Creating an input node in C

Before you start

Ensure that you have read and understood the following topics:

A loadable implementation library, or a LIL, is the implementation module for a C node (or parser). A LIL is implemented as a dynamic link library (DLL). It does not have the file extension .dll but .lil.

The implementation functions that have to be written by the developer are listed in C node implementation functions. The utility functions that are provided by WebSphere Message Broker to aid this process are listed in C node utility functions.

WebSphere Message Broker provides the source for two sample user-defined nodes called SwitchNode and TransformNode. You can use these nodes in their current state, or you can modify them.

Declaring and defining the node

To declare and define a user-defined node to the broker you must include an initialization function, bipGetMessageflowNodeFactory, in your LIL. The following steps outline how the broker calls your initialization function and how your initialization function declares and defines the user-defined node:

  1. The initialization function, bipGetMessageflowNodeFactory, is called by the broker after the LIL has been loaded and initialized by the operating system. The broker calls this function to understand what your LIL is able to do and how the broker should call the LIL. For example:
    CciFactory LilFactoryExportPrefix * LilFactoryExportSuffix
    bipGetMessageflowNodeFactory()
  2. The bipGetMessageflowNodeFactory function must then call the utility function cniCreateNodeFactory. This function passes back a unique factory name (or group name) for all the nodes that your LIL supports. Every factory name (or group name) passed back must be unique throughout all the LILs in the broker.
  3. The LIL must then call the utility function cniDefineNodeClass to pass the unique name of each node, and a virtual function table of the addresses of the implementation functions.
    For example, the following code declares and defines a single node called InputxNode:
    {
    	CciFactory* factoryObject;
    	int rc = 0;
    	CciChar factoryName[] = L"MyNodeFactory";
    	CCI_EXCEPTION_ST exception_st;
    
    	/* Create the Node Factory for this node */
    	factoryObject = cniCreateNodeFactory(0, factoryName);
    	if (factoryObject == CCI_NULL_ADDR) {
    		
    		/* Any local error handling can go here */
    	}
    	else {
    		/* Define the nodes supported by this factory */
    		static CNI_VFT vftable = {CNI_VFT_DEFAULT};
    
    	/* Setup function table with pointers to node implementation functions */
    	vftable.iFpCreateNodeContext = _createNodeContext;
    	vftable.iFpDeleteNodeContext = _deleteNodeContext;
    	vftable.iFpGetAttributeName2 = _getAttributeName2;
    	vftable.iFpSetAttribute      = _setAttribute;
    	vftable.iFpGetAttribute2     = _getAttribute2;
    	vftable.iFpRun               = _run;
    
    	cniDefineNodeClass(0, factoryObject, L"InputxNode", &vftable);
    	}
    
    	/* Return address of this factory object to the broker */
    	return(factoryObject);
    }

    A user-defined node identifies itself as providing the capability of an input node by implementing the cniRun implementation function.

    User-defined nodes have to either implement a cniRun or a cniEvaluate implementation function, otherwise the broker does not load the user-defined node, and the cniDefineNodeClass utility function fails, returning CCI_MISSING_IMPL_FUNCTION.

    For example:
    int cniRun(                       
      CciContext* context,                
      CciMessage* destinationList,        
      CciMessage* exceptionList,          
      CciMessage* message
    ){          
      ...
      /* Get data from external source */
      return CCI_SUCCESS_CONTINUE;                                    
    }
    The return value should be used to return control periodically to the broker.

    When a message flow containing a user-defined input node is deployed successfully, the node's cniRun function is called for each message propagated to the node.

    For the minimum code required to compile a C user-defined node, see C skeleton code.

