I am running workflow on the AWS console. I saw there is a Event Type called "TimerFired" in the console. What does it do?
TimerFired event is generated when timer scheduled through StartTimer decision fires. It also causes a new decision being scheduled to give decider chance to handle the timer event.
Related
Having read Timely (and Stateful) Processing with Apache Beam and looked at the JavaDoc for a Timer, I have successfully setup a timer to be fired. However, I am missing how to request that a timer be cancelled before it has fired. Is this a semantic that is possible / allowable in Beam architecture?
My back-story is that I wish to register a timer to fire a callback if something has not occurred ... but if it HAS occurred prior to the timer firing, then I wish to cancel the timer from being fired.
I don't see a way to cancel the time. However, you can use a Boolean value state to skip the processing onTime when timer fires.
You can cancel timers as of Beam SDK 2.32.0.
Hi I have been asked to maintain a Delphi 5 based program for someone, and the program is using a timer object to tick every 50 milli-seconds and upon each time-up it runs the block of single threaded code. I am just wondering, what would happen if the time taken to execute this block of code is longer than the timer tick interval, would this be bad? For example could it cause problems like access violation? How does Delphi handle this kind of situation by default? Thanks a lot.
The critical part of this question is :
... what would happen if the time taken to execute this block of code is longer than the timer tick interval, would this be bad?
It's not great, but it's not a show stopper and it certainly cannot cause access violations. Delphi's TTimer is implemented using the WinAPI SetTimer function.
You might naively think that if your timer's handler took longer than the interval to process that the timer would continue to pile up messages in the message queue and your program would effectively lock up with a flood of timer messages that had no hope of ever all being processed. Thankfully, this isn't exactly how timers work. The documentation can shed some light.
WM_TIMER message
The WM_TIMER message is a low-priority message. The GetMessage and PeekMessage functions post this message only when no other higher-priority messages are in the thread's message queue.
Now, there isn't really a concept of "high" and "low" priority messages in a windows application, and while this statement is a little ambiguous we can take the context to mean that WM_TIMER is a message that is not posted to the application's message queue but rather generated in response to a GetMessage or PeekMessage call when a timer has been set with SetTimer, when that timer's interval has elapsed, and when there are no other messages already in the queue.
So, while the timer interval may elapse during your handler's processing, any other messages that come in while this is happening will still enter the queue normally and will be processed once your handler completes. Only once the queue has been emptied again will another WM_TIMER message be generated.
The timer events, therefore, will execute either at the rate of the tick interval or as fast as your application can process them, whichever ends up being longest. If you do have timer messages coming in too quickly, however, and your timer handler's processing time is long then your application's responsiveness can suffer. It won't become unresponsive, but all other message processing will be restricted to being processed at the interval of your timer's event handler execution time. This can make your application feel sluggish.
Example
To demonstrate, create a new forms application and add a TTimer component with an interval set to 10. Then attach this handler :
procedure TForm1.Timer1Timer(Sender: TObject);
begin
sleep(200);
end;
While the program is running, try moving the window around. What we have done is to essentially quantize the application's message processing to a 200ms interval (the duration of the timer's event handler execution).
Ticks of the timer do not interrupt your code.
Timer ticks are delivered in the form of window messages. Window messages can only arrive when you check the message queue for new messages. That happens automatically when your timer event handler returns and your program resumes its event loop, but you can trigger it explicitly by calling Application.ProcessMessages. Don't call that, though; it seldom solves problems in the long run.
If you don't check the message queue in your timer-tick handler, then your handler will never start running a second time while it's still handling a previous tick.
Even if you do check the queue, all that happens is that the tick handler will be called recursively. After all, it's all running in a single thread. Recursive timer handling probably isn't what you want to happen, though, so I'll again counsel against checking for messages in a message handler.
Furthermore, timer messages can never "pile up" if your timer handler takes a long time to run. Timer messages are "fake" in that they don't actually get added to the message queue at regular intervals. Instead, the OS will synthesize a timer message at the time your program checks the queue for more messages. If there are no higher-priority messages in the queue, and the timer interval has elapsed, then the OS will return a wm_Timer message. If you don't check for more messages, then there will be no timer message in the queue. In particular, there will not be multiple timer messages in the queue.
Further reading:
Stack Overflow: How does the message queue work in Win32?
Dr. Dobbs: Inside the Windows Messaging System
I'm new to Quartz.Net and I'm wondering what happens if a job is scheduled in the past? Is an exception thrown? Is it run immediately? I'm wondering if I need to guard for this condition when I schedule jobs.
You will get an exception if the trigger you are adding would never fire. There is a distinction here to note: you can schedule start time to be in the past but there needs to be some future time for the trigger to fire. So having start time of NOW()-1d is fine if you have firing happening after the time you add the trigger (EndTimeUtc in the future AND schedule that permits futher fires from now).
So you should check that GetNextFireTimeUtc returns non-null value for trigger you are adding.
I have a TTimer in my application that fires every 2 seconds and calls my event handler, HandleTimerEvent(). The HandleTimerEvent() function modifies shared resources and can take 10's of seconds to execute before returning. Furthermore, I call Sleep() in the event handler to relinquish the processor at times.
I'm not sure how C++ builder's TTimer object works when it comes to calling events, so the scenario I just explained has got me thinking, particularly, whether HandleTimerEvent() gets called before a prior call has returned.
The question comes down to a couple of things.
Does the TTimer object queue the events?
Can the TTimer object call my event handler before a prior call has returned?
This reply assumes that TTimer is still implemented to use WM_Timer messages. If the implementation has changed (since 2005), please disregard.
No, the TTimer object does not queue events. It is driven by the Windows WM_Timer message, and Windows does not let WM_TIMER messages stack up in the message queue. If the next timer interval occurs and Windows sees that a WM_Timer message is already in the app's message queue, it does not add another WM_Timer messsage to the queue. (Same for WM_Paint, btw)
Yes, it is possible for a TTimer.OnTimer event to be fired even while a prior event handler is still executing. If you do anything in your event handler that allows the app to process messages, then your timer event can be reentered. The obvious one is if your event handler calls Application.ProcessMessages, but it can be much more subtle than that - if anything you call in your event handler internally calls Application.ProcessMessages, or calls PeekMessage/GetMessage + DispatchMessage, or opens a modal dialog, or calls a COM interface that is bound to an out-of-process COM object, then messages in your app message queue will be processed and that could include your next WM_Timer message.
A simple solution is to disable the timer object when you enter your timer event handler, and reenable it when you exit your timer event handler. This will prevent timer messages from firing while your event handler is still working, regardless of the message handling characteristics of your code.
I use TTimer extensively. It does not queue events. If you want it to hand off to an event handler, then create a TThread that handles your events so the Timer can continue with it's work. The timer does not operate asychronously but rather synchronously.
i want implement a windows services scheduled task.
I already created windows service. In a service i have implemented a timer.The timer is initialized at class interval. The timers interval is set in the start method of service and also it is enabled in the start method of the service. After timers elapsed event is fire i have done some actions.
My problem is that, i am in a dilemma. Lets say the action i have done in Elapsed event, lets say take one hour and the timers interval is set to half an hour. so there are chances that even if the previous call to elapsed event has not ended new call to elapsed event will occur.
my question will there be any conflict or is it ok or shall i use threads.
please give some advice
You can stop the timer at the begining of your long running method, then start it again at the end:
Timer.Change(Timeout.Infinite, Timeout.Infinite)
// do long task ...
Timer.Change(dueTime,period)