tutorial_server_datasource.c

/* This work is licensed under a Creative Commons CCZero 1.0 Universal License.
 * See http://creativecommons.org/publicdomain/zero/1.0/ for more information. */

/**
 * Connecting a Variable with a Physical Process
 * ---------------------------------------------
 *
 * In OPC UA-based architectures, servers are typically situated near the source
 * of information. In an industrial context, this translates into servers being
 * near the physical process and clients consuming the data at runtime. In the
 * previous tutorial, we saw how to add variables to an OPC UA information
 * model. This tutorial shows how to connect a variable to runtime information,
 * for example from measurements of a physical process. For simplicity, we take
 * the system clock as the underlying "process".
 *
 * The following code snippets are each concerned with a different way of
 * updating variable values at runtime. Taken together, the code snippets define
 * a compilable source file.
 *
 * Updating variables manually
 * ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 * As a starting point, assume that a variable for a value of type
 * :ref:`datetime` has been created in the server with the identifier
 * "ns=1,s=current-time". Assuming that our application gets triggered when a
 * new value arrives from the underlying process, we can just write into the
 * variable. */

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

static void
updateCurrentTime(UA_Server *server) {
    UA_DateTime now = UA_DateTime_now();
    UA_Variant value;
    UA_Variant_setScalar(&value, &now, &UA_TYPES[UA_TYPES_DATETIME]);
    UA_NodeId currentNodeId = UA_NODEID_STRING(1, "current-time-value-callback");
    UA_Server_writeValue(server, currentNodeId, value);
}

static void
addCurrentTimeVariable(UA_Server *server) {
    UA_DateTime now = 0;
    UA_VariableAttributes attr = UA_VariableAttributes_default;
    attr.displayName = UA_LOCALIZEDTEXT("en-US", "Current time - value callback");
    attr.accessLevel = UA_ACCESSLEVELMASK_READ | UA_ACCESSLEVELMASK_WRITE;
    UA_Variant_setScalar(&attr.value, &now, &UA_TYPES[UA_TYPES_DATETIME]);

    UA_NodeId currentNodeId = UA_NODEID_STRING(1, "current-time-value-callback");
    UA_QualifiedName currentName = UA_QUALIFIEDNAME(1, "current-time-value-callback");
    UA_NodeId parentNodeId = UA_NS0ID(OBJECTSFOLDER);
    UA_NodeId parentReferenceNodeId = UA_NS0ID(ORGANIZES);
    UA_NodeId variableTypeNodeId = UA_NS0ID(BASEDATAVARIABLETYPE);
    UA_Server_addVariableNode(server, currentNodeId, parentNodeId,
                              parentReferenceNodeId, currentName,
                              variableTypeNodeId, attr, NULL, NULL);

    updateCurrentTime(server);
}

/**
 * Variable Value Callback
 * ^^^^^^^^^^^^^^^^^^^^^^^
 *
 * When a value changes continuously, such as the system time, updating the
 * value in a tight loop would take up a lot of resources. Value callbacks allow
 * to synchronize a variable value with an external representation. They attach
 * callbacks to the variable that are executed before every read and after every
 * write operation. */

static void
beforeReadTime(UA_Server *server,
               const UA_NodeId *sessionId, void *sessionContext,
               const UA_NodeId *nodeid, void *nodeContext,
               const UA_NumericRange *range, const UA_DataValue *data) {
    updateCurrentTime(server);
}

static void
afterWriteTime(UA_Server *server,
               const UA_NodeId *sessionId, void *sessionContext,
               const UA_NodeId *nodeId, void *nodeContext,
               const UA_NumericRange *range, const UA_DataValue *data) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND,
                "The variable was updated");
}

static void
addValueCallbackToCurrentTimeVariable(UA_Server *server) {
    UA_NodeId currentNodeId = UA_NODEID_STRING(1, "current-time-value-callback");
    UA_ValueCallback callback ;
    callback.onRead = beforeReadTime;
    callback.onWrite = afterWriteTime;
    UA_Server_setVariableNode_valueCallback(server, currentNodeId, callback);
}

/**
 * Variable Data Sources
 * ^^^^^^^^^^^^^^^^^^^^^
 *
 * With value callbacks, the value is still stored in the variable node.
 * So-called data sources go one step further. The server redirects every read
 * and write request to a callback function. Upon reading, the callback provides
 * a copy of the current value. Internally, the data source needs to implement
 * its own memory management. */

static UA_StatusCode
readCurrentTime(UA_Server *server,
                const UA_NodeId *sessionId, void *sessionContext,
                const UA_NodeId *nodeId, void *nodeContext,
                UA_Boolean sourceTimeStamp, const UA_NumericRange *range,
                UA_DataValue *dataValue) {
    UA_DateTime now = UA_DateTime_now();
    UA_Variant_setScalarCopy(&dataValue->value, &now,
                             &UA_TYPES[UA_TYPES_DATETIME]);
    dataValue->hasValue = true;
    return UA_STATUSCODE_GOOD;
}

static UA_StatusCode
writeCurrentTime(UA_Server *server,
                 const UA_NodeId *sessionId, void *sessionContext,
                 const UA_NodeId *nodeId, void *nodeContext,
                 const UA_NumericRange *range, const UA_DataValue *data) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_USERLAND,
                "Changing the system time is not implemented");
    return UA_STATUSCODE_BADINTERNALERROR;
}

static void
addCurrentTimeDataSourceVariable(UA_Server *server) {
    UA_VariableAttributes attr = UA_VariableAttributes_default;
    attr.displayName = UA_LOCALIZEDTEXT("en-US", "Current time - data source");
    attr.accessLevel = UA_ACCESSLEVELMASK_READ | UA_ACCESSLEVELMASK_WRITE;

    UA_NodeId currentNodeId = UA_NODEID_STRING(1, "current-time-datasource");
    UA_QualifiedName currentName = UA_QUALIFIEDNAME(1, "current-time-datasource");
    UA_NodeId parentNodeId = UA_NS0ID(OBJECTSFOLDER);
    UA_NodeId parentReferenceNodeId = UA_NS0ID(ORGANIZES);
    UA_NodeId variableTypeNodeId = UA_NS0ID(BASEDATAVARIABLETYPE);

    UA_DataSource timeDataSource;
    timeDataSource.read = readCurrentTime;
    timeDataSource.write = writeCurrentTime;
    UA_Server_addDataSourceVariableNode(server, currentNodeId, parentNodeId,
                                        parentReferenceNodeId, currentName,
                                        variableTypeNodeId, attr,
                                        timeDataSource, NULL, NULL);
}

static UA_DataValue *externalValue;

static void
addCurrentTimeExternalDataSource(UA_Server *server) {
    UA_NodeId currentNodeId = UA_NODEID_STRING(1, "current-time-external-source");

    UA_ValueBackend valueBackend;
    valueBackend.backendType = UA_VALUEBACKENDTYPE_EXTERNAL;
    valueBackend.backend.external.value = &externalValue;

    UA_Server_setVariableNode_valueBackend(server, currentNodeId, valueBackend);
}

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

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

    addCurrentTimeVariable(server);
    addValueCallbackToCurrentTimeVariable(server);
    addCurrentTimeDataSourceVariable(server);
    addCurrentTimeExternalDataSource(server);

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