BookSleeve is very good in performance because it use async IO as much as could.
But the problem is, it may throw fatal exception from asynchronous operation and crash my application.
I know that keyword await can help, but I can't use await for every call.
Is there a way to catch the unhandled exception globally?
What version of the library are you using? BookSleeve internally observes its own exceptions specifically so that they should not cause any crashes, however, you can also:
handle the .Error event
handle the TaskScheduler.UnobservedTaskException event
strictly, neither of these should be required. It is the latter than would prevent app closures, but again: this should not matter - unless there is a bug in some of the code and it is missing one or more tasks
But on a more general level, it is good practice to check what happens to your tasks. If you don't want to await them, it would probably still be worth subscribing a ContinueWith which checks for exceptions. Also, you don't necessarily need to await; in many cases, connection.Wait(result) is fine too - although this does tie up a primary thread for a little longer (but note: it doesn't tie up the multiplexer).
Related
I'm writing web services in C++/CLI (not my choice) using Microsoft's Web API. A lot of functions in Web API are async, but because I'm using C++/CLI, I don't get the async/await support of C# or VB. So the fallback position is to use ContinueWith() to schedule a continuation delegate for reading the async task's result safely.
However, because C++/CLI also doesn't support inline anonymous delegates or managed lambdas, every delegate continuation must be written as a separate function somewhere. That quickly turns into spaghetti with the number of async functions in Web API.
So, to avoid the deadlock issues of Task<T>::Result, I've been trying this:
[HttpGet, Route( "get/some/dto" )]
Task< SomeDTO ^ > ^ MyActionMethod()
{
return Task::Run( gcnew Func< SomeDTO ^ >( this, &MyController::MyActionMethod2 ) );
}
SomeDTO ^ MyActionMethod2()
{
// execute code and use any task->Result calls I need without deadlocking
}
Okay, so I know this isn't great, but how bad is it? I don't yet understand enough of the guts of Web API or ASP.NET to comprehend the performance or scaling ramifications this will have.
Also, what other consequences may this have that aren't necessarily related to performance? For example, exceptions get wrapped in an extra AggregateException, which represents additional complexity and work for handling exceptions.
Your memory usage will increase with your application's parallelism. For every concurrent call to MyActionMethod you will need a separate thread with its own stack. That will cost you about 1 MB of RAM for each concurrent call. If MyActionMethod runs long enough so that 10000 instances run at once, you're looking at 10 GB of RAM. There is also CPU overhead in setting up each thread.
If concurrency is low, dropping async support won't be a problem. In that case, don't bother with Task::Run. Just change MyActionMethod to return SomeDTO^ (no Task wrapper).
Another potential concern is that lose easy use of cancellation tokens. However, for Web API it's usually fine to just let an exception propagate back to Web API, which ends up cancelling the synchronous call anyway.
Finally, if you were planning on performing any operation within your action method in parallel, you'll still need to use ContinueWith to accomplish that. Going non-async by default means you'll always perform one operation at a time. Fortunately, it's often just fine to do so.
Okay, so I know this isn't great, but how bad is it?
It's difficult to answer this without load-testing your specific scenario. But you can walk through the known semantics (taken largely from my blog).
First, when a request comes in, ASP.NET executes your handler on a thread pool thread within that request context. Your request handler calls Task.Run, which takes another thread from the thread pool and executes the actual request logic on it. The handler then returns the task returned from Task.Run; this releases the original request thread back to the thread pool.
Then, the Task.Run delegate will block on any asynchronous parts. So, this pattern has the scaling disadvantages of a regular synchronous handler, plus an extra thread context switch. Also, it uses a thread from the ASP.NET thread pool, which is not necessarily a bad thing, but in some scenarios it may throw off the ASP.NET thread pool heuristics.
Also, what other consequences may this have that aren't necessarily related to performance? For example, exceptions get wrapped in an extra AggregateException, which represents additional complexity and work for handling exceptions.
Yes, the exceptions from any .Result or Wait() calls will be wrapped in AggregateException. You may be able to avoid this by calling .GetAwaiter().GetResult() instead.
Another important consideration is that the code executing within the Task.Run is executing without a request context. So, ambient data like HttpContext.Current, current culture, thread principal, etc. are not going to be set correctly. You'll have to capture any important data before calling Task.Run and pass it down manually.
