In our web application, for each http-request there is a lot of computation that happens on back end. Output can vary from 10 sec - 1 Hour. In the mean time when it is computed, "Waiting.." is shown on the website for the respective user.
But it so happens, that a user might cut down the service in between. So what all can be done on the back end so that the computation can be stopped in between to save resources? What different tactics can be applied here?
And if better (instead of killing the thread directly), then a graceful termination policy should make wonders.
I'm not sure if this fits your scenario but here is how I have tackled this issue in the past. We were generating pdf reports for a web-app. Most reports could be generated in under 5 seconds but some would take up to an hour.
When the User clicks on generate button we redirect them to a "Generating..." dialog screen which has a sort of progress bar and a Cancel button. This also launches the generate process on the server in a separate thread (we have a worker pool). The browser then polls the server regularly via ajax to check on the progress (either update the progress bar or redirect to the display page when finished).
The synchronization at the server between the generating process and the ajax process was done via a process synchronization object. The sync-obj was a very simple class instance which could be retrieved quickly from any thread at any time via some unique string.
Both processes could update this shared sync-obj. As the report generated the repgen thread would update the sync-obj which the ajax thread would inform the browser. If the User clicked the Cancel button then the ajax thread would set the "cancel" flag in the sync-ob and the repgen thread would pick that up and break out of the generate loop.
Clearly the responsiveness of the whole process depends a lot on how frequently the repgen thread checks the sync-obj and that often comes down to how the individual report was coded.
Finally, to answer your question, if the User gets bored and goes "back" and clicks the generate button again we do not cancel the first report and start a second but rather realise that it is the same report (and the same sync-obj id) and so just let the report continue. However if that does not suit your scenario then starting a generate process could cancel the first in the same manner that the User could via the Cancel button.
Related
I'm working on a standalone countdown App for Apple Watch. When the timer is over, the watch rings or vibrate.
There are three different situations where this can happen:
The app screen is active on the user's wrist: that's the easiest part; when the timer is over, the app runs a sound/haptic feedback, no need for anything running in the background. No problem with that.
The app is in the background: Using notifications seems to be the obvious choice, and it already works very well.
The app is in the foreground, but the screen turns off either because it reached the maximum wake duration (15 or 70 seconds) or because the user rotated his wrist back in rest position, which automatically turns the apple watch's screen off.
This third and last situation is where I'm confused. Neither the first nor the second situation works here: when the app is active, but the screen turns off, the app stops running, but the notifications are still not triggered since the app is technically not in the background.
Are there any straightforward ways around this problem?
Use Extended Runtime Sessions to create smart alarms
like this:
func notifyUser(hapticType type: WKHapticType,
repeatHandler: ((UnsafeMutablePointer<WKHapticType>) -> Time
Interval)? = nil)
Apple Documentation
For schedulable sessions such as smart alarms, call this method during the session to alert the user. When you call the method, the system plays repeating haptic feedback. If the app isn’t active, the system also displays a system alarm alert on the watch.
The haptic feedback repeats at the interval specified by the repeatHandler, and continues to repeat until the application or system alert invalidates the session.
If the app isn’t active, the user can tap the Stop button to invalidate the session or tap the Open button to activate the app.
If the app is active, the app must invalidate the session by calling its invalidate method.
Only call this method on a schedulable session that’s running: you must schedule the session using the startAtDate: method, and the session’s state must equal WKExtendedRuntimeSessionStateRunning. During a smart alarm session, your app must call this method before the session expires.
Also you should check out the life cycle of watchOS app
I am executing a external job using DBMS_SCHEDULER through apex page by clicking a button in below manner.(Dynamic action=>Execute PlSql)
dbms_scheduler.run_job(job_name => 'APEXDATA.myJobName', use_current_session=> TRUE);
Its executing the external job correctly.(taking 1-2 minutes).My issue is that, in between the time while its executing i can not able to access any other page or can not able to login with new session nothing.showing below error in every task i am performing.
**503 Service Unavailable
The connection pool named: |apex|| is not correctly configured, due to the following error(s):
Exception occurred while getting connection: oracle.ucp.UniversalConnectionPoolException:
All connections in the Universal Connection Pool are in use**
Is this the general or known issue?if yes how to resolve the issue,because in same time other user also has to perform any other task or other may login same time.
Thank You.
I think you're mixing 2 things that hard to combine:
Dynamic actions are designed to submit code from the page without a page submit so the user can continue to work on the page after he has done something (eg run pl/sql code)
Running a process in the database that takes up the database session until it is completed ( use_current_session=> TRUE). Your dbms_scheduler.run_job process will run in the current session and as long as that job is running no other operations can be run in that database session (the connection is in use as shown in the error message).