Creating an instance of the node

The following procedure shows you how to instantiate your node:

  1. When the broker has received the table of function pointers, it calls the function cniCreateNodeContext for each instantiation of the user-defined node. If you have three message flows that are using your user-defined node, your cniCreateNodeContext function is called for each of them. This function should allocate memory for that instantiation of the user-defined node to hold the values for the configured attributes. For example:
    1. Call the cniCreateNodeContext function:
      CciContext* _createNodeContext(
        CciFactory* factoryObject,
        CciChar*    nodeName,
        CciNode*    nodeObject
      ){
        static char* functionName = (char *)"_createNodeContext()";
        NODE_CONTEXT_ST* p;
        CciChar          buffer[256];
      
    2. Allocate a pointer to the local context and clear the context area:
        p = (NODE_CONTEXT_ST *)malloc(sizeof(NODE_CONTEXT_ST));
      
        if (p) {
           memset(p, 0, sizeof(NODE_CONTEXT_ST));
    3. Save the node object pointer in the context:
         p->nodeObject = nodeObject;
    4. Save the node name:
       CciCharNCpy((CciChar*)&p->nodeName, nodeName, MAX_NODE_NAME_LEN);
    5. Return the node context:
      return (CciContext*) p;
  2. An input node has a number of output terminals associated with it, but does not typically have any input terminals. Use the utility function cniCreateOutputTerminal to add output terminals to an input node when the node is instantiated. These functions must be invoked within the cniCreateNodeContext implementation function. For example, to define an input node with three output terminals:
       {
          const CciChar* ucsOut = CciString("out", BIP_DEF_COMP_CCSID) ;
          insOutputTerminalListEntry(p, (CciChar*)ucsOut);
          free((void *)ucsOut) ;
        }
        {
          const CciChar* ucsFailure = CciString("failure", BIP_DEF_COMP_CCSID) ;
          insOutputTerminalListEntry(p, (CciChar*)ucsFailure);
          free((void *)ucsFailure) ;
        }    
        {
          const CciChar* ucsCatch = CciString("catch", BIP_DEF_COMP_CCSID) ;
          insOutputTerminalListEntry(p, (CciChar*)ucsCatch);
          free((void *)ucsCatch) ;    }
    For the minimum code required to compile a C user-defined node, see C skeleton code.

Setting attributes

Attributes are set whenever you start the broker, or when you redeploy the message flow with new values.

Following the creation of output terminals, the broker calls the cniSetAttribute function to pass the values for the configured attributes of the user-defined node. For example:
    {
      const CciChar* ucsAttr = CciString("nodeTraceSetting", BIP_DEF_COMP_CCSID) ;
      insAttrTblEntry(p, (CciChar*)ucsAttr, CNI_TYPE_INTEGER);
      _setAttribute(p, (CciChar*)ucsAttr, (CciChar*)constZero);
      free((void *)ucsAttr) ;
    }
    {
      const CciChar* ucsAttr = CciString("nodeTraceOutfile", BIP_DEF_COMP_CCSID) ;
      insAttrTblEntry(p, (CciChar*)ucsAttr, CNI_TYPE_STRING);
      _setAttribute(p, (CciChar*)ucsAttr, (CciChar*)constSwitchTraceLocation);
      free((void *)ucsAttr) ;
    }
Start of changeThere is no limit to the number of configuration attributes that a node can have. However, a plug-in node must not implement an attribute that is already implemented as a base configuration attribute. These base attributes are:
  • label
  • userTraceLevel
  • traceLevel
  • userTraceFilter
  • traceFilter
End of change

Implementing the node functionality

When the broker knows that it has an input node, it calls the cniRun function of this node at regular intervals. The cniRun function must then decide what course of action it should take. If data is available for processing, the cniRun function should attempt to propagate the message. If no data is available for processing, the cniRun function should return with CCI_TIMEOUT so that the broker can continue configuration changes.

For example, to configure the node to call cniDispatchThread and process the message, or return with CCI_TIMEOUT:
If ( anything to do )
	CniDispatchThread;

   /* do the work */

	If ( work done O.K.)
		Return CCI_SUCCESS_CONTINUE;
	Else
		Return CCI_FAILURE_CONTINUE;
Else
  Return CCI_TIMEOUT;

Overriding the default message parser attributes (optional)

An input node implementation normally determines what message parser initially parses an input message. For example, the primitive MQInput node dictates that an MQMD parser is required to parse the MQMD header. A user-defined input node can select an appropriate header or message parser, and the mode in which the parsing is controlled, by using the following attributes that are included as default, which you can override:

rootParserClassName
Defines the name of the root parser that parses message formats supported by the user-defined input node. It defaults to GenericRoot, a supplied root parser that causes the broker to allocate and chain parsers together. It is unlikely that a node would need to modify this attribute value.
firstParserClassName
Defines the name of the first parser, in what might be a chain of parsers that are responsible for parsing the bitstream. It defaults to XML.
messageDomainProperty
An optional attribute that defines the name of the message parser required to parse the input message. The supported values are the same as those supported by the MQInput node. (See MQInput node for more information about the MQInput node.)
messageSetProperty
An optional attribute that defines the message set identifier, or the message set name, in the Message Set field, only if the MRM parser was specified by the messageDomainProperty attribute.
messageTypeProperty
An optional attribute that defines the identifier of the message in the MessageType field, only if the MRM parser was specified by the messageDomainProperty attribute.
messageFormatProperty
An optional attribute that defines the format of the message in the Message Format field, only if the MRM parser was specified by the messageDomainProperty attribute.
If you have written a user-defined input node that always begins with data of a known structure, you can ensure that a certain parser deals with the start of the data. For example, the MQInputNode only reads data from WebSphere MQ queues, so this data always has an MQMD at the beginning, and the MQInputNode sets firstParserClassName to MQHMD. If your user-defined node always deals with data that begins with a structure that can be parsed by a certain parser, say "MYPARSER", you set firstParserClassName to MYPARSER as follows:
  1. Declare the implementation functions:
    CciFactory LilFactoryExportPrefix * LilFactoryExportSuffix bipGetMessageflowNodeFactory()
    {
      ....
      CciFactory*      factoryObject;
      ....
      factoryObject = cniCreateNodeFactory(0, (unsigned short *)constPluginNodeFactory);
      ...
      vftable.iFpCreateNodeContext = _createNodeContext;
      vftable.iFpSetAttribute      = _setAttribute;
      vftable.iFpGetAttribute      = _getAttribute;
      ...  
      cniDefineNodeClass(&rc, factoryObject, (CciChar*)constSwitchNode, &vftable);
      ...
      return(factoryObject);
    }
  2. Set the attribute in the cniCreateNodeContext implementation function:
    CciContext* _createNodeContext(
      CciFactory* factoryObject,
      CciChar*    nodeName,
      CciNode*    nodeObject
    ){
      NODE_CONTEXT_ST* p;
      ...
    
        /* Allocate a pointer to the local context */
        p = (NODE_CONTEXT_ST *)malloc(sizeof(NODE_CONTEXT_ST));
        /* Create attributes and set default values */
        {
          const CciChar* ucsAttrName  = CciString("firstParserClassName", BIP_DEF_COMP_CCSID);
          const CciChar* ucsAttrValue = CciString("MYPARSER", BIP_DEF_COMP_CCSID) ;
          insAttrTblEntry(p, (CciChar*)ucsAttrName, CNI_TYPE_INTEGER);
          /*see sample BipSampPluginNode.c for implementation of insAttrTblEntry*/
    
          _setAttribute(p, (CciChar*)ucsAttrName, (CciChar*)ucsAttrValue);
          free((void *)ucsAttrName) ;
          free((void *)ucsAttrValue) ;
        }
    In the code example above, the insAttrTblEntry method is called. This method is declared in the SwitchNode and TransformNode sample user-defined nodes.

Deleting an instance of the node

Nodes are destroyed when a message flow is redeployed, or when the execution group process is stopped (using the mqsistop command). When a node is destroyed, it should free any used memory and release any held resources. You do this using the cniDeleteNodeContext function. For example:

void _deleteNodeContext(
  CciContext* context
){
  static char* functionName = (char *)"_deleteNodeContext()";

  return;
}