Actions

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 TSNetConnect - which takes TSCont as an argument to call back when the connection is established. TSNetConnect might not call the continuation back immediately and will return an TSAction structure that the caller can use to cancel the operation. Cancelling the operation does not necessarily mean that the operation will not occur; it simply means that the continuation passed into the operation will not be called back. In such an example, the connection might still occur if the action is cancelled; however, the continuation that initiated the connection would not be called back.

In the preceding example, it is also possible that the connection will complete and call back the continuation before TSNetConnect returns. If that occurs, then TSNetConnect returns a special action that causes TSActionDone to return 1. This specifies that the operation has already completed, so it’s pointless to try to cancel the operation. Also 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 TSNetConnect is made by a continuation and that function returns a valid action (TSActionDone 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 because the continuation may have already been destroyed.

Below is an example of typical usage for an action:

#include <ts/ts.h>
static int
handler (TSCont contp, TSEvent event, void *edata)
{
    if (event == TS_EVENT_IMMEDIATE) {
        TSAction actionp = TSNetConnect (contp, 127.0.0.1, 9999);
        if (!TSActionDone (actionp)) {
            TSContDataSet (contp, actionp);
        } else {
            /* We've already been called back... */
            return 0;
        }
    } else if (event == TS_EVENT_NET_CONNECT) {
        /* Net connection succeeded */
        TSContDataSet (contp, NULL);
        return 0;
    } else if (event == TS_EVENT_NET_CONNECT_FAILED) {
        /* Net connection failed */
        TSContDataSet (contp, NULL);
        return 0;
    }
    return 0;
}

void
TSPluginInit (int argc, const char *argv[])
{
    TSCont contp;

    contp = TSContCreate (handler, TSMutexCreate ());

    /* We don't want to call things out of TSPluginInit
       directly since it's 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. */
    TSContSchedule (contp, 0);
}

The example above shows a simple plugin that creates a continuation and then schedules it to be called immediately. When the plugin’s handler function is called the first time, the event is TS_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 in cider notation to further clarify what is going on; also note that the above won’t actually compile until the IP address is modified. The action returned from TSNetConnect 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 timeout expires first, then the plugin would cancel the action. The following sample code implements this:

#include <ts/ts.h>
static int
handler (TSCont contp, TSEvent event, void *edata)
{
    switch (event) {
        case (TS_EVENT_IMMEDIATE):
            TSContSchedule (contp, 30000);
            TSAction actionp = TSNetConnect(contp, 127.0.0.1, 9999);
            if (!TSActionDone (actionp)) {
                TSContDataSet (contp, actionp);
            } else {
                /* We've already been called back ... */
            }
            break;

        case (TS_EVENT_TIMEOUT):
            TSAction actionp = TSContDataGet (contp);
            if (!TSActionDone(actionp)) {
                TSActionCancel (actionp);
            }
            break;

        case (TS_EVENT_NET_CONNECT):
            /* Net connection succeeded */
            TSContDataSet (contp, NULL);
            break;

        case (TS_EVENT_NET_CONNECT_FAILED):
            /* Net connection failed */
            TSContDataSet (contp, NULL);
            break;

    }
    return 0;
}

void
TSPluginInit (int argc, const char *argv[])
{
    TSCont contp;

    contp = TSContCreate (handler, TSMutexCreate ());
    /* We don't want to call things out of TSPluginInit
       directly since it's 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. */
    TSContSchedule (contp, 0, TS_THREAD_POOL_DEFAULT);
}

The action functions are: