Working with Objects and Object Types

Using objects to structure information models

Assume a situation where we want to model a set of pumps and their runtime state in an OPC UA information model. Of course, all pump representations should follow the same basic structure, For example, we might have graphical representation of pumps in a SCADA visualisation that shall be resuable for all pumps.

Following the object-oriented programming paradigm, every pump is represented by an object with the following layout:

digraph tree {

fixedsize=true;
node [width=2, height=0, shape=box, fillcolor="#E5E5E5", concentrate=true]

node_root [label=< <I>ObjectNode</I><BR/>Pump >]

{ rank=same
  point_1 [shape=point]
  node_1 [label=< <I>VariableNode</I><BR/>ManufacturerName >] }
node_root -> point_1 [arrowhead=none]
point_1 -> node_1 [label="hasComponent"]

{ rank=same
  point_2 [shape=point]
  node_2 [label=< <I>VariableNode</I><BR/>ModelName >] }
point_1 -> point_2 [arrowhead=none]
point_2 -> node_2 [label="hasComponent"]

{  rank=same
   point_4 [shape=point]
   node_4 [label=< <I>VariableNode</I><BR/>Status >] }
point_2 -> point_4 [arrowhead=none]
point_4 -> node_4 [label="hasComponent"]

{  rank=same
   point_5 [shape=point]
   node_5 [label=< <I>VariableNode</I><BR/>MotorRPM >] }
point_4 -> point_5 [arrowhead=none]
point_5 -> node_5 [label="hasComponent"]

}

The following code manually defines a pump and its member variables. We omit setting constraints on the variable values as this is not the focus of this tutorial and was already covered.

#include <open62541/plugin/log_stdout.h>
#include <open62541/server.h>
#include <open62541/server_config_default.h>

#include <signal.h>

static void
manuallyDefinePump(UA_Server *server) {
    UA_NodeId pumpId; /* get the nodeid assigned by the server */
    UA_ObjectAttributes oAttr = UA_ObjectAttributes_default;
    oAttr.displayName = UA_LOCALIZEDTEXT("en-US", "Pump (Manual)");
    UA_Server_addObjectNode(server, UA_NODEID_NULL,
                            UA_NODEID_NUMERIC(0, UA_NS0ID_OBJECTSFOLDER),
                            UA_NODEID_NUMERIC(0, UA_NS0ID_ORGANIZES),
                            UA_QUALIFIEDNAME(1, "Pump (Manual)"), UA_NODEID_NUMERIC(0, UA_NS0ID_BASEOBJECTTYPE),
                            oAttr, NULL, &pumpId);

    UA_VariableAttributes mnAttr = UA_VariableAttributes_default;
    UA_String manufacturerName = UA_STRING("Pump King Ltd.");
    UA_Variant_setScalar(&mnAttr.value, &manufacturerName, &UA_TYPES[UA_TYPES_STRING]);
    mnAttr.displayName = UA_LOCALIZEDTEXT("en-US", "ManufacturerName");
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "ManufacturerName"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), mnAttr, NULL, NULL);

    UA_VariableAttributes modelAttr = UA_VariableAttributes_default;
    UA_String modelName = UA_STRING("Mega Pump 3000");
    UA_Variant_setScalar(&modelAttr.value, &modelName, &UA_TYPES[UA_TYPES_STRING]);
    modelAttr.displayName = UA_LOCALIZEDTEXT("en-US", "ModelName");
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "ModelName"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), modelAttr, NULL, NULL);

    UA_VariableAttributes statusAttr = UA_VariableAttributes_default;
    UA_Boolean status = true;
    UA_Variant_setScalar(&statusAttr.value, &status, &UA_TYPES[UA_TYPES_BOOLEAN]);
    statusAttr.displayName = UA_LOCALIZEDTEXT("en-US", "Status");
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "Status"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), statusAttr, NULL, NULL);

    UA_VariableAttributes rpmAttr = UA_VariableAttributes_default;
    UA_Double rpm = 50.0;
    UA_Variant_setScalar(&rpmAttr.value, &rpm, &UA_TYPES[UA_TYPES_DOUBLE]);
    rpmAttr.displayName = UA_LOCALIZEDTEXT("en-US", "MotorRPM");
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "MotorRPMs"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), rpmAttr, NULL, NULL);
}

Object types, type hierarchies and instantiation

Building up each object manually requires us to write a lot of code. Furthermore, there is no way for clients to detect that an object represents a pump. (We might use naming conventions or similar to detect pumps. But that’s not exactly a clean solution.) Furthermore, we might have more devices than just pumps. And we require all devices to share some common structure. The solution is to define ObjectTypes in a hierarchy with inheritance relations.

