I want to implement such logic in my asp-net-mvc application:
user clicks a button ->
server executes some time-consuming logic in ~15 threads (i get data from really slow independent sources) ->
when all work is done, server merges the results and passes it back to user
The other day i've seen an article which explained why creating new Threads in asp-net application is highly not recommended, and ThreadPool is the one that should be used.
What are best practices for mvc in this case? Why shouldnt i create my threads, backgroundworkers, tasks, whatever by myself and use threadpool? The Application will be hosted on a public server, if it matters.
This seems like a really good place for using the new AsycController in ASP.NET MVC 2. It is really easy to use and lets you run queries against multiple independent sources without blocking request threads.
MSDN has a great example where they are querying a news service, a weather service, and a sports service.
You can see in the original code that they are querying each source sequentially, but in the final version, all the tasks run in parallel and control returns to the controller when they are all completed:
public void IndexAsync(string city)
{
AsyncManager.OutstandingOperations.Increment(3);
NewsService newsService = new NewsService();
newsService.GetHeadlinesCompleted += (sender, e) =>
{
AsyncManager.Parameters["headlines"] = e.Value;
AsyncManager.OutstandingOperations.Decrement();
};
newsService.GetHeadlinesAsync();
SportsService sportsService = new SportsService();
sportsService.GetScoresCompleted += (sender, e) =>
{
AsyncManager.Parameters["scores"] = e.Value;
AsyncManager.OutstandingOperations.Decrement();
};
sportsService.GetScoresAsync();
WeatherService weatherService = new WeatherService();
weatherService.GetForecastCompleted += (sender, e) =>
{
AsyncManager.Parameters["forecast"] = e.Value;
AsyncManager.OutstandingOperations.Decrement();
};
weatherService.GetForecastAsync();
}
public ActionResult IndexCompleted(string[] headlines, string[] scores, string[] forecast)
{
return View("Common", new PortalViewModel {
NewsHeadlines = headlines,
SportsScores = scores,
Weather = forecast
});
}
Yes JQuery and some AJAX will do it most properly.
Load the Page, then send ~15 separate Ajax queries back to the Server and let them finish asynchronously. This way you can let the web-server handle Threading (which is does well) and concentrate on displaying either a ticker or a virtual progress bar to the user while waiting.
If you're using .Net 4, I would even recommend looking at the parallel namespaces. They make this even simpler and do a better job of utilizing all your CPU cores.
I would also look at offloading this from your main web app altogether. Having a separate set of services or a message queue to handle this long running request will let you scale a lot more easily and will allow your web app to worry about servicing page requests, not performing long running logic. Google up something like "iis long running request" to get started.
Because a ThreadPool is designed to do that:
It provides a pool of threads that can be used to execute tasks, post work items, process asynchronous I/O, wait on behalf of other threads, and process timers.
I guess it's better to store the results on your server in a database when they are done.
And then you could use a timer and AJAX to periodically request if the process is done, if so, get data.
You shouldn't create your own threads because this has significant overhead. Creating a new thread is expensive. So the question should be: why shouldn't you use the threadpool? It has a number of threads ready for use. An even better approach would be to use the task parallel library, it provides a layer of abstraction above the threadpool that makes it a lot easier to work with threads.
You should realize that the only (easy) way to accomplish this, is by having the user wait until all work is done and then serve the page. A more complicated but also more responsive approach would be to use Ajax (via jQuery for example) to ask the server periodically how far the work has progressed. This way you can also provide some progress indicator.
Related
I have a .NET Core Web API application. The app contains models for db access and a way to send emails to users. My end goal is to call a method nightly (to email users that their registration expired and to mark it expired in the database).
So, in short, I can build an endpoint and call it manually every night. Or build a windows service to call the endpoint. Or build a windows service to do the work. But I want to keep the logic in one application.
My ideal solution would be to have a timer running inside my app and calling a method in a service every 24 hours. Of course, that's not possible, so I am looking at Hangfire. The official documentation seems to indicate that there is a lot of overhead.
