Generating events

To make sense of the many things going on in a server, monitoring items can be useful. Though in many cases, data change does not convey enough information to be the optimal solution. Events can be generated at any time, hold a lot of information and can be filtered so the client only receives the specific attributes of interest.

Emitting events by calling methods

The following example will be based on the server method tutorial. We will be creating a method node which generates an event from the server node.

The event we want to generate should be very simple. Since the BaseEventType is abstract, we will have to create our own event type. EventTypes are saved internally as ObjectTypes, so add the type as you would a new ObjectType.

static UA_NodeId eventType;

static UA_StatusCode
addNewEventType(UA_Server *server) {
    UA_ObjectTypeAttributes attr = UA_ObjectTypeAttributes_default;
    attr.displayName = UA_LOCALIZEDTEXT("en-US", "SimpleEventType");
    attr.description = UA_LOCALIZEDTEXT("en-US", "The simple event type we created");
    return UA_Server_addObjectTypeNode(server, UA_NODEID_NULL,
                                       UA_NS0ID(BASEEVENTTYPE), UA_NS0ID(HASSUBTYPE),
                                       UA_QUALIFIEDNAME(0, "SimpleEventType"),
                                       attr, NULL, &eventType);
}

Creating an event

In order to set up the event, we can first use UA_Server_createEvent to give us a node representation of the event. All we need for this is our EventType. Once we have our event node, which is saved internally as an ObjectNode, we can define the attributes the event has the same way we would define the attributes of an object node. It is not necessary to define the attributes EventId, ReceiveTime, SourceNode or EventType since these are set automatically by the server. In this example, we will be setting the fields ‘Message’ and ‘Severity’ in addition to Time which is needed to make the example UaExpert compliant.

static UA_StatusCode
createEventMethodCallback(UA_Server *server,
                          const UA_NodeId *sessionId, void *sessionHandle,
                          const UA_NodeId *methodId, void *methodContext,
                          const UA_NodeId *objectId, void *objectContext,
                          size_t inputSize, const UA_Variant *input,
                          size_t outputSize, UA_Variant *output) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND, "Creating event");

    UA_UInt16 severity = 100;
    UA_LocalizedText message = UA_LOCALIZEDTEXT("en-US", "An event has been generated.");
    UA_StatusCode res = UA_Server_createEvent(server, UA_NS0ID(SERVER), eventType,
                                              severity, message, NULL, NULL, NULL);
    if(res != UA_STATUSCODE_GOOD) {
        UA_LOG_WARNING(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND,
                       "Creating event failed. StatusCode %s",
                       UA_StatusCode_name(res));
    }
    return res;
}

Now, all that is left to do is to create a method node which triggers the event creation for the example.

int main(void) {
    UA_Server *server = UA_Server_new();

    addNewEventType(server);

    UA_MethodAttributes generateAttr = UA_MethodAttributes_default;
    generateAttr.description = UA_LOCALIZEDTEXT("en-US","Generate an event.");
    generateAttr.displayName = UA_LOCALIZEDTEXT("en-US","Generate Event");
    generateAttr.executable = true;
    generateAttr.userExecutable = true;
    UA_Server_addMethodNode(server, UA_NODEID_NUMERIC(1, 62541),
                            UA_NS0ID(OBJECTSFOLDER), UA_NS0ID(HASCOMPONENT),
                            UA_QUALIFIEDNAME(1, "Generate Event"),
                            generateAttr, &createEventMethodCallback,
                            0, NULL, 0, NULL, NULL, NULL);

    UA_Server_runUntilInterrupt(server);
    UA_Server_delete(server);
    return 0;
}