I was working on a project and there is bulk e-mail sending part of it and when user clicks on the button, he/she gets the "Thanks the e-mails have been sent" immediately as a Response and the same method is firing an async thread as well.
ThreadPool.QueueUserWorkItem(SendEMail, _message);
This thread is queued when user clicks on the send button but since this is default Background Thread I was expecting when the page response ended, this thread would be terminated but it didn't happen because the current thread which fires this Thread was also a Background Thread and a Worker Thread, so which means there is an unfinished foreground thread(s) (Could be MainThread or Worker Threads) still alive but I don't know when they finish because their finish time will effect my background worker threads; When the last foreground thread ends, it causes the process to be terminated, so do background threads.
Should I be afraid of this or Asp.NET can handle it automatically and I am kinda confused because I've read a lot of things about it and now everything is mixed up.
Could you please clarify things a little bit ?
Thanks in advance.
Using the ThreadPool for long-running tasks will negatively influence the performance of your application (because ASP.NET also uses the ThreadPool to handle incoming requests).
Creating threads manually for each task can also become a problem if too much threads are created.
One technique I've used in the past, is to create a custom ThreadPool when starting the application, and enqueuing tasks on that ThreadPool. A simple custom ThreadPool could consist of a single Thread and a Queue of tasks.
When you call QueueUserWorkItem a new thread will be drawn from the thread pool if available and execute the callback function on this thread. If no threads are available, the function will block until a thread is freed. Once it finishes the execution of the method the thread will be suspended and returned to the pool for further reuse. Your only concern should be the fact that threads from the pool are also used to service requests, so if you perform lengthy tasks on them you could jeopardize the whole system request servicing capabilities because there's a limited number of threads in the pool and if all of them are busy performing lengthy operations future HTTP requests will be queued. As an alternative you could consider creating threads manually: new Thread(state => { }).Start();
You can use MSMQ for creating your email queue and have a single thread process the queue.
The following post might be useful as it fits your problem domain. Although it does not use MSMQ but is a good post on processing scheduled tasks using ASP.net.
Simulate a Windows Service using ASP.NET to run scheduled jobs
Related
I was reading the documentation of Microsoft specifically the Async programming article and I didn't understand this section while he is explaining the work of the server's threads when using Async code.
because it(The server) uses async and await, each of its threads is freed up when the I/O-bound work starts, rather than when it finishes.
Could anyone help what does it mean by the threads r freed up when the I/O starts??
Here is the article : https://learn.microsoft.com/en-us/dotnet/standard/async-in-depth
When ASP.NET gets an HTTP request, it takes a thread from the thread pool and uses that to execute the handler for that request (e.g., a specific controller action).
For synchronous actions, the thread stays assigned to that HTTP request until the action completes. For asynchronous actions, the await in the action method may cause the thread to return an incomplete task to the ASP.NET runtime. In this case, ASP.NET will free up the thread to handle other requests while the I/O is in flight.
Further reading about the difference between synchronous and asynchronous request handling and how asynchronous work doesn't require a thread at all times.
When your application makes a call to an external resource like Database or HttpClient thread, that initiated connection needs to wait.
Until it gets a response, it waits idly.
In the asynchronous approach, the thread gets released as soon as the app makes an external call.
Here is an article about how it happens:
https://medium.com/#karol.rossa/asynchronous-programming-73b4f1988cc6
And performance comparison between async and sync apporach
https://medium.com/#karol.rossa/asynchronous-performance-1be01a71925d
Here's an analogy for you: have you ever ordered at a restaurant with a large group and had someone not be ready to order when the waiter came to them? Did they bring in a different waiter to wait for him or did the waiter just come back to him after he took other people's orders?
The fact that the waiter is allowed to come back to him later means that he's freed up immediately after calling on him rather than having to wait around until he's ready.
Asynchronous I/O works the same way. When you do a web service call, for example, the slowest part (from the perspective of the client at least) is waiting for the result to come back: most of the delay is introduced by the network (and the other server), during which time the client thread would otherwise have nothing to do but wait. Async allows the client to do other things in the background.
I'd like to start using asynchronous processing in IIS. Edit: I'm talking about using the task parallel library.
For example, on certain page loads I want to log a bunch of crap, send an email, update some tables, etc. But I don't want to make the user wait for me to log all that crap.
So normally what I do is I have a static Queue that I push the log info onto, and then I have a cron job that calls a special page every 10 minutes whose OnLoad flushes out the queue. This works, but it's kind of clunky to setup, especially when you want to log 50 things. I'd rather do this:
Task.CreateNew(() => Log(theStuff));
However I'm terrified of running tasks in IIS because one slip up and your entire website goes down.
So now I have
SafeTask.FireAndForget(() => Log(theStuff));
This wraps the delegate in some try/catch and passes it into Task.CreateNew. So if someone changes something that affects something else that generates an exception somewhere else that accidentally gets thrown on the task thread, we get a notification instead of a crashed website. Also, the error notification inside the catch is also inside its own try/catch, and the catch for that also has a try/catch that tries to log in a different way.
Now that I can safely run stuff asynchronously in IIS, what other things do I need to worry about before I can start using my SafeTask class?
Every request in IIS and .net is processed in one thread by default. This thread comes from a thread pool called the "Application Pool". Existing threads are reused so you can't really use them for thread state unless you clear or set it every time. You define the size of this thread pool using a formula from MSDN in the machine.config or even your web.config.
Now, every async function call is put on a different thread. This includes async web service calls, async page functions, async delegates, etc. This thread comes from the "application pool" thus reducing the number of thread available for IIS to service new requests.
Most likely, your application will work just fine while using async function calls. In case you are worried or you have a lot of async tasks then you may want to create your own thread pool or look at SmartThreadPool on codeplex.
Hope this helps.
Consider using the page's OnUnload event. Read about it here: http://msdn.microsoft.com/en-us/library/ms178472.aspx
This event fires after the content is sent to the user (so the user isn't blocked while you do work), and should completely satisfy your requirement without introducing additional threads.
Specific to your question, you should be concerned about thread pool exhaustion only if your load and performance testing suggests you're running up against thread limits. If you're not then what you propose is certainly reasonable.
I'm not talking about asynchronous pages or asynchronous handlers, I just want to know if I should be afraid of any side effect when I invoke an asynchronous method that will end after the page is finished rendering.
Example given: Each time that a user do login, I have to launch a heavy and time consuming SQL operation, but the user doesn't need to know the result of that operation, so I can execute the query using BeginExecuteNonQuery without pass any callback, and finish rendering the page.
My concern is, what happen if the HTTP call ends (because the page is served) and whatever I've executed asynchronously is already running? is ASP.NET or IIS going to cut, destroy, void anything?
Cheers.
That operation will run, even when the request has finished. However, please note that the ASP.NET host aggressively kills threads. When IIS has any reason for unloading or recycling the AppDomain, your background thread will be killed. Unloading happens in several situations. For instance when no new requests have come in for a certain period of time. Or when too many exceptions are fired from the application within a certain period of time. Or when the memory pressure gets too high.
If you need the guarantee, that the operation will finish, I think there are three things you can do:
Speed up the operation so that it can run synchronously, or
Move that that heavy operation to a Windows Service and let that execute it, or
You can hook onto the HostingEnvironment.RegisterObject method (as Phill Haack explains here) (demands full trust) to prevent the AppDomain to go down while that thread is running.
If you have a callback registered, the process will comeback to notify the callback otherwise it will still complete the job. AFAIK - neither ASP.NET or IIS will cut/destroy or void anything as the execution was already ordered and it has to complete.
I have a req where in i have to create a worker thread and keep it alive throughout my application life time to perform some back ground tasks . so is there any way i can stack tasks to this worker thread when ever needed by my application .?
Update: Even though you've indicated in comments you have to do this in Asp.Net, I'll leave my original content below, as it has some useful links.
Since Asp.Net uses the thread pool to schedule incoming requests, running your background task on the thread pool will take one thread off of it and will impact Asp.Net performance. Thus, you will have to use the Thread class.
To achieve your scenario, you can create a new Thread instance, set its IsBackground property to true and start it. Once started, the thread will wait for an AutoResetEvent (using the WaitOne method) to be set by an incoming request (using the Set met6hod), which will signal the background thread that its task should be processed. Once the task is finished, the background thread will again wait on the event.
This is the simplest implementation, which does not allow passing parameters between the request and the background thread and does not allow more than one tasks to be queued at a time. If you need support for parameters or queueing, you will have to keep a reference to the thread object somewhere it ill be accessible to the incoming requests.
You will also have to consider that your background thread can be killed at any point in time, if IIS decides to recycle the Asp.Net worker process. Also, throwing an exception inside the background thread will cause IIS to recycle the Asp.Net worker process.
There are also some considerations around the identity of the background thread. In particular, a background thread can't easily impersonate the identity of the user on the current incoming request. It is possible, but it will require you to pass the user identity each time a new task is scheduled by a request.
It would be useful if you tell us what language and what platform you are writing your code in.
If it happens to be a Windows platform, there is a thread pool you can "borrow" threads from for your tasks. You can schedule your task on the thread pool by using either the QueueUserWorkItem API (C++) or the ThreadPool.QueueUserWorkItem (C#/.Net). Note there are some implications if your task will be running for a longer time.
You can also create your own thread using either the Thread class (C#/.Net) or the _beginthreadex or the CreateThread API (C++). In this case, you will have to implement a queue for the foreground thread to schedule the tasks on and you will have a loop on the background thread to pick the new tasks and execute them. And of course, you will have to synchronize the access to that queue from both threads using some synchronization primitive like a CRITICAL_SECTION (C++) or the lock statement (C#/.Net).