digraph tree {

fixedsize=true;
node [width=2, height=0, shape=box, fillcolor="#E5E5E5", concentrate=true]

node_root [label=< <I>ObjectTypeNode</I><BR/>Device >]

{ rank=same
  point_1 [shape=point]
  node_1 [label=< <I>VariableNode</I><BR/>ManufacturerName<BR/>(mandatory) >] }
node_root -> point_1 [arrowhead=none]
point_1 -> node_1 [label="hasComponent"]

{ rank=same
  point_2 [shape=point]
  node_2 [label=< <I>VariableNode</I><BR/>ModelName >] }
point_1 -> point_2 [arrowhead=none]
point_2 -> node_2 [label="hasComponent"]

{  rank=same
   point_3 [shape=point]
   node_3 [label=< <I>ObjectTypeNode</I><BR/>Pump >] }
point_2 -> point_3 [arrowhead=none]
point_3 -> node_3 [label="hasSubtype"]

{  rank=same
   point_4 [shape=point]
   node_4 [label=< <I>VariableNode</I><BR/>Status<BR/>(mandatory) >] }
node_3 -> point_4 [arrowhead=none]
point_4 -> node_4 [label="hasComponent"]

{  rank=same
   point_5 [shape=point]
   node_5 [label=< <I>VariableNode</I><BR/>MotorRPM >] }
point_4 -> point_5 [arrowhead=none]
point_5 -> node_5 [label="hasComponent"]

}

Children that are marked mandatory are automatically instantiated together with the parent object. This is indicated by a hasModellingRule reference to an object that representes the mandatory modelling rule.

/* predefined identifier for later use */
UA_NodeId pumpTypeId = {1, UA_NODEIDTYPE_NUMERIC, {1001}};

static void
defineObjectTypes(UA_Server *server) {
    /* Define the object type for "Device" */
    UA_NodeId deviceTypeId; /* get the nodeid assigned by the server */
    UA_ObjectTypeAttributes dtAttr = UA_ObjectTypeAttributes_default;
    dtAttr.displayName = UA_LOCALIZEDTEXT("en-US", "DeviceType");
    UA_Server_addObjectTypeNode(server, UA_NODEID_NULL,
                                UA_NODEID_NUMERIC(0, UA_NS0ID_BASEOBJECTTYPE),
                                UA_NODEID_NUMERIC(0, UA_NS0ID_HASSUBTYPE),
                                UA_QUALIFIEDNAME(1, "DeviceType"), dtAttr,
                                NULL, &deviceTypeId);

    UA_VariableAttributes mnAttr = UA_VariableAttributes_default;
    mnAttr.displayName = UA_LOCALIZEDTEXT("en-US", "ManufacturerName");
    UA_NodeId manufacturerNameId;
    UA_Server_addVariableNode(server, UA_NODEID_NULL, deviceTypeId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "ManufacturerName"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), mnAttr, NULL, &manufacturerNameId);
    /* Make the manufacturer name mandatory */
    UA_Server_addReference(server, manufacturerNameId,
                           UA_NODEID_NUMERIC(0, UA_NS0ID_HASMODELLINGRULE),
                           UA_EXPANDEDNODEID_NUMERIC(0, UA_NS0ID_MODELLINGRULE_MANDATORY), true);


    UA_VariableAttributes modelAttr = UA_VariableAttributes_default;
    modelAttr.displayName = UA_LOCALIZEDTEXT("en-US", "ModelName");
    UA_Server_addVariableNode(server, UA_NODEID_NULL, deviceTypeId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "ModelName"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), modelAttr, NULL, NULL);

    /* Define the object type for "Pump" */
    UA_ObjectTypeAttributes ptAttr = UA_ObjectTypeAttributes_default;
    ptAttr.displayName = UA_LOCALIZEDTEXT("en-US", "PumpType");
    UA_Server_addObjectTypeNode(server, pumpTypeId,
                                deviceTypeId, UA_NODEID_NUMERIC(0, UA_NS0ID_HASSUBTYPE),
                                UA_QUALIFIEDNAME(1, "PumpType"), ptAttr,
                                NULL, NULL);

    UA_VariableAttributes statusAttr = UA_VariableAttributes_default;
    statusAttr.displayName = UA_LOCALIZEDTEXT("en-US", "Status");
    statusAttr.valueRank = UA_VALUERANK_SCALAR;
    UA_NodeId statusId;
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpTypeId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "Status"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), statusAttr, NULL, &statusId);
    /* Make the status variable mandatory */
    UA_Server_addReference(server, statusId,
                           UA_NODEID_NUMERIC(0, UA_NS0ID_HASMODELLINGRULE),
                           UA_EXPANDEDNODEID_NUMERIC(0, UA_NS0ID_MODELLINGRULE_MANDATORY), true);