I have a standard, non-async action like:
[HttpPost]
public JsonResult StartGeneratePdf(int id)
{
PdfGenerator.Current.GenerateAsync(id);
return Json(null);
}
The idea being that I know this PDF generation could take a long time, so I just start the task and return, not caring about the result of the async operation.
In a default ASP.Net MVC 4 app this gives me this nice exception:
System.InvalidOperationException: An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%# Page Async="true" %>.
Which is all kinds of irrelevant to my scenario. Looking into it I can set a flag to false to prevent this Exception:
<appSettings>
<!-- Allows throwaway async operations from MVC Controller actions -->
<add key="aspnet:AllowAsyncDuringSyncStages" value="true" />
</appSettings>
https://stackoverflow.com/a/15230973/176877
http://msdn.microsoft.com/en-us/library/hh975440.aspx
But the question is, is there any harm by kicking off this Async operation and forgetting about it from a synchronous MVC Controller Action? Everything I can find recommends making the Controller Async, but that isn't what I'm looking for - there would be no point since it should always return immediately.
Relax, as Microsoft itself says (http://msdn.microsoft.com/en-us/library/system.web.httpcontext.allowasyncduringsyncstages.aspx):
This behavior is meant as a safety net to let you know early on if
you're writing async code that doesn't fit expected patterns and might
have negative side effects.
Just remember a few simple rules:
Never await inside (async or not) void events (as they return immediately). Some WebForms Page events support simple awaits inside them - but RegisterAsyncTask is still the highly preferred approach.
Don't await on async void methods (as they return immediately).
Don't wait synchronously in the GUI or Request thread (.Wait(), .Result(), .WaitAll(), WaitAny()) on async methods that don't have .ConfigureAwait(false) on root await inside them, or their root Task is not started with .Run(), or don't have the TaskScheduler.Default explicitly specified (as the GUI or Request will thus deadlock).
Use .ConfigureAwait(false) or Task.Run or explicitly specify TaskScheduler.Default for every background process, and in every library method, that does not need to continue on the synchronization context - think of it as the "calling thread", but know that it is not one (and not always on the same one), and may not even exist anymore (if the Request already ended). This alone avoids most common async/await errors, and also increases performance as well.
Microsoft just assumed you forgot to wait on your task...
UPDATE: As Stephen clearly (pun not intended) stated in his answer, there is an inherit but hidden danger with all forms of fire-and-forget when working with application pools, not solely specific to just async/await, but Tasks, ThreadPool, and all other such methods as well - they are not guaranteed to finish once the request ends (app pool may recycle at any time for a number of reasons).
You may care about that or not (if it's not business-critical as in the OP's particular case), but you should always be aware of it.
The InvalidOperationException is not a warning. AllowAsyncDuringSyncStages is a dangerous setting and one that I would personally never use.
The correct solution is to store the request to a persistent queue (e.g., an Azure queue) and have a separate application (e.g., an Azure worker role) processing that queue. This is much more work, but it is the correct way to do it. I mean "correct" in the sense that IIS/ASP.NET recycling your application won't mess up your processing.
If you absolutely want to keep your processing in-memory (and, as a corollary, you're OK with occasionally "losing" reqeusts), then at least register the work with ASP.NET. I have source code on my blog that you can drop in your solution to do this. But please don't just grab the code; please read the entire post so it's clear why this is still not the best solution. :)
The answer turns out to be a bit more complicated:
If what you're doing, as in my example, is just setting up a long-running async task and returning, you don't need to do more than what I stated in my question.
But, there is a risk: If someone expanded this Action later where it made sense for the Action to be async, then the fire and forget async method inside it is going to randomly succeed or fail. It goes like this:
The fire and forget method finishes.
Because it was fired from inside an async Task, it will attempt to rejoin that Task's context ("marshal") as it returns.
If the async Controller Action has completed and the Controller instance has since been garbage collected, that Task context will now be null.
Whether it is in fact null will vary, because of the above timings - sometimes it is, sometimes it isn't. That means a developer can test and find everything working correctly, push to Production, and it explodes. Worse, the error this causes is:
A NullReferenceException - very vague.
Thrown inside .Net Framework code you can't even step into inside of Visual Studio - usually System.Web.dll.