Hangfire keeps background jobs and other information that relates to the processing inside a persistent storage. Persistence helps background jobs to survive on application restarts, server reboots, etc.
Do I need this if I just want to call a method?
Background jobs are processed by Hangfire Server. It is implemented as a set of dedicated (not thread pool’s) background threads that fetch jobs from a storage and process them. Server is also responsible to keep the storage clean and remove old data automatically.
Do I even need jobs?
Is there a way to JUST call a method without all this overhead with Hangfire?
tl;dr: Are there options to opt out of the dashboard, database connectivity, etc and just have Hangfire work as a timer?
My ideal solution would be to have a timer running inside my app and calling a method in a service every 24 hours. Of course, that's not possible...
It's very possible, actually, using IHostedService. You should take some time to read the full documentation, but simply, for your scenario, you'd just need something like:
internal class NightlyEmailHostedService : IHostedService, IDisposable
{
private Timer _timer;
public Task StartAsync(CancellationToken cancellationToken)
{
_timer = new Timer(DoWork, null, TimeSpan.Zero,
TimeSpan.FromHours(24));
return Task.CompletedTask;
}
private void DoWork(object state)
{
// send email
}
public Task StopAsync(CancellationToken cancellationToken)
{
_timer?.Change(Timeout.Infinite, 0);
return Task.CompletedTask;
}
public void Dispose()
{
_timer?.Dispose();
}
}
Then, in Startup.cs just add:
services.AddHostedService<NightlyEmailHostedService>();
Now, that's an extremely naive approach. It basically just kicks off a timer that will run once every 24 hours, but depending on when your app started, it may not always be at night. In reality, you'd likely want to have the timer run every minute or so, and check against a particular time you actually want the email to go out. There's an interesting implementation of handling cron-style times via an IHostedService you might want to reference.
The long and short is that it's very possible to do this all in your app, without requiring anything additional like Hangfire. However, you have to a do a bit more work than you would have to using something like Hangfire, of course.
I have an MVC controller action that needs to make several web requests. So in an attempt to free this thread to handle other incoming web requests I wrote something like this:
(NOTE: this is an over simplification of the real code)
public async Task<ViewResult> Index()
{
MyObj o = await Task.Factory.StartNew<MyObj>(() =>
{
WebClient c = new WebClient();
var res1 = c.DownloadString("...");
var res2 = c.DownloadString("..."); //Not shown but res1 required for this call.
return new MyObj(res1, res2);
}
return View(o);
}
My question is have I made things better or worse by spinning up a new thread to do this work. My intention is to free this .Net thread to handle other incoming requests while the network requests get made. But after looking at it seems to me I'm still making a .Net thread block, just maybe a different one from the thread pool instead of the original, so I'm really no better off. So is my suspicion correct and the above code making things worse?
The benefit of Task.Factory.StartNew() if it worked... is that it simplifies the work inside from having to be done all async. My question though is: Does doing it that way in fact free a thread for handling incoming web requests or is it still tying up the same number of threads from the thread pool?
Yes, you are just blocking a different thread now.
If this was good, MVC would just wrap all action methods in a new task automatically.
To unblock threads you need async IO somewhere under the covers. The BCL usually provides this. If it says "async" on the box it usually is async IO.
That said, few web apps have the problem of running out of threads. The thread-pool starts tons of threads if necessary. Very few web apps are limited in throughput by the number of threads available. Do you really need to process 100s of concurrent requests? Can your backend services even handle this load? If any of these questions is answered by "no" you don't need async.
The code you made isn't really optimal, since both long operations block each other:
var result1 = c.DownloadString("...");
will run first and when done, it runs
var result2 = c.DownloadString("...");
Adding an extra thread where this runs in will not improve performance, nor will it free any open requests.
You don't need to the factory block. You can simplify your example to:
public async Task<ViewResult> Index()
{
WebClient client = new WebClient();
var result1 = await client.DownloadStringAsync("...");
var result2 = await client.DownloadStringAsync("...");
/* whatever you're doing with result1 & result2 */
return View();
}
I need to execute an infinite while loop and want to initiate the execution in global.asax.