For Linux or OS X you might look into POSIX threads. I have not done much *nix style programming, so there might be even better alternatives. If you are targeting one of these platforms, add that info to your question and I am sure there will be helpful answers in no time.
Creating a thread is typically done by calling a special primitive (CreateThread() in Win32) and passing it an entry function. That function code in invoked in the newly spawned thread and can do whatever it wishes - for example, it could start a loop and peek tasks from a queue created in advance. Your main thread could post tasks to that queue so that the worker thread processes them. When there're no tasks in the queue the worker thread could simply block on a synchronization primitive (an event for example) waiting for new tasks to be posted.
Is it possible to use BackGroundWorker thread in ASP.NET 2.0 for the following scenario, so that the user at the browser's end does not have to wait for long time?
Scenario
The browser requests a page, say SendEmails.aspx
SendEmails.aspx page creates a BackgroundWorker thread, and supplies the thread with enough context to create and send emails.
The browser receives the response from the ComposeAndSendEmails.aspx, saying that emails are being sent.
Meanwhile, the background thread is engaged in a process of creating and sending emails which could take some considerable time to complete.
My main concern is about keeping the BackgroundWorker thread running, trying to send, say 50 emails while the ASP.NET workerprocess threadpool thread is long gone.
If you don't want to use the AJAX libraries, or the e-mail processing is REALLY long and would timeout a standard AJAX request, you can use an AsynchronousPostBack method that was the "old hack" in the .net 1.1 days.
Essentially what you do is have your submit button begin the e-mail processing in an asynchronous state, while the user is taken to an intermediate page. The benefit to this is that you can have your intermediate page refresh as much as needed, without worrying about hitting the standard timeouts.
When your background process is complete, it will put a little "done" flag in the database/application variable/whatever. When your intermediate page does a refresh of itself, it detects this flag and automatically redirects the user to the "done" page.
Again, AJAX makes all of this moot, but if for some reason you have a very intensive or timely process that has to be done over the web, this solution will work for you. I found a nice tutorial on it here and there are plenty more out there.
I had to use a process like this when we were working on a "web check-in" type application that was interfacing with a third party application and their import API was hideously slow.
EDIT: GAH! Curse you Guzlar and your god-like typing abilities 8^D.
You shouldn't do any threading from ASP.NET pages. Any thread that is long running is in danger of being killed when the worker process recycles. You can't predict when this will happen. Any long-running processes need to be handled by a windows service. You can kick off these processes by dropping a message in MSMQ, for example.
ThreadPool.QueueUserWorkItem(delegateThatSendsEmails)
or on System.Net.Mail.SmtpServer use the SendAsync method.
You want to put the email sending code on another thread, because then it will return the the user immediately, and will just process, no matter how long it takes.
It is possible. Once you start a new thread asynchronously from page, page request will proceed and send the page back to the user. The async thread will continue to run on the server but will no longer have access to the session.
If you have to show task progress, consider some Ajax techniques.
What you need to use for this scenario is Asynchronous Pages, a feature that was added in ASP.NET 2.0
Asynchronous pages offer a neat
solution to the problems caused by
I/O-bound requests. Page processing
begins on a thread-pool thread, but
that thread is returned to the thread
pool once an asynchronous I/O
operation begins in response to a
signal from ASP.NET. When the
operation completes, ASP.NET grabs
another thread from the thread pool
and finishes processing the request.
Scalability increases because
thread-pool threads are used more
efficiently. Threads that would
otherwise be stuck waiting for I/O to
complete can now be used to service
other requests. The direct
beneficiaries are requests that don't
perform lengthy I/O operations and can
therefore get in and out of the
pipeline quickly. Long waits to get
into the pipeline have a
disproportionately negative impact on
the performance of such requests.
http://msdn.microsoft.com/en-us/magazine/cc163725.aspx
If you want using multitheading in your ASP page, you might using simple threading model like this:
{
System.Threading.Thread _thread = new Thread(new ThreadStart(Activity_DoWork));
_thred.Start();
}
Activity_DoWork()
{
/*Do some things...
}
This method is correct working with ASP pages. The ASP page with BackgroundWorker will not start while BackgroundWorker will finish.
5 years later, but problems the same… If you want to perform fire-and-forget operations from your application and forget about all difficulties related to background job processing in ASP.NET applications, you can use http://hangfire.io.
It does not loose your jobs on recycling process, because it uses persistent storage to keep information about background jobs.
It automatically retries your background jobs that were aborted or failed due to transient exception (SMTP Server connectivity errors).
It allows you to easily debug background jobs through the integrated web interface.
It is very easy to install/configure/use HangFire.
There is also tutorial Sending Mail in Background with ASP.NET MVC for using HangFire with Postal.