Subscriptions

Subscriptions in OPC UA are asynchronous. That is, the client sends several PublishRequests to the server. The server returns PublishResponses with notifications. But only when a notification has been generated. The client does not wait for the responses and continues normal operations.

Note the difference between Subscriptions and MonitoredItems. Subscriptions are used to report back notifications. MonitoredItems are used to generate notifications. Every MonitoredItem is attached to exactly one Subscription. And a Subscription can contain many MonitoredItems.

The client automatically processes PublishResponses (with a callback) in the background and keeps enough PublishRequests in transit. The PublishResponses may be recieved during a synchronous service call or in UA_Client_runAsync.

/* Callbacks defined for Subscriptions */
typedef void (*UA_Client_DeleteSubscriptionCallback)
    (UA_Client *client, UA_UInt32 subId, void *subContext);

typedef void (*UA_Client_StatusChangeNotificationCallback)
    (UA_Client *client, UA_UInt32 subId, void *subContext,
     UA_StatusChangeNotification *notification);

/* Provides default values for a new subscription.
 *
 * RequestedPublishingInterval:  500.0 [ms]
 * RequestedLifetimeCount: 10000
 * RequestedMaxKeepAliveCount: 10
 * MaxNotificationsPerPublish: 0 (unlimited)
 * PublishingEnabled: true
 * Priority: 0 */
static UA_INLINE UA_CreateSubscriptionRequest
UA_CreateSubscriptionRequest_default(void) {
    UA_CreateSubscriptionRequest request;
    UA_CreateSubscriptionRequest_init(&request);

    request.requestedPublishingInterval = 500.0;
    request.requestedLifetimeCount = 10000;
    request.requestedMaxKeepAliveCount = 10;
    request.maxNotificationsPerPublish = 0;
    request.publishingEnabled = true;
    request.priority = 0;
    return request;
}

UA_CreateSubscriptionResponse
UA_Client_Subscriptions_create(UA_Client *client,
                               const UA_CreateSubscriptionRequest request,
                               void *subscriptionContext,
                               UA_Client_StatusChangeNotificationCallback statusChangeCallback,
                               UA_Client_DeleteSubscriptionCallback deleteCallback);

UA_ModifySubscriptionResponse
UA_Client_Subscriptions_modify(UA_Client *client, const UA_ModifySubscriptionRequest request);

UA_DeleteSubscriptionsResponse
UA_Client_Subscriptions_delete(UA_Client *client,
                               const UA_DeleteSubscriptionsRequest request);

/* Delete a single subscription */
UA_StatusCode
UA_Client_Subscriptions_deleteSingle(UA_Client *client, UA_UInt32 subscriptionId);

static UA_INLINE UA_SetPublishingModeResponse
UA_Client_Subscriptions_setPublishingMode(UA_Client *client,
                                          const UA_SetPublishingModeRequest request) {
    UA_SetPublishingModeResponse response;
    __UA_Client_Service(client, &request,
                        &UA_TYPES[UA_TYPES_SETPUBLISHINGMODEREQUEST], &response,
                        &UA_TYPES[UA_TYPES_SETPUBLISHINGMODERESPONSE]);
    return response;
}

MonitoredItems

MonitoredItems for Events indicate the EventNotifier attribute. This indicates to the server not to monitor changes of the attribute, but to forward Event notifications from that node.

During the creation of a MonitoredItem, the server may return changed adjusted parameters. Check the returned UA_CreateMonitoredItemsResponse to get the current parameters.

/* Provides default values for a new monitored item. */
static UA_INLINE UA_MonitoredItemCreateRequest
UA_MonitoredItemCreateRequest_default(UA_NodeId nodeId) {
    UA_MonitoredItemCreateRequest request;
    UA_MonitoredItemCreateRequest_init(&request);
    request.itemToMonitor.nodeId = nodeId;
    request.itemToMonitor.attributeId = UA_ATTRIBUTEID_VALUE;
    request.monitoringMode = UA_MONITORINGMODE_REPORTING;
    request.requestedParameters.samplingInterval = 250;
    request.requestedParameters.discardOldest = true;
    request.requestedParameters.queueSize = 1;
    return request;
}

The clientHandle parameter can’t be set by the user, any value will be replaced by the client before sending the request to the server.