My question is how exactly should I do it? Should I start a new Thread or should I use Async and Task or anything else? Inside the while loop I need to do await TaskEx.Delay(5000);
How do I do this so it will not block any other processes and will not create memory leaks?
I use VS10,AsyncCTP3,MVC4
EDIT:
public void SignalRConnectionRecovery()
{
while (true)
{
Clients.SetConnectionTimeStamp(DateTime.UtcNow.ToString());
await TaskEx.Delay(5000);
}
}
All I need to do is to run this as a singleton instance globally as long as application is available.
EDIT:SOLVED
This is the final solution in Global.asax
protected void Application_Start()
{
Thread signalRConnectionRecovery = new Thread(SignalRConnectionRecovery);
signalRConnectionRecovery.IsBackground = true;
signalRConnectionRecovery.Start();
Application["SignalRConnectionRecovery"] = signalRConnectionRecovery;
}
protected void Application_End()
{
try
{
Thread signalRConnectionRecovery = (Thread)Application["SignalRConnectionRecovery"];
if (signalRConnectionRecovery != null && signalRConnectionRecovery.IsAlive)
{
signalRConnectionRecovery.Abort();
}
}
catch
{
///
}
}
I found this nice article about how to use async worker: http://www.dotnetfunda.com/articles/article613-background-processes-in-asp-net-web-applications.aspx
And this:
http://code.msdn.microsoft.com/CSASPNETBackgroundWorker-dda8d7b6
But I think for my needs this one will be perfect:
http://forums.asp.net/t/1433665.aspx/1
ASP.NET is not designed to handle this kind of requirement. If you need something to run constantly, you would be better off creating a windows service.
Update
ASP.NET is not designed for long running tasks. It's designed to respond quickly to HTTP requests. See Cyborgx37's answer or Can I use threads to carry out long-running jobs on IIS? for a few reasons why.
Update
Now that you finally mentioned you are working with SignalR, I see that you are trying to host SignalR within ASP.NET, correct? I think you're going about this the wrong way, see the example NuGet package referenced on the project wiki. This example uses an IAsyncHttpHandler to manage tasks.
You can start a thread in your global.asax, however it will only run till your asp.net process get recycled. This will happen at least once a day, or when no one uses of your site. If the process get recycled, the only way the thread is restarted agian, is when you have a hit on your site. So the thread is not running continueuosly.
To get a continues process it is better to start a windows service.
If you do the 'In process' solution, it realy depends on what your are doing. The Thread itself will not cause you any problems in memory or deadlocks. You should add a meganism to stop your thread when the application stops. Otherwise restarting will take a long time, because it will wait for your thread to stop.
This is an old post, but as I was seraching for this, I would like to report that in .NET 4.5.2 there is a native way to do it with QueueBackgroundWorkItem.
Take a look at this post: https://blogs.msdn.microsoft.com/webdev/2014/06/04/queuebackgroundworkitem-to-reliably-schedule-and-run-background-processes-in-asp-net/
MarianoC
It depends what you are trying to accomplish in your while loop, but in general this is the kind of situation where a Windows Service is the best answer. Installing a Windows Service is going to require that you have admin privileges on the web server.
With an infinite loop you end up with a lot of issues regard the Windows message pump. This is the thing that keeps a Windows application alive even when the application isn't "doing" anything. Without it, a program simply ends.
The problem with an infinite loop is that the application is stuck "doing" something, which prevents other applications (or threads) from "doing" their thing. There have been a few workarounds, such as the DoEvents in Windows Forms, but they all have some serious drawbacks when it comes to responsiveness and resource management. (Acceptable on a small LOB application, maybe not on a web server.) Even if the while-loop is on a separate thread, it will use up all available processing power.
Asynchronus programming is really designed more for long-running processes, such as waiting for a database to return a result or waiting for a printer to come online. In these cases, it's the external process that is taking a long time, not a while-loop.
If a Window Service is not possible, then I think your best bet is going to be setting up a separate thread with its own message pump, but it's a bit complicated. I've never done it on a web server, but you might be able to start an Application. This will provide a message pump for you and allow you to respond to Windows events, etc. The only problem is that this is going to start a Windows application (either WPF or WinForms), which may not be desirable on a web server.
What are you trying to accomplish? Is there another way you might go about it?
I found this nice article about how to use async worker, will give it a try. http://www.dotnetfunda.com/articles/article613-background-processes-in-asp-net-web-applications.aspx
And this:
http://code.msdn.microsoft.com/CSASPNETBackgroundWorker-dda8d7b6
But I think for my needs this one will be perfect:
http://forums.asp.net/t/1433665.aspx/1
Since the very begining of writing ASP.NET applications when I wanted to add a threading there are 3 simple ways I can accomplish threading within my ASP.NET application :
Using the System.Threading.ThreadPool.
Using a custom delegate and calling its BeginInvoke method.
Using custom threads with the aid of System.Threading.Thread class.
The first two methods offer a quick way to fire off worker threads for your application. But unfortunately, they hurt the overall performance of your application since they consume threads from the same pool used by ASP.NET to handle HTTP requests.
Then I wanted to use a new Task or async/await to write IHttpAsyncHandler. One example you can find is what Drew Marsh explains here : https://stackoverflow.com/a/6389323/261950
My guess is that using Task or async/await still consume the thread from the ASP.NET thread pool and I don't want for the obvious reason.
Could you please tell me if I can use Task (async/await) on the background thread like with System.Threading.Thread class and not from thread pool ?
Thanks in advance for your help.
Thomas
This situation is where Task, async, and await really shine. Here's the same example, refactored to take full advantage of async (it also uses some helper classes from my AsyncEx library to clean up the mapping code):
// First, a base class that takes care of the Task -> IAsyncResult mapping.
// In .NET 4.5, you would use HttpTaskAsyncHandler instead.
public abstract class HttpAsyncHandlerBase : IHttpAsyncHandler
{
public abstract Task ProcessRequestAsync(HttpContext context);
IAsyncResult IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
{
var task = ProcessRequestAsync(context);
return Nito.AsyncEx.AsyncFactory.ToBegin(task, cb, extraData);
}
void EndProcessRequest(IAsyncResult result)
{
Nito.AsyncEx.AsyncFactory.ToEnd(result);
}
void ProcessRequest(HttpContext context)
{
EndProcessRequest(BeginProcessRequest(context, null, null));
}
public virtual bool IsReusable
{
get { return true; }
}
}
// Now, our (async) Task implementation
public class MyAsyncHandler : HttpAsyncHandlerBase
{
public override async Task ProcessRequestAsync(HttpContext context)
{
using (var webClient = new WebClient())
{
var data = await webClient.DownloadDataTaskAsync("http://my resource");
context.Response.ContentType = "text/xml";
context.Response.OutputStream.Write(data, 0, data.Length);
}
}
}
(As noted in the code, .NET 4.5 has a HttpTaskAsyncHandler which is similar to our HttpAsyncHandlerBase above).
The really cool thing about async is that it doesn't take any threads while doing the background operation:
An ASP.NET request thread kicks off the request, and it starts downloading using the WebClient.
While the download is going, the await actually returns out of the async method, leaving the request thread. That request thread is returned back to the thread pool - leaving 0 (zero) threads servicing this request.
When the download completes, the async method is resumed on a request thread. That request thread is briefly used just to write the actual response.
This is the optimal threading solution (since a request thread is required to write the response).
The original example also uses threads optimally - as far as the threading goes, it's the same as the async-based code. But IMO the async code is easier to read.
If you want to know more about async, I have an intro post on my blog.
I've been looking for information through internet for a couple of days. Let me sum up what I found until now :
ASP.NET ThreadPool facts
As Andres said: When async/await will not consume an additional ThreadPool thread ? Only in the case you are using BCL Async methods. that uses an IOCP thread to execute the IO bound operation.
Andres continues with ...If you are trying to async execute some sync code or your own library code, that code will probably use an additional ThreadPool thread unless you explicitely use the IOCP ThreadPool or your own ThreadPool.
But as far as I know you can't chose whetever you want to use a IOCP thread, and making correct implementation of the threadPool is not worth the effort. I doubt someone does a better one that already exists.
ASP.NET uses threads from a common language runtime (CLR) thread pool to process requests. As long as there are threads available in the thread pool, ASP.NET has no trouble dispatching incoming requests.
Async delegates use the threads from ThreadPool.
When you should start thinking about implementing asynchronous execution ?
When your application performs relatively lengthy I/O operations (database queries, Web service calls, and other I/O operations)
If you want to do I/O work, then you should be using an I/O thread (I/O Completion Port) and specifically you should be using the async callbacks supported by whatever library class you're using. Theirs names start with the words Begin and End.
If requests are computationally cheap to process, then parallelism is probably an unnecessary overhead.
If the incoming request rate is high, then adding more parallelism will likely yield few benefits and could actually decrease performance, since the incoming rate of work may be high enough to keep the CPUs busy.
Should I create new Threads ?
Avoid creating new threads like you would avoid the plague.
If you are actually queuing enough work items to prevent ASP.NET from processing further requests, then you should be starving the thread pool! If you are running literally hundreds of CPU-intensive operations at the same time, what good would it do to have another worker thread to serve an ASP.NET request, when the machine is already overloaded.
And the TPL ?
TPL can adapt to use available resources within a process. If the server is already loaded, the TPL can use as little as one worker and make forward progress. If the server is mostly free, they can grow to use as many workers as the ThreadPool can spare.
Tasks use threadpool threads to execute.
References
http://msdn.microsoft.com/en-us/magazine/cc163463.aspx
http://blogs.msdn.com/b/pfxteam/archive/2010/02/08/9960003.aspx
https://stackoverflow.com/a/2642789/261950
Saying that "0 (zero) threads will be servicing this request" is not accurate entirely.
I think you mean "from the ASP.NET ThreadPool", and in the general case that will be correct.
When async/await will not consume an additional ThreadPool thread?
Only in the case you are using BCL Async methods (like the ones provided by WebClient async extensions) that uses an IOCP thread to execute the IO bound operation.
If you are trying to async execute some sync code or your own library code, that code will probably use an additional ThreadPool thread unless you explicitely use the IOCP ThreadPool or your own ThreadPool.
Thanks,
Andrés.
The Parallel Extensions team has a blog post on using TPL with ASP.NET that explains how TPL and PLINQ use the ASP.NET ThreadPool. The post even has a decision chart to help you pick the right approach.
In short, PLINQ uses one worker thread per core from the threadpool for the entire execution of the query, which can lead to problems if you have high traffic.
The Task and Parallel methods on the other hand will adapt to the process's resources and can use as little as one thread for processing.
As far as the Async CTP is concerned, there is little conceptual difference between the async/await construct and using Tasks directly. The compiler uses some magic to convert awaits to Tasks and Continuations behind the scenes. The big difference is that your code is much MUCH cleaner and easier to debug.
Another thing to consider is that async/await and TPL (Task) are not the same thing.
Please read this excellent post http://blogs.msdn.com/b/ericlippert/archive/2010/11/04/asynchrony-in-c-5-0-part-four-it-s-not-magic.aspx to understand why async/await doesn't mean "using a background thread".
Going back to our topic here, in your particular case where you want to perform some expensive calculations inside an AsyncHandler you have three choices:
1) leave the code inside the Asynchandler so the expensive calculation will use the current thread from the ThreadPool.
2) run the expensive calculation code in another ThreadPool thread by using Task.Run or a Delegate
3) Run the expensive calculation code in another thread from your custom thread pool (or IOCP threadPool).
The second case MIGHT be enough for you depending on how long your "calculation" process run and how much load you have. The safe option is #3 but a lot more expensive in coding/testing. I also recommend always using .NET 4 for production systems using async design because there are some hard limits in .NET 3.5.
There's a nice implementation of HttpTaskAsyncHandler for the .NET 4.0 in the SignalR project. You may want to ckeck it out: http://bit.ly/Jfy2s9
I have a web service that receives requests from users and returns some json. I need to save the json string in the database so for the moment, the write query occurs before the response is sent back.
Is there a way to send the response first and then do the write query, after the response left the web service?
Thanks.
There's a couple of different options here - they all have tradeoffs, though, and would be pretty esoteric. You don't mention why you want to do this, so I'm guessing performance. If that's the case, I think you're barking up the wrong tree - a simple write is almost certainly not your performance problem.
So, off the top of my head:
Queuing, as Ragesh mentions, would be a nice approach. This gets you similar semantics of a transaction, while off loading the write. You still have to write to the queue, though, which may be about the same overhead as writing to the DB.
You could spawn a new thread (using either the ThreadPool or System.Threading.Thread - there's some debates about which is preferable in ASP.NET) to handle the write. This can generally work, but you may have issues with unhandled exceptions, app domain restarts, etc.
You could store the JSON data into a static or Application variable, then use a Timer to periodically write them to the DB. This will be multithreaded code, so you will need to synchronize read/writes to the collection.
Similar to #3, store the JSON data into Cache and use the invalidation callback to write to the DB.
Lots of variations on store somewhere (memory, disk, flat DB table, etc.), process later (ASP.NET, scheduled task, Windows Service, Sql Agent, etc.).
#frenchie says: a response starts by reading the json string from the db and ends with writing it back. In other words, if the user sends a request, the json string that's going to be read must be the one that was written in the previous response.
That complicates things, since inherent in async work is not knowing when something is done. If you require the async portion (writing back to the DB) to be done before handling the next request, you'll have to execute a wait to make sure it actually completed. In order to do that, you'll need to keep server side state on the client - not exactly a best practice as far as services go (though, it sounds like you're already doing that with these JSON request/response pairs).
Given the complications, I would make sure that you've done your profiling and determined it is indeed a performance problem.
You can do schedule a query work like
ThreadPool.QueueUserWorkItem(state =>
this.AsynchronousExecuteReference());
// and run
static void AsynchronousExecuteReference()
{
// run here your sql update
}
One other example using Thread inside an class and you can pass parameters to it.
public class RunThreadProcess
{
// Some parametres
public int cProductID;
// my thread
private Thread t = null;
// start it
public Thread Start()
{
t = new Thread(new ThreadStart(this.work));
t.IsBackground = true;
t.SetApartmentState(ApartmentState.MTA);
t.Start();
return t;
}
// actually work
private void work()
{
// do thread work
all parametres are available here
}
}
And here is how I run it
var OneAction = new RunThreadProcess();
OneAction.cProductID = 100;
OneAction.Start();
Do not worry about memory, CG knows that this process is used until the thread ends, so I have check it and CG not delete it and wait the thread to ends.
You should look at using message queues like MSMQ, ActiveMQ or RabbitMQ to do this. When you receive your request, you'll put the relevant data in to the queue, and send your response to the client. At the other end of the queue, you'll have some process that reads from the queue and inserts data in to your database.
this is missing the point of a request/response. unless you want to get into async commands like a service bus, but that's pub/sub, not request/response. the point of request/response is to do the work on the server after receiving the request and before sending the response. even if the work is sending an async message to a service bus.
You could try moving your web service URL to an ASPX page where the lifecycles come in to play.
In the code-behind, call your routine that does the main portion of the work in Page_Load or Page_Prerender (or whenever is appropriate prior to the response being sent) and then do your DB work in the Page_Unload event which occurs after the response has been sent (http://msdn.microsoft.com/en-us/library/ie/ms178472.aspx).