Not captured by any try/catch because the part of the Task Parallel Library that lets you marshal back into existing try/catch contexts is the part that's failing.
So, you'll get a mystery error where things just don't occur - Exceptions are being thrown but you're likely not privy to them. Not good.
The clean way to prevent this is:
[HttpPost]
public JsonResult StartGeneratePdf(int id)
{
#pragma warning disable 4014 // Fire and forget.
Task.Run(async () =>
{
await PdfGenerator.Current.GenerateAsync(id);
}).ConfigureAwait(false);
return Json(null);
}
So, here we have a synchronous Controller with no issues - but to ensure it still won't even if we change it to async later, we explicitly start a new Task via Run, which by default puts the Task on the main ThreadPool. If we awaited it, it would attempt to tie it back to this context, which we don't want - so we don't await it, and that gets us a nuisance warning. We disable the warning with the pragma warning disable.
In my 3.5 .net web application I have a background thread that does a lot of work (the application is similar to mint.com in that it does a lot of account aggregation on background threads). I do extensive exception handling within the thread performing the aggregation but there's always the chance an unhandled exception will be thrown and my entire application will die. I've read some articles about this topic but they all seem fairly outdated and none of them implement a standard approach. Is there a standard approach to this nowadays? Is there any nicer way to handle this in ASP.NET 4.0?
Arguably, the entire application should die if you have an unhandled exception. An unhandled exception means that your program is in an unknown/indeterminate state, and any further processing or user interaction could cause corruption of the program's state, or worse, data corruption.
You're doing the right thing handling exceptions within your thread work. As far as I know, there is no way for a .NET application to "gracefully" deal with unhandled exceptions on background threads - they will always terminate the process.
Certain .NET Framework classes, such as the BackgroundWorker component and the Task Parallel Library in .NET 4 make multithreading easier and handle a lot of the dirty work of exception handling for you, so if it's possible for you to use those instead of implementing your own multi-threaded code, then you should definitely do so. But if those aren't able to help you in a given circumstance, if you must use the ThreadPool or a pure Thread, then be sure not to let any unhandled exceptions escape.
You can always put a try/catch block around your worker thread at a very high level... like right when the thread starts. I'm assuming this is what you're doing already, or something like it. But just keep in mind that you definitely don't want to turn an unknown error into a silent unknown error, because then it's going to be much harder to track down when something goes wrong. Be sure you are logging the exception to the EventLog or your custom app log if you want to just catch it and forget it.
Like Aaronaught says, the application should die when something unexpected happens. But I don't see a problem with just letting your background thread exit/die instead of bringing down the whole application process (in fact, I don't think Aaron is correct here, it won't kill the entire process) I think your question can basically be translated as "is there something magical in ASP.NET that will suddenly solve issues I don't even know about yet?" and the answer to that of course, is no. But you already knew that. :)
For the last couple of days of my X-mas holidays, I have been struggling with an UnathorizedAccessException when trying to READ a XML file on a remote share through my ASP.NET application using an implementation of the IHttpAsyncHandler a long with the IRequiresSessionState.
After much headache, and concluding that the code OUTSIDE the handler worked flawlessly (see: access granted), I thought it might be some threading issue, so I changed the IHttpAsyncHandler to IHttpHandler, and the problem disappears.
What is troubling me here is, that for test purposes, I did not actually make use if the IHttpAsyncHandler implementation (hence, I did not use BeginProcessRequest and EndProcessRequest - only the sync. version, ProcessRequest.
Can anyone try to explain the issue at hand?
There are some beneficial matters in using the handler asynchronously, as I could pre-cache the values to be delivered later in the application, but for that to work, I have to get pass the security issues that seems to only manifest when implementing the IHttpAsyncHandler.
Thanks in advance for your kind help - and happy holidays :-)
The ASP.NET infrastructure calls an async handler differently (regardless if the impl is truly async). Is it possible that you were relying on impersonation to access the network resource? My guess would be that the necessary WindowsIdentity didn't flow to the threadpool thread that actually handled the request (I've never tried using impersonation + async handler, but I've gotten nailed with other thread state flow issues in the past).
Regardless, a true async handler is expensive to implement correctly. Unless you're building on top of a lot of other async infrastructure (async file i/o, async DB client, etc), it doesn't do you any good (in fact, even in the best cases, async handlers hurt raw performance). I'd look to see if your performance needs really justify the extra hassle and overhead of an async handler (eg, you need to service many more concurrent requests than threads in the process, etc).
I admit it: I don't bother with too much exception handling. I know I should do more but I can never wrap my head around where to start and where to stop. I'm not being lazy. Far from it. It's that I'm overwrought with exception handling ambivalence. It just seems that there is a seemingly infinite number of places in even the smallest app where exception handling can be applied and it can begin to feel like overkill.
I've gotten by with careful testing, validating, and silent prayer but this is a bad programming accident waiting to happen.
So, what are your exception handling best practices? In particular, where are the most obvious/critical places where exception handling should be applied and where are places where it should be considered?
Sorry for the vague the question but I really want to close the book on this once and for all.
Microsoft's Patterns & Practices team did a good job incorporating best practices of exception management into Enterprise Library Exception Handling Application Block
Event if wouldn't use Enterprise Library, I highly recommend you to read their documentation. P&P team describes common scenarios and best practices for exceptions handling.
To get you started I recommend read following articles:
Exception Handling on MSDN
Exception Management in .NET on MSDN
Exception Handling Best Practices in .NET on CodeProject
ASP.NET specific articles:
User Friendly ASP.NET Exception Handling
Global Exception Handling with
ASP.NET
Exception handling in C# and ASP
.Net
The golden rule with exception handling is:
"Only catch what you know how to handle"
I've seen too many try-catch blocks where the catch does nothing but rethrow the exception. This adds no value. Just because you call a method that has the potential to throw an exception doesn't mean you have to deal with the possible exception in the calling code. It is often perfectly acceptable to let exceptions propagate up the call stack to some other code that does know what to do.
In some cases, it is valid to let exceptions propagate all the way up to the user interface layer then catch and display the message to the user. It might be that no code is best-placed to know how to handle the situation and the user must decide the course of action.
I recommend you start by adding a good error page that catches all exceptions and prints a slightly less unfriendly message to the user. Be sure to log all details available of the exception and revise that. Let the user know that you have done this, and give him a link back to a page that will (probably) work.
Now, use that log to detect where special exception handling should be put in place. Remember that there is no use in catching an exception unless you plan to do something with it. If you have the above page in place, there is no use in catching database exceptions individually on all db operations, unless you have some specific way to recover at that specific point.
Remember: The only thing worse than not catching exceptions, is catching them and not doing nothing. This will only hide the real problems.
Might be more about exception handling in general than ASP.NET speific but:
Try to catch exceptions as close to
the cause as possible so that you
can record (log) as much information
about the exception as possible.
Include some form of catch all, last
resort exception handler at the
entry points to your program. In
ASP.NET this could be the
Application level error handler.
If you don't know how to "correctly" handle an exception let it bubble up to the catch all handler where you can treat it as an "unexpected" exception.
Use the Try***** methods in .NET
for things like accessing a
Dictionary. This helps avoid major
performance problems (exception
handling is relatively slow) if you
throw multiple exceptions in say a
loop.
Don't use exception handling to
control normal logic of your
program, e.g. exiting from a loop via
a throw statement.
Start off with a global exception handler such as http://code.google.com/p/elmah/.
Then the question comes down to what kind of application are you writting and what kind of user experience do you need to provide. The more rich the user experience the better exception handling you'll want to provide.
As an example consider a photo hosting site which has disk quotas, filesize limits, image dimension limits, etc. For each error you could simply return "An error has occured. Please try again". Or you could get into detailed error handling:
"Your file is to large. Maximum
filesizes is 5mb."
"Your image is is
to large. Maximum dimensions are
1200x1200."
"Your album is full.
Maximum storage capacity is 1gb".
"There was an error with your
upload. Our hampsters are unhappy.
Please come back later."
etc. etc.
There is no one size fits all for exception handling.
Well at the very basic level you should be handling the HttpApplication.Error event in the Global.asax file. This should log any exception that occurs to a single place so you can review the stack trace of the exception.
Apart from this basic level you should ideally be handling exceptions where you know you can recover from them - for example if you expect a file might be locked then handling the IOException and reporting the error back to the user would be a good idea.