Custom PubSub Message Handling

The PubSub Stack and SDK integration allows custom PubSub message handling. Such custom message handling is mainly necessary for real-time communication parts. Since the OPC UA server is typically running with lower priorities than real-time components, also the PubSub communication is executed in the same priority.

OPC UA PubSub has different transport profiles (message mapping and transport protocol mapping). Most of them do not need custom handling and the default SDK implementation can be used. Such an example is UADP over MQTT or UADP over UDP multi-cast.

If transport profile like UADP over Ethernet has stricter timing requirements, the communication needs typically higher priority and optimized processing. Such requirements can be fulfilled by doing the following custom implementations:

• Custom data integration to encode the data to be sent by a DataSetWriter directly from the data source into the network message or to decode the data received by a DataSetReader directly from the network message to the data target.
• Custom timing of WriterGroup for sampling and publishing of network message at the configured offsets
• Custom network integration for optimized buffer management, prioritized sending and receiving of network messages

These three custom handlers are typically implemented together.

For the custom data integration and custom timing, the SDK provides a callback interface where the application is called for every PubSub object during the start-up of the PubSub configuration. For each DataSetWriter, DataSetReader and WriterGroup, the application can decide if it wants to handle the processing instead of the default processing provided by the SDK. The UaServer_PubSub_SetCallback function is used to register the callback interface.

• For each DataSetWriter in the configuration the callback UaServer_pfServer_PubSub_WriterData is called. If the application provides a callback, the callback UaServer_pfServer_PubSub_WriterDataCB is always called if the PubSub stack creates the network message that contains the DataSet message for the DataSetWriter
• For each DataSetReader in the configuration the callback UaServer_pfServer_PubSub_ReaderData is called. If the application provides a callback, the callback UaServer_pfServer_PubSub_ReaderDataCB is always called if the PubSub stack receives a network message that contains a DataSet message for the DataSetReader
• For each WriterGroup in the configuration the callback UaServer_pfServer_PubSub_WriterGroupProcessing is called. If the application handles the timing for a WriterGroup, the application is responsible for the cyclic call of UaServer_PubSub_WriterGroup_Sample for the creation of the network message and UaServer_PubSub_WriterGroup_Publish for the sending of the network message. The functions must be called for each handled WriterGroup in each publishing interval. For exact timing the WriterGroup setting SamplingOffset is used for the timing of UaServer_PubSub_WriterGroup_Sample. The WriterGroup setting PublishingOffset is used for UaServer_PubSub_WriterGroup_Publish.

The following sample code can be found in the file demoserver.c in the function ServerMain of the ANSI C SDK Demo Server.

UaServer_PubSubCallbackInterface pubSubCbInterface;

OpcUa_MemSet(&pubSubCbInterface, 0, sizeof(pubSubCbInterface));
pubSubCbInterface.WriterData = PubSub_WriterData;
pubSubCbInterface.ReaderData = PubSub_ReaderData;
pubSubCbInterface.WriterGroupProcessing = PubSub_WriterGroupProcessing;
uStatus = UaServer_PubSub_SetCallback(&g_UaServer, &pubSubCbInterface);
OpcUa_GotoErrorIfBad(uStatus);

The file demoserver.c in the ANSI C SDK Demo Server contains also sample code for the callback implementations.