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Parallelizing synchronous tasks while retaining the HttpContext.Current in ASP.NET

I've scoured the SO for answers but found none that pertain to the problem at hand, although this one nails it on "why", but isn't solving it.
I have a REST endpoint that needs to gather data from other endpoints - in doing so, it accesses the HttpContext (setting authentication, headers, etc... all done with 3rd party lib I don't have access to).
Unfortunately, this library for service communication is made to be synchronous, and we want to parallelize its use.
In the following example (abstracted) code, the issue is that CallEndpointSynchronously unfortunately uses some built in authentication, which throws null exception when HttpContext isn't set:
public class MyController: ApiController
//...
[HttpPost]
public async Task<IHttpActionResult> DoIt(IEnumerable<int> inputs)
{
var tasks = inputs.Select(i =>
Task.Run(()=>
{
/* call some REST endpoints, pass some arguments, get the response from each.
The obvious answer (HttpContext.Current = parentContext) can't work because
there's some async code underneath (for whatever reasons), and that would cause it
to sometimes not return to the same thread, and basically abandon the Context,
again resulting in null */
var results = Some3rdPartyTool.CallEndpointSynchronously(MyRestEndpointConfig[i]);
return results;
});
var outcome = await Task.WhenAll(tasks);
// collect outcome, do something with it, render outputs...
}
Is there a cure for this?
We want to optimize for single requests, not interested in maximizing parallel users at this moment.

Unfortunately, this library for service communication is made to be synchronous, and we want to parallelize its use.
throws null exception when HttpContext isn't set:
The obvious answer (HttpContext.Current = parentContext) can't work because there's some async code underneath (for whatever reasons), and that would cause it to sometimes not return to the same thread, and basically abandon the Context, again resulting in null
There's an important part of your question in the example code comment. :)
Normally, HttpContext shouldn't be shared across threads. It's just not threadsafe at all. But you can set HttpContext.Current (for some reason), so you can choose to live dangerously.
The more insidious problem here is that the library has a synchronous API and is doing sync-over-async - but somehow without deadlocking (?). At this point, I must be honest and say the best approach is to fix the library: make the vendor fix it, or submit a PR, or just rewrite it if you have to.
However, there is a tiny chance that you can get this kinda sorta working by adding Even More Dangerous code.
So, here's the information you need to know:
ASP.NET (pre-Core) uses an AspNetSynchronizationContext. This context:
Ensures that only one thread runs in this context at a time.
Sets HttpContext.Current for any thread that is running in the context.
Now, you could capture the SynchronizationContext.Current and install it on the thread pool threads, but in addition to being Very Dangerous, it would not achieve your actual goal (parallelization), since the AspNetSynchronizationContext only allows one thread in at a time. The first portion of the 3rd-party code would be able to run in parallel, but anything queued to the AspNetSynchronizationContext would run one thread at a time.
So, the only way I can think of making this work is to use your own custom SynchronizationContext that resumes on the same thread, and set HttpContext.Current on that thread. I have an AsyncContext class that can be used for this:
[HttpPost]
public async Task<IHttpActionResult> DoIt(IEnumerable<int> inputs)
{
var context = HttpContext.Current;
var tasks = inputs.Select(i =>
Task.Run(() =>
AsyncContext.Run(() =>
{
HttpContext.Current = context;
var results = Some3rdPartyTool.CallEndpointSynchronously(MyRestEndpointConfig[i]);
return results;
})));
var outcome = await Task.WhenAll(tasks);
}
So for each input, a thread is grabbed from the thread pool (Task.Run), a custom single-threaded synchronization context is installed (AsyncContext.Run), HttpContext.Current is set, and then the code in question is run. This may or may not work; it depends on how exactly Some3rdPartyTool uses its SynchronizationContext and HttpContext.
Note that there are several bad practices in this solution:
Using Task.Run on ASP.NET.
Accessing the same HttpContext instance simultaneously from multiple threads.
Using AsyncContext.Run on ASP.NET.
Blocking on asynchronous code (done by AsyncContext.Run and also presumably Some3rdPartyTool.
In conclusion, I again recommend updating/rewriting/replacing Some3rdPartyTool. But this pile of hacks might work.

Async/await and resource access

I’m wondering how to implement resource access nicely using async/await. I have singleton service in web application that is acting as a proxy to LDAP and have to buffer all data on first access – all invocation after that is done via cache but after some time cache is invalidated and data should be get again. Now my implementation looks like this but it is not meet my requirements
public async Task<string> GetUserDisplayName(string username)
{
var users = await GetCachedUsers();
// code using users from cache
}
private async Task<IEnumerable<LdapUser>> GetCachedUsers()
{
var users = _Cache.Get<IEnumerable<LdapUser>>();
if (users == null)
{
users = await _Connector.GetAllUsers();
_Cache.Add(users, TimeSpan.FromHours(USER_CACHE_VALID_HOURS));
}
return users;
}
I’m wondering how to implement this in this way that when couple request go to the service first time they should be awaited on the same task but not blocked and download from LDAP should go only once. I could do this traditionally and lock the resource but that threads will be blocked and I want them to back to threadpool in async way like in async/wait pattern.

SemaphoreSlim has a WaitAsync method that will let you create a critical section in asynchronous code. You can use that semaphore to prevent multiple invocations of the method from generating the value together without actually blocking any of the threads.