/* Callback for the deletion of a MonitoredItem */
typedef void (*UA_Client_DeleteMonitoredItemCallback)
    (UA_Client *client, UA_UInt32 subId, void *subContext,
     UA_UInt32 monId, void *monContext);

/* Callback for DataChange notifications */
typedef void (*UA_Client_DataChangeNotificationCallback)
    (UA_Client *client, UA_UInt32 subId, void *subContext,
     UA_UInt32 monId, void *monContext,
     UA_DataValue *value);

/* Callback for Event notifications */
typedef void (*UA_Client_EventNotificationCallback)
    (UA_Client *client, UA_UInt32 subId, void *subContext,
     UA_UInt32 monId, void *monContext,
     size_t nEventFields, UA_Variant *eventFields);

/* Don't use to monitor the EventNotifier attribute */
UA_CreateMonitoredItemsResponse
UA_Client_MonitoredItems_createDataChanges(UA_Client *client,
            const UA_CreateMonitoredItemsRequest request, void **contexts,
            UA_Client_DataChangeNotificationCallback *callbacks,
            UA_Client_DeleteMonitoredItemCallback *deleteCallbacks);

UA_MonitoredItemCreateResult
UA_Client_MonitoredItems_createDataChange(UA_Client *client, UA_UInt32 subscriptionId,
          UA_TimestampsToReturn timestampsToReturn, const UA_MonitoredItemCreateRequest item,
          void *context, UA_Client_DataChangeNotificationCallback callback,
          UA_Client_DeleteMonitoredItemCallback deleteCallback);

/* Monitor the EventNotifier attribute only */
UA_CreateMonitoredItemsResponse
UA_Client_MonitoredItems_createEvents(UA_Client *client,
            const UA_CreateMonitoredItemsRequest request, void **contexts,
            UA_Client_EventNotificationCallback *callback,
            UA_Client_DeleteMonitoredItemCallback *deleteCallback);

UA_MonitoredItemCreateResult
UA_Client_MonitoredItems_createEvent(UA_Client *client, UA_UInt32 subscriptionId,
          UA_TimestampsToReturn timestampsToReturn, const UA_MonitoredItemCreateRequest item,
          void *context, UA_Client_EventNotificationCallback callback,
          UA_Client_DeleteMonitoredItemCallback deleteCallback);

UA_DeleteMonitoredItemsResponse
UA_Client_MonitoredItems_delete(UA_Client *client, const UA_DeleteMonitoredItemsRequest);

UA_StatusCode
UA_Client_MonitoredItems_deleteSingle(UA_Client *client, UA_UInt32 subscriptionId, UA_UInt32 monitoredItemId);

/* The clientHandle parameter will be filled automatically */
UA_ModifyMonitoredItemsResponse
UA_Client_MonitoredItems_modify(UA_Client *client,
                                const UA_ModifyMonitoredItemsRequest request);

The following service calls go directly to the server. The MonitoredItem settings are not stored in the client.

static UA_INLINE UA_SetMonitoringModeResponse
UA_Client_MonitoredItems_setMonitoringMode(UA_Client *client,
                                           const UA_SetMonitoringModeRequest request) {
    UA_SetMonitoringModeResponse response;
    __UA_Client_Service(client,
                        &request, &UA_TYPES[UA_TYPES_SETMONITORINGMODEREQUEST],
                        &response, &UA_TYPES[UA_TYPES_SETMONITORINGMODERESPONSE]);
    return response;
}

static UA_INLINE UA_SetTriggeringResponse
UA_Client_MonitoredItems_setTriggering(UA_Client *client,
                                       const UA_SetTriggeringRequest request) {
    UA_SetTriggeringResponse response;
    __UA_Client_Service(client,
                        &request, &UA_TYPES[UA_TYPES_SETTRIGGERINGREQUEST],
                        &response, &UA_TYPES[UA_TYPES_SETTRIGGERINGRESPONSE]);
    return response;
}

#endif