Solutions:
use_current_session=> FALSE so the job runs in the background
In the dynamic action, set "Wait for result" to true, so the user is forced to wait until the job completes.
Execute the job on page submit which will also force the user to wait for the job to be completed.
Since your job takes 1-2 mins to complete, options 2 and 3 are probably not feasible because the user experience is not optimal. If you execute the job in the background, then you probably need to write some additional code to prevent the user from clicking a couple of times and submitting the job multiple times. You could do that by checking if the job is running before you submit it and not submit it if it is currently running.
I am using wxWizard, in my 3rd page I need to call a function from backend, when that function takes long time to send response, my app hangs and shows not responding in title. Once that response is recieved from backend function, app behaves normally. why this is happening? Am I doing something wrong? How should I make that app not to hang? Response from backend function is delayed because of some network issue or long processing time in backend.
See this answer. Following is an extract:
An application gets the events from a queue provided by Windows.
If the application doesn't poll the eventqueue for a while (5 seconds), for example when doing a long calculation, then Windows assumes that the application is hung and alerts the user.
To avoid that applications should push expensive calculations to worker threads or split up processing and make sure the queue gets polled regularly.
So, the problem with your code is that, In your program there is only one thread,the main thread. It is taking care of all the activity e.g. UI update, event handling, responding users etc (which are very less time consuming). But when use it comes to handle connection with backend server, it is time consuming. So, you should use another thread to handle network operations. In this way main thread will be available for its normal work and not show that it is not responding.
In order to prevent application's data from losing caused by "End Task" from Task Manager, I am trying to save data at the function handler of WM_CLOSE event.
The app saves data successfully in case I closed my app via Alt+F4 or "close" button. But when I killed it via the Task Manager, the saving data process couldn't be done properly. It seems that the saving progress was terminated in middle.
I tried to debug it via VS2015 IDE, the debugger intercepted a break point in the WM_CLOSE handler successfully but it could not go further, hitting F10 to step over caused my app closes immediately.
Is there any way to delay the termination progress until my application saves data completely?
I found two links below but they didn't help.
How to handle "End Task" from Windows Task Manager on a background process?
How does task manager kill my program?
The task manager might decide that your application isn't responding, and terminate it. You can do nothing against it.
If you want to ensure that your data is always saved, you should save constantly (with some heuristics, like at least once every minute, preferrably after no change happened in a few seconds) in the background. It's more complex but has the advantage of working even when you won't receive WM_CLOSE at all, for example in the case of power loss.
In my Django app, I need to implement this "timer-based" functionality:
User creates some jobs and for each one defines when (in the same unit the timer works, probably seconds) it will take place.
User starts the timer.
User may pause and resume the timer whenever he wants.
A job is executed when its time is due.
This does not fit a typical cron scenario as time of execution is tied to a timer that the user can start, pause and resume.
What is the preferred way of doing this?
This isn't a Django question. It is a system architecture problem. The http is stateless, so there is no notion of times.
My suggestion is to use Message Queues such as RabbitMQ and use Carrot to interface with it. You can put the jobs on the queue, then create a seperate consumer daemon which will process jobs from the queue. The consumer has the logic about when to process.
If that it too complex a system, perhaps look at implementing the timer in JS and having it call a url mapped to a view that processes a unit of work. The JS would be the timer.
Have a look at Pinax, especially the notifications.
Once created they are pushed to the DB (queue), and processed by the cron-jobbed email-sending (2. consumer).
In this senario you won't stop it once it get fired.
That could be managed by som (ajax-)views, that call system process....
edit
instead of cron-jobs you could use a twisted-based consumer:
write jobs to db with time-information to the db
send a request for consuming (or resuming, pausing, ...) to the twisted server via socket
do the rest in twisted
You're going to end up with separate (from the web server) processes to monitor the queue and execute jobs. Consider how you would build that without Django using command-line tools to drive it. Use Django models to access the the database.
When you have that working, layer on on a web-based interface (using full Django) to manipulate the queue and report on job status.
I think that if you approach it this way the problem becomes much easier.
I used the probably simplest (crudest is more appropriate, I'm afraid) approach possible: 1. Wrote a model featuring the current position and the state of the counter (active, paused, etc), 2. A django job that increments the counter if its state is active, 3. An entry to the cron that executes the job every minute.
Thanks everyone for the answers.
You can always use a client based jquery timer, but remember to initialize the timer with a value which is passed from your backend application, also make sure that the end user didn't edit the time (edit by inspecting).
So place a timer start time (initial value of the timer) and timer end time or timer pause time in the backend (DB itself).
Monitor the duration in the backend and trigger the job ( in you case ).
Hope this is clear.