    UA_VariableAttributes rpmAttr = UA_VariableAttributes_default;
    rpmAttr.displayName = UA_LOCALIZEDTEXT("en-US", "MotorRPM");
    rpmAttr.valueRank = UA_VALUERANK_SCALAR;
    UA_Server_addVariableNode(server, UA_NODEID_NULL, pumpTypeId,
                              UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT),
                              UA_QUALIFIEDNAME(1, "MotorRPMs"),
                              UA_NODEID_NUMERIC(0, UA_NS0ID_BASEDATAVARIABLETYPE), rpmAttr, NULL, NULL);
}

Now we add the derived ObjectType for the pump that inherits from the device object type. The resulting object contains all mandatory child variables. These are simply copied over from the object type. The object has a reference of type hasTypeDefinition to the object type, so that clients can detect the type-instance relation at runtime.

static void
addPumpObjectInstance(UA_Server *server, char *name) {
    UA_ObjectAttributes oAttr = UA_ObjectAttributes_default;
    oAttr.displayName = UA_LOCALIZEDTEXT("en-US", name);
    UA_Server_addObjectNode(server, UA_NODEID_NULL,
                            UA_NODEID_NUMERIC(0, UA_NS0ID_OBJECTSFOLDER),
                            UA_NODEID_NUMERIC(0, UA_NS0ID_ORGANIZES),
                            UA_QUALIFIEDNAME(1, name),
                            pumpTypeId, /* this refers to the object type
                                           identifier */
                            oAttr, NULL, NULL);
}

Often we want to run a constructor function on a new object. This is especially useful when an object is instantiated at runtime (with the AddNodes service) and the integration with an underlying process cannot be manually defined. In the following constructor example, we simply set the pump status to on.

static UA_StatusCode
pumpTypeConstructor(UA_Server *server,
                    const UA_NodeId *sessionId, void *sessionContext,
                    const UA_NodeId *typeId, void *typeContext,
                    const UA_NodeId *nodeId, void **nodeContext) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND, "New pump created");

    /* Find the NodeId of the status child variable */
    UA_RelativePathElement rpe;
    UA_RelativePathElement_init(&rpe);
    rpe.referenceTypeId = UA_NODEID_NUMERIC(0, UA_NS0ID_HASCOMPONENT);
    rpe.isInverse = false;
    rpe.includeSubtypes = false;
    rpe.targetName = UA_QUALIFIEDNAME(1, "Status");

    UA_BrowsePath bp;
    UA_BrowsePath_init(&bp);
    bp.startingNode = *nodeId;
    bp.relativePath.elementsSize = 1;
    bp.relativePath.elements = &rpe;

    UA_BrowsePathResult bpr =
        UA_Server_translateBrowsePathToNodeIds(server, &bp);
    if(bpr.statusCode != UA_STATUSCODE_GOOD ||
       bpr.targetsSize < 1)
        return bpr.statusCode;

    /* Set the status value */
    UA_Boolean status = true;
    UA_Variant value;
    UA_Variant_setScalar(&value, &status, &UA_TYPES[UA_TYPES_BOOLEAN]);
    UA_Server_writeValue(server, bpr.targets[0].targetId.nodeId, value);
    UA_BrowsePathResult_clear(&bpr);

    /* At this point we could replace the node context .. */

    return UA_STATUSCODE_GOOD;
}

static void
addPumpTypeConstructor(UA_Server *server) {
    UA_NodeTypeLifecycle lifecycle;
    lifecycle.constructor = pumpTypeConstructor;
    lifecycle.destructor = NULL;
    UA_Server_setNodeTypeLifecycle(server, pumpTypeId, lifecycle);
}

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

static volatile UA_Boolean running = true;
static void stopHandler(int sign) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_SERVER, "received ctrl-c");
    running = false;
}

int main(void) {
    signal(SIGINT, stopHandler);
    signal(SIGTERM, stopHandler);

    UA_Server *server = UA_Server_new();
    UA_ServerConfig_setDefault(UA_Server_getConfig(server));

    manuallyDefinePump(server);
    defineObjectTypes(server);
    addPumpObjectInstance(server, "pump2");
    addPumpObjectInstance(server, "pump3");
    addPumpTypeConstructor(server);
    addPumpObjectInstance(server, "pump4");
    addPumpObjectInstance(server, "pump5");

    UA_StatusCode retval = UA_Server_run(server, &running);

    UA_Server_delete(server);
    return retval == UA_STATUSCODE_GOOD ? EXIT_SUCCESS : EXIT_FAILURE;
}