Traffic Server Software Developers Kit

An action is a handle to an operation initiated by a plugin that has not yet completed. For example: when a plugin connects to a remote server, it uses the call INKNetConnect which takes an INKCont as an argument to call back when the connection is established. INKNetConnect might not call the continuation back immediately and will return an INKAction structure that the caller can use to cancel the operation. Cancelling the operation does not necessarily mean that the operation will not occur; it means that the continuation passed in to the operation will not be called back. In the above example, the connection might still occur if the action is cancelled, but the continuation that initiated the connection would not be called back when that occurred.

In the preceding example, it is possible that the connection will complete and callback the continuation before INKNetConnect returns. If this occurs, then INKNetConnect returns a special action that causes INKActionDone to return 1. Basically, this specifies that the operation has already completed; there is no point in trying to cancel the operation. Note that an action will never change from non-completed to completed. When the operation actually succeeds and the continuation is called back, the continuation must zero out its action pointer to indicate to itself that the operation succeeded.

The asynchronous nature of all operations in Traffic Server necessitates actions. You should notice from the above discussion that once a call to a function like INKNetConnect is made by a continuation and that function returns a valid action (INKActionDone returns 0), it is not safe for the continuation to do anything else except return from its handler function. It is not safe to modify or examine the continuation’s data since the continuation may have already been destroyed.

Below is an example of typical usage for an action:

#include “InkAPI.h”
static int
handler (INKCont contp, INKEvent event, void *edata)
{
   if (event == INK_EVENT_IMMEDIATE) {
      INKAction actionp = INKNetConnect (contp, 127.0.0.1, 9999);
      if (!INKActionDone (actionp)) {
   INKContDataSet (contp, actionp);
   } else {
          /* we've already been called back... */
   return 0;
   }
   } else if (event == INK_EVENT_NET_CONNECT) {
      /* net connection succeeded */
      INKContDataSet (contp, NULL);
      return 0;
   } else if (event == INK_EVENT_NET_CONNECT_FAILED) {
   /* net connection failed */
   INKContDataSet (contp, NULL);
   return 0;
   } 
   return 0;
}

void
INKPluginInit (int argc, const char *argv[])
{
    INKCont contp;

    contp = INKContCreate (handler, INKMutexCreate ());

    /* We don't want to call things out of INKPluginInit
       directly since it is called before the rest of the
       system is initialized. We'll simply schedule an event
       on the continuation to occur as soon as the rest of
       the system is started up. */
    INKContSchedule (contp, 0);
}

The example above shows a simple plugin that creates a continuation and schedules it to be called immediately. When the plugin’s handler function is called the first time, the event will be INK_EVENT_IMMEDIATE. The plugin then tries to open a net connection to port 9999 on localhost (127.0.0.1). The IP description was left cider notation to make it clearer what is going on; also note that the above won’t actually compile until the IP address is modified. The action returned from INKNetConnect is examined by the plugin. If the operation has not completed, then the plugin stores the action in its continuation. Otherwise, the plugin knows it has already been called back and there is no reason to store the action pointer.

A final question might be why would a plugin want to cancel an action. In the above example, a valid reason could be to place a limit on the length of time it takes to open a connection. The plugin could schedule itself to get called back in 30 seconds and then initiate the net connection. If the time-out expires first, then the plugin would cancel the action. The following sample code implements this:

#include “InkAPI.h”
static int
handler (INKCont contp, INKEvent event, void *edata)
{
   switch (event) {
	case (INK_EVENT_IMMEDIATE):
      INKContSchedule (contp, 30000);
      INKAction actionp = INKNetConnect(contp, 127.0.0.1, 9999);
      if (!INKActionDone (actionp)) {
         INKContDataSet (contp, actionp);
      } else {
         /* we’ve already been called back ... */
      }
      break;

   case (INK_EVENT_TIMEOUT):
      INKAction actionp = INKContDataGet (contp);
      if (!INKActionDone(actionp)) {
         INKActionCancel (actionp);
      }
      break;

   case (INK_EVENT_NET_CONNECT):
   /* net connection succeeded */
      INKContDataSet (contp, NULL);
      break;

   case (INK_EVENT_NET_CONNECT_FAILED):
   /* net connection failed */
      INKContDataSet (contp, NULL);
      break;

   } 
   return 0;
}

void
INKPluginInit (int argc, const char *argv[])
{
    INKCont contp;

    contp = INKContCreate (handler, INKMutexCreate ());
    /* We don't want to call things out of INKPluginInit
       directly since it is called before the rest of the
       system is initialized. We'll simply schedule an event
       on the continuation to occur as soon as the rest of
       the system is started up. */
    INKContSchedule (contp, 0);
}

The action functions are: