In my application I have the following:
db2.CreateTable<CategoryGroup>();
db2.CreateTable<Category>();
db2.CreateTable<CategoryGroupSource>();
db2.CreateTable<CategorySource>();
db2.CreateTable<Phrase>();
db2.CreateTable<PhraseSource>();
db2.CreateTable<Score>();
db2.CreateTable<Setting>();
From what I understand there is an Async way to do this also:
database.CreateTableAsync<TodoItem>().Wait();
Can someone explain if there is any advantage in me using the Async way and do people normally always use the Async?
Also are there likely to be benefits if I use this type of Async query:
public Task<TodoItem> GetItemAsync(int id)
{
return database.Table<TodoItem>().Where(i => i.ID == id).FirstOrDefaultAsync();
}
When calling the methods on the main (UI) thread everything on the UI stops for as long as it takes that method to execute. If db2.CreateTable<CategoryGroup>() doesn't take up much time when doing it's thing, it shouldn't be a problem.
Doing a lot of time consuming actions straight after each other might affect your UI and make it freeze.
Calling the *Async variant of the method moves the work to a background thread, via the task API. Calling Wait() on that task, though, makes the current thread (in this case the UI thread) wait for the task to finish, and you're stuck with the same problem.
You should always await tasks: await database.CreateTableAsync<TodoItem>(). This will let it execute on a background thread and not make the current thread wait for it to finish. The next line in your code won't be executed until the Task is finished though. When you write the code, it makes the `Async variant look like it's behaving like the regular version.
Personally, I'd probably move all the methods into a task and just await that. That way you're not returning to the UI thread between each task to execute the next one:
await Task.Run(() =>
{
db2.CreateTable<CategoryGroup>();
db2.CreateTable<Category>();
db2.CreateTable<CategoryGroupSource>();
db2.CreateTable<CategorySource>();
db2.CreateTable<Phrase>();
db2.CreateTable<PhraseSource>();
db2.CreateTable<Score>();
db2.CreateTable<Setting>();
}
In this case you're making the database do all it's work on a background thread (and not freezing the UI while it's doing it). It then returns the result to the UI thread to enable you to update UI.
public Task<TodoItem> GetItemAsync(int id)
{
return database.Table<TodoItem>().Where(i => i.ID == id).FirstOrDefaultAsync();
}
Related
I'm trying to achieve something like this:
[HttpPost]
public ActionResult PostData()
{
// 1. Need to trigger an async operation for some long processes
// Trying to trigger SideProcess() async controller
// Return to view to allow user do other stuff without waiting for above process to complete
return View("Listing", "Users");
}
public async Task<ActionResult> SideProcess()
{
...
}
Can I use async task method like above? Because I definitely have to call PostData() first on a button click. Otherwise most likely I have to use a different method.
You can, by not await-ing the result. This has some drawbacks though. (E.g. what happens if an error occurrs?)
It is better to run the async operation in via Task.Run() in such cases, since unhandled exceptions will trigger the TaskScheduler.UnobservedTaskException and don't mess with your controller action.
Task.Run(SideProcess);
I tend to use Hangfire for such problems in all of my projects and never had an issue. I would recommend and prefer it over the above solution. Comes with very handy oob features for managing BackgroundJobs (including a dashboard, automatic retry mechanisms and many more) while beeing testable too.
Doing that, you won't have any guarantees that your long running process will finish. ASP.NET was purposely not built for that.
But if you have to queue some background work, use HostingEnvironment.QueueBackgroundWorkItem.
Beware of the remarks:
Differs from a normal ThreadPool work item in that ASP.NET can keep track of how many work items registered through this API are currently running, and the ASP.NET runtime will try to delay AppDomain shutdown until these work items have finished executing. This API cannot be called outside of an ASP.NET-managed AppDomain. The provided CancellationToken will be signaled when the application is shutting down.
I often have to execute code on a separate thread that is long running, blocking, instable and\or has a potential to hang forever. Since the existence of TPL the internet is full of examples that nicely cancel a task with the cancellation token but I never found an example that kills a task that hangs. Code that hangs forever is likely to be expected as soon as you communicate with hardware or call some third party code. A task that hangs cannot check the cancellation token and is doomed to stay alive forever. In critical applications I equip those tasks with alive signals that are sent on regular time intervals. As soon as a hanging task is detected, it is killed and a new instance is started.
The code below shows an example task that calls a long running placeholder method SomeThirdPartyLongOperation() which has the potential to hang forever. The StopTask() first checks if the task is still running an tries to cancel it with the cancellation token. If that doesn’t work, the task hangs and the underlying thread is interrupted\aborted old school style.
private Task _task;
private Thread _thread;
private CancellationTokenSource _cancellationTokenSource;
public void StartTask()
{
_cancellationTokenSource = new CancellationTokenSource();
_task = Task.Factory.StartNew(() => DoWork(_cancellationTokenSource.Token), _cancellationTokenSource.Token, TaskCreationOptions.LongRunning, TaskScheduler.Default);
}
public void StopTask()
{
if (_task.Status == TaskStatus.RanToCompletion)
return;
_cancellationTokenSource.Cancel();
try
{
_task.Wait(2000); // Wait for task to end and prevent hanging by timeout.
}
catch (AggregateException aggEx)
{
List<Exception> exceptions = aggEx.InnerExceptions.Where(e => !(e is TaskCanceledException)).ToList(); // Ignore TaskCanceledException
foreach (Exception ex in exceptions)
{
// Process exception thrown by task
}
}
if (!_task.IsCompleted) // Task hangs and didn't respond to cancellation token => old school thread abort
{
_thread.Interrupt();
if (!_thread.Join(2000))
{
_thread.Abort();
}
}
_cancellationTokenSource.Dispose();
if (_task.IsCompleted)
{
_task.Dispose();
}
}
private void DoWork(CancellationToken cancellationToken)
{
if (string.IsNullOrEmpty(Thread.CurrentThread.Name)) // Set thread name for debugging
Thread.CurrentThread.Name = "DemoThread";
_thread = Thread.CurrentThread; // Save for interrupting/aborting if thread hangs
for (int i = 0; i < 10; i++)
{
cancellationToken.ThrowIfCancellationRequested();
SomeThirdPartyLongOperation(i);
}
}
Although I’ve been using this construct for some years now, I want to know if there are some potential mistakes in it. I’ve never seen an example of a task that saves the underlying thread or gives it a name to simplify debugging, so I’m a bit unsure if this is the right way to go. Comment on any detail is welcome!
Code that hangs forever is likely to be expected as soon as you communicate with hardware or call some third party code.
Communication: absolutely not. There's always a way to timeout with communication APIs, so even with misbehaving hardware, there's no need to force-kill an I/O operation.
Third-party code: only if you're paranoid (or have high demands such as 24x7 automation).
Here's the bottom line:
There's no way to force-kill a task.
You can force-kill a thread, but this can easily cause serious problems with application state, possibility if introducing deadlocks in other parts of the code, and resource leaks.
You can force-kill an appdomain, which solves a large portion of app state / deadlock issues with killing threads. However, it doesn't solve them all, and there's still the problem of resource leaks.
You can force-kill a process. This is the only truly clean and reliable solution.
So, if you choose to trust the third-party code, I recommend that you just call it like any other API. If you require 100% reliability regardless of third-party libraries, you'll need to wrap the third-party dll into a separate process and use cross-process communication to call it.
Your current code force-kills a thread pool thread, which is certainly not recommended; those threads belong to the thread pool, not to you, and this is still true even if you specify LongRunning. If you go the kill-thread route (which is not fully reliable), then I recommend using an explicit thread.
The question is why is this task even hanging at all? I think there's no universal solution to this problem but you should focus on the task to be always responsible and not on forcing to interrupt it.
In this code, it looks like you're looking for a simple thread rather than a task - you shouldn't link tasks to threads - it's very likely that the task will switch to another thread after some async operations and you will end up on killing an innoccent thread that is not connected to your task anymore. If you really need to kill the whole thread then make a dedicated one just for this job.
You shouldn't also name or do anything with any thread that is used for tasks' default pool. Consider this code:
static void Main(string[] args)
{
Task.Run(sth);
Console.Read();
}
static async Task sth()
{
Thread.CurrentThread.Name = "My name";
Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
await Task.Delay(1);
Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
Console.WriteLine(Thread.CurrentThread.Name ?? "No name");
}
the output is:
3
4
No name
New to async and trying to understand when it makes sense to use it.
We are going to have lots methods in webapi2 calling legacy webservices.
We have lots of low level dlls (Company.Dal.dll,Company.Biz.dll) etc.. that have methods that are not async
Question
Does async has to be all the way really ?
Is there any benefit of having an high level dll (all method async) calling low level dlls (dal,biz etc legacy code) where none of the method are async?
Is it there any benefit in having just the high level component to be async and the rest syncronous?
Many thanks for clarification
Any good tutorials explaning this concept
Using async only makes sense if you actually await something. If you don't, the async method will actually be completely synchronous (and you get a warning from the compiler about it).
In this case, async doesn't have any advantages, only disadvantages: the code is more complex and less efficient.
A thread can only do one thing at a time. If procedures keep your thread busy, there is no sense in making them async.
However if there are periods where the thread in your procedure has to wait for something else to finish, your thread might do something useful instead. In those circumstances async-await becomes useful.
Eric lippert once explained async-await with a restaurant metaphor (search on the page for async-await). If you have a cook who has to wait until the bread is toasted, this cook could do something else, like cooking an egg, and get back to the toaster when the "something else" is finished, or when has to wait for something, like await for the egg to be cooked.
In software the things where your thread typically will do nothing except waiting for something to finish is when reading / writing to disk, sending or receiving data over the network etc. Those are typically actions where you can find async versions as well as non-async versions of the procedure. See for instance classes like Stream, TextReader, WebClient, etc.
If your thread has to do a lot of calculations, it is not useful to make the function async, because there is no moment your thread will not do anything but wait, so your thread won't have time to do other things.
However, if your thread could do something useful while the calculations are done, consider letting another thread do those calculations while your thread is doing the other useful stuff:
private async Task<int> MyLengthyProcedure(...)
{
Task<int> calculationTask = Task.Run( () => DoCalculations(...));
// while one of the threads is doing the calculations,
// your thread could do other useful things:
int i = DoOtherCalculations();
// or if there are calculations that could be performed
// by separate threads simultaneously, start a second task
Task<int> otherCalculationTask = Task.Run( () => DoEvenMoreCalculations(...));
// now two threads are doing calculations. This thread is still free to
// do other things
// after a while you need the result of both calculations:
await Task.WhenAll( new Task[] {calculationTask, otherCalculationTask});
// the int return value of DoCalculations and DoOtherCalculations are in
// the Result property of the task object:
int j = calculationTask.Result;
int k = otherCalculationTask.Result;
return i + j + k;
;
We recently developed a site based on SOA but this site ended up having terrible load and performance issues when it went under load. I posted a question related this issue here:
ASP.NET website becomes unresponsive under load
The site is made of an API (WEB API) site which is hosted on a 4-node cluster and a web site which is hosted on another 4-node cluster and makes calls to the API. Both are developed using ASP.NET MVC 5 and all actions/methods are based on async-await method.
After running the site under some monitoring tools such as NewRelic, investigating several dump files and profiling the worker process, it turned out that under a very light load (e.g. 16 concurrent users) we ended up having around 900 threads which utilized 100% of CPU and filled up the IIS thread queue!
Even though we managed to deploy the site to the production environment by introducing heaps of caching and performance amendments many developers in our team believe that we have to remove all async methods and covert both API and the web site to normal Web API and Action methods which simply return an Action result.
I personally am not happy with approach because my gut feeling is that we have not used the async methods properly otherwise it means that Microsoft has introduced a feature that basically is rather destructive and unusable!
Do you know any reference that clears it out that where and how async methods should/can be used? How we should use them to avoid such dramas? e.g. Based on what I read on MSDN I believe the API layer should be async but the web site could be a normal no-async ASP.NET MVC site.
Update:
Here is the async method that makes all the communications with the API.
public static async Task<T> GetApiResponse<T>(object parameters, string action, CancellationToken ctk)
{
using (var httpClient = new HttpClient())
{
httpClient.BaseAddress = new Uri(BaseApiAddress);
var formatter = new JsonMediaTypeFormatter();
return
await
httpClient.PostAsJsonAsync(action, parameters, ctk)
.ContinueWith(x => x.Result.Content.ReadAsAsync<T>(new[] { formatter }).Result, ctk);
}
}
Is there anything silly with this method? Note that when we converted all method to non-async methods we got a heaps better performance.
Here is a sample usage (I've cut the other bits of the code which was related to validation, logging etc. This code is the body of a MVC action method).
In our service wrapper:
public async static Task<IList<DownloadType>> GetSupportedContentTypes()
{
string userAgent = Request.UserAgent;
var parameters = new { Util.AppKey, Util.StoreId, QueryParameters = new { UserAgent = userAgent } };
var taskResponse = await Util.GetApiResponse<ApiResponse<SearchResponse<ProductItem>>>(
parameters,
"api/Content/ContentTypeSummary",
default(CancellationToken));
return task.Data.Groups.Select(x => x.DownloadType()).ToList();
}
And in the Action:
public async Task<ActionResult> DownloadTypes()
{
IList<DownloadType> supportedTypes = await ContentService.GetSupportedContentTypes();
Is there anything silly with this method? Note that when we converted
all method to non-async methods we got a heaps better performance.
I can see at least two things going wrong here:
public static async Task<T> GetApiResponse<T>(object parameters, string action, CancellationToken ctk)
{
using (var httpClient = new HttpClient())
{
httpClient.BaseAddress = new Uri(BaseApiAddress);
var formatter = new JsonMediaTypeFormatter();
return
await
httpClient.PostAsJsonAsync(action, parameters, ctk)
.ContinueWith(x => x.Result.Content
.ReadAsAsync<T>(new[] { formatter }).Result, ctk);
}
}
Firstly, the lambda you're passing to ContinueWith is blocking:
x => x.Result.Content.ReadAsAsync<T>(new[] { formatter }).Result
This is equivalent to:
x => {
var task = x.Result.Content.ReadAsAsync<T>(new[] { formatter });
task.Wait();
return task.Result;
};
Thus, you're blocking a pool thread on which the lambda is happened to be executed. This effectively kills the advantage of the naturally asynchronous ReadAsAsync API and reduces the scalability of your web app. Watch out for other places like this in your code.
Secondly, an ASP.NET request is handled by a server thread with a special synchronization context installed on it, AspNetSynchronizationContext. When you use await for continuation, the continuation callback will be posted to the same synchronization context, the compiler-generated code will take care of this. OTOH, when you use ContinueWith, this doesn't happen automatically.
Thus, you need to explicitly provide the correct task scheduler, remove the blocking .Result (this will return a task) and Unwrap the nested task:
return
await
httpClient.PostAsJsonAsync(action, parameters, ctk).ContinueWith(
x => x.Result.Content.ReadAsAsync<T>(new[] { formatter }),
ctk,
TaskContinuationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext()).Unwrap();
That said, you really don't need such added complexity of ContinueWith here:
var x = await httpClient.PostAsJsonAsync(action, parameters, ctk);
return await x.Content.ReadAsAsync<T>(new[] { formatter });
The following article by Stephen Toub is highly relevant:
"Async Performance: Understanding the Costs of Async and Await".
If I have to call an async method in a sync context, where using await
is not possible, what is the best way of doing it?
You almost never should need to mix await and ContinueWith, you should stick with await. Basically, if you use async, it's got to be async "all the way".
For the server-side ASP.NET MVC / Web API execution environment, it simply means the controller method should be async and return a Task or Task<>, check this. ASP.NET keeps track of pending tasks for a given HTTP request. The request is not getting completed until all tasks have been completed.
If you really need to call an async method from a synchronous method in ASP.NET, you can use AsyncManager like this to register a pending task. For classic ASP.NET, you can use PageAsyncTask.
At worst case, you'd call task.Wait() and block, because otherwise your task might continue outside the boundaries of that particular HTTP request.
For client side UI apps, some different scenarios are possible for calling an async method from synchronous method. For example, you can use ContinueWith(action, TaskScheduler.FromCurrentSynchronizationContext()) and fire an completion event from action (like this).
async and await should not create a large number of threads, particularly not with just 16 users. In fact, it should help you make better use of threads. The purpose of async and await in MVC is to actually give up the thread pool thread when it's busy processing IO bound tasks. This suggests to me that you are doing something silly somewhere, such as spawning threads and then waiting indefinitely.
Still, 900 threads is not really a lot, and if they're using 100% cpu, then they're not waiting.. they're chewing on something. It's this something that you should be looking into. You said you have used tools like NewRelic, well what did they point to as the source of this CPU usage? What methods?
If I were you, I would first prove that merely using async and await are not the cause of your problems. Simply create a simple site that mimics the behavior and then run the same tests on it.
Second, take a copy of your app, and start stripping stuff out and then running tests against it. See if you can track down where the problem is exactly.
There is a lot of stuff to discuss.
First of all, async/await can help you naturally when your application has almost no business logic. I mean the point of async/await is to do not have many threads in sleep mode waiting for something, mostly some IO, e.g. database queries (and fetching). If your application does huge business logic using cpu for 100%, async/await does not help you.
The problem of 900 threads is that they are inefficient - if they run concurrently. The point is that it's better to have such number of "business" threads as you server has cores/processors. The reason is thread context switching, lock contention and so on. There is a lot of systems like LMAX distruptor pattern or Redis which process data in one thread (or one thread per core). It's just better as you do not have to handle locking.
How to reach described approach? Look at disruptor, queue incoming requests and processed them one by one instead of parallel.
Opposite approach, when there is almost no business logic, and many threads just waits for IO is good place where to put async/await into work.
How it mostly works: there is a thread which reads bytes from network - mostly only one. Once some some request arrive, this thread reads the data. There is also limited thread pool of workers which processes requests. The point of async is that once one processing thread is waiting for some thing, mostly io, db, the thread is returned in poll and can be used for another request. Once IO response is ready, some thread from pool is used to finish the processing. This is the way how you can use few threads to server thousand request in a second.
I would suggest that you should draw some picture how your site is working, what each thread does and how concurrently it works. Note that it's necessary to decide whether throughput or latency is important for you.
in the last hours I've struggled with delegates and accessing Windows Forms controls (C++) where I've used this tutorial (the first thread safe method): http://msdn.microsoft.com/en-us/library/ms171728.aspx#Y190
Changing TextBoxes and Labels works perfectly but when I want to show or hide the whole GUI from another thread this fails.
I use the following methode (which is part of the GUI class):
System::Void UI::showUI(boolean value) {
if (this->InvokeRequired) {
SetTextDelegate^ d = gcnew SetTextDelegate(this, &UI::showUI);
this->Invoke(d, gcnew array<Object^> { value });
} else {
if (value == true)
this->Show();
else
this->Hide();
}
}
In the first call the if-clause is true so Invoke is called. But usually the showUI method should be called a second time automatically where the if-clause returns false, but this is not happening. So the GUI is neither shown nor hiden.
Is it necessary to show/hide the GUI with a delegate or can I do it from every possible thread? If a delegate is necessary, why is showUI not executed a second time?
Thanks,
Martin
edit: okay the name SetTextDelegate is not appropriate but this is not the point...
This is a pretty standard case of deadlock, not uncommon with Control::Invoke(). It can only proceed if the UI thread is not busy. Use Debug + Windows + Threads and double-click the Main thread. Look at the call stack to see what it is doing. The typical case is that it is blocking, waiting for the thread to finish the job. That will never happen since the thread can't complete until the Invoke() call returns.
Don't block the UI thread.
Consider using BackgroundWorker, its RunworkerCompleted event is nice to do stuff after the thread completes, removing the need to block.