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 he is interested in.

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_NODEID_NUMERIC(0, UA_NS0ID_BASEEVENTTYPE),
                                       UA_NODEID_NUMERIC(0, UA_NS0ID_HASSUBTYPE),
                                       UA_QUALIFIEDNAME(0, "SimpleEventType"),
                                       attr, NULL, &eventType);

Setting up 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
setUpEvent(UA_Server *server, UA_NodeId *outId) {
    UA_StatusCode retval = UA_Server_createEvent(server, eventType, outId);
    if (retval != UA_STATUSCODE_GOOD) {
                       "createEvent failed. StatusCode %s", UA_StatusCode_name(retval));
        return retval;

    /* Set the Event Attributes */
    /* Setting the Time is required or else the event will not show up in UAExpert! */
    UA_DateTime eventTime = UA_DateTime_now();
    UA_Server_writeObjectProperty_scalar(server, *outId, UA_QUALIFIEDNAME(0, "Time"),
                                         &eventTime, &UA_TYPES[UA_TYPES_DATETIME]);

    UA_UInt16 eventSeverity = 100;
    UA_Server_writeObjectProperty_scalar(server, *outId, UA_QUALIFIEDNAME(0, "Severity"),
                                         &eventSeverity, &UA_TYPES[UA_TYPES_UINT16]);

    UA_LocalizedText eventMessage = UA_LOCALIZEDTEXT("en-US", "An event has been generated.");
    UA_Server_writeObjectProperty_scalar(server, *outId, UA_QUALIFIEDNAME(0, "Message"),
                                         &eventMessage, &UA_TYPES[UA_TYPES_LOCALIZEDTEXT]);

    UA_String eventSourceName = UA_STRING("Server");
    UA_Server_writeObjectProperty_scalar(server, *outId, UA_QUALIFIEDNAME(0, "SourceName"),
                                         &eventSourceName, &UA_TYPES[UA_TYPES_STRING]);


Triggering an event

First a node representing an event is generated using setUpEvent. Once our event is good to go, we specify a node which emits the event - in this case the server node. We can use UA_Server_triggerEvent to trigger our event onto said node. Passing NULL as the second-last argument means we will not receive the EventId. The last boolean argument states whether the node should be deleted.

static UA_StatusCode
generateEventMethodCallback(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");

    /* set up event */
    UA_NodeId eventNodeId;
    UA_StatusCode retval = setUpEvent(server, &eventNodeId);
    if(retval != UA_STATUSCODE_GOOD) {
                       "Creating event failed. StatusCode %s", UA_StatusCode_name(retval));
        return retval;

    retval = UA_Server_triggerEvent(server, eventNodeId,
                                    UA_NODEID_NUMERIC(0, UA_NS0ID_SERVER),
                                    NULL, UA_TRUE);
    if(retval != UA_STATUSCODE_GOOD)
                       "Triggering event failed. StatusCode %s", UA_StatusCode_name(retval));

    return retval;

Now, all that is left to do is to create a method node which uses our callback. We do not require any input and as output we will be using the EventId we receive from triggerEvent. The EventId is generated by the server internally and is a random unique ID which identifies that specific event.

This method node will be added to a basic server setup.

static void
addGenerateEventMethod(UA_Server *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_NODEID_NUMERIC(0, UA_NS0ID_OBJECTSFOLDER),
                            UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                            UA_QUALIFIEDNAME(1, "Generate Event"),
                            generateAttr, &generateEventMethodCallback,
                            0, NULL, 0, NULL, NULL, NULL);

It follows the main server code, making use of the above definitions.

static volatile UA_Boolean running = true;
static void stopHandler(int sig) {
    running = false;

int main (void) {
    /* default server values */
    signal(SIGINT, stopHandler);
    signal(SIGTERM, stopHandler);

    UA_Server *server = UA_Server_new();


    UA_StatusCode retval = UA_Server_run(server, &running);