPC UA Clients can search and interpret this structural information."
Therefor, regarding data consistency, it is also important that your OPC-UA SDK (client) is modern enough to be able to "search and interpret this structural information".

Answer 3

The answer is provided by Flippo & Kevin in the above comments. So this will be a recap for everyone to quickly spot:
OPC UA guarantees atomicity only for simple types (Integer, Double Integer etc)
Longer structures' atomicity depends on the specific manufacturer.