Correct way to use HttpContext.Current.User with async await

I am working with async actions and use the HttpContext.Current.User like this
public class UserService : IUserService
{
public ILocPrincipal Current
{
get { return HttpContext.Current.User as ILocPrincipal; }
}
}
public class ChannelService : IDisposable
{
// In the service layer
public ChannelService()
: this(new Entities.LocDbContext(), new UserService())
{
}
public ChannelService(Entities.LocDbContext locDbContext, IUserService userService)
{
this.LocDbContext = locDbContext;
this.UserService = userService;
}
public async Task<ViewModels.DisplayChannel> FindOrDefaultAsync(long id)
{
var currentMemberId = this.UserService.Current.Id;
// do some async EF request …
}
}
// In the controller
[Authorize]
[RoutePrefix("channel")]
public class ChannelController : BaseController
{
public ChannelController()
: this(new ChannelService()
{
}
public ChannelController(ChannelService channelService)
{
this.ChannelService = channelService;
}
// …
[HttpGet, Route("~/api/channels/{id}/messages")]
public async Task<ActionResult> GetMessages(long id)
{
var channel = await this.ChannelService
.FindOrDefaultAsync(id);
return PartialView("_Messages", channel);
}
// …
}
I have the code recently refactored, previously I had to give the user on each call to the service.
Now I read this article https://www.trycatchfail.com/2014/04/25/using-httpcontext-safely-after-async-in-asp-net-mvc-applications/ and I’m not sure if my code still works.
Has anyone a better approach to handle this? I don’t want to give the user on every request to the service.

As long as your web.config settings are correct, async/await works perfectly well with HttpContext.Current. I recommend setting httpRuntime targetFramework to 4.5 to remove all "quirks mode" behavior.
Once that is done, plain async/await will work perfectly well. You'll only run into problems if you're doing work on another thread or if your await code is incorrect.
First, the "other thread" problem; this is the second problem in the blog post you linked to. Code like this will of course not work correctly:
async Task FakeAsyncMethod()
{
await Task.Run(() =>
{
var user = _userService.Current;
...
});
}
This problem actually has nothing to do with asynchronous code; it has to do with retrieving a context variable from a (non-request) thread pool thread. The exact same problem would occur if you try to do it synchronously.
The core problem is that the asynchronous version is using fake asynchrony. This inappropriate, especially on ASP.NET. The solution is to simply remove the fake-asynchronous code and make it synchronous (or truly asynchronous, if it actually has real asynchronous work to do):
void Method()
{
var user = _userService.Current;
...
}
The technique recommended in the linked blog (wrapping the HttpContext and providing it to the worker thread) is extremely dangerous. HttpContext is designed to be accessed only from one thread at a time and AFAIK is not threadsafe at all. So sharing it among different threads is asking for a world of hurt.
If the await code is incorrect, then it causes a similar problem. ConfigureAwait(false) is a technique commonly used in library code to notify the runtime that it doesn't need to return to a specific context. Consider this code:
async Task MyMethodAsync()
{
await Task.Delay(1000).ConfigureAwait(false);
var context = HttpContext.Current;
// Note: "context" is not correct here.
// It could be null; it could be the correct context;
// it could be a context for a different request.
}
In this case, the problem is obvious. ConfigureAwait(false) is telling ASP.NET that the rest of the current method does not need the context, and then it immediately accesses that context. When you start using context values in your interface implementations, though, the problem is not as obvious:
async Task MyMethodAsync()
{
await Task.Delay(1000).ConfigureAwait(false);
var user = _userService.Current;
}
This code is just as wrong but not as obviously wrong, since the context is hidden behind an interface.
So, the general guideline is: use ConfigureAwait(false) if you know that the method does not depend on its context (directly or indirectly); otherwise, do not use ConfigureAwait. If it's acceptable in your design to have interface implementations use the context in their implementation, then any method that calls an interface method should not use ConfigureAwait(false):
async Task MyMethodAsync()
{
await Task.Delay(1000);
var user = _userService.Current; // works fine
}
As long as you follow that guideline, async/await will work perfectly with HttpContext.Current.

Async is fine. The problem is when you post the work to a different thread. If your application is setup as 4.5+, the asynchronous callback will be posted in the original context, so you'll also have the proper HttpContext etc.
You don't want to access shared state in a different thread anyway, and with Tasks, you rarely need to handle that explicitly - just make sure you put all your inputs as arguments, and only return a response, rather than reading or writing to a shared state (e.g. HttpContext, static fields etc.)

There is no problem, if your ViewModels.DisplayChannel is a simple object without additional logic.
A problem may occur, if the result of your Task references to "some context objects", f.e. to HttpContext.Current. Such objects are often attached to the thread, but entire code after await may be executed in another thread.
Keep in mind, that UseTaskFriendlySynchronizationContext doesn't solve all your problems. If we are talking about ASP.NET MVC, this setting ensures that Controller.HttpContext contains correct value as before await as after. But it doesn't ensure that HttpContext.Current contains correct value, and after await it still can be null.

HttpContext.Current.Items after an Async operation

Consider the following ASP.NET Web API Delegating Handler:
public class MyHandler : DelegatingHandler
{
protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
{
var guid = Guid.NewGuid();
HttpContext.Current.Items["foo"] = guid;
// An Async operation
var result = await base.SendAsync(request, cancellationToken);
//All code from this point is not gauranteed to run on the same thread that started the handler
var restoredGuid = (Guid)HttpContext.Current.Items["foo"];
//Is this gauranteed to be true
var areTheSame = guid == restoredGuid;
return result;
}
}
The above example is in a delegating handler, the same problem I am trying to fix applies in Controllers, Business Objects, etc.
I am ultimately trying to provide some simple in-memory shared state between various objects per HTTP Request
As I understand it during Async operations the ASP.NET thread originally running the operation is returned to the thread pool and a different thread may be used to finish the request after the Async operation has completed.
Does this affect the HttpContext.Current.Items collection?
Is an item that was in the Items collection guaranteed to be there when the Request resumes?
I'm aware that using HttpContext.Current is often frowned upon by
the wider community these days for reasons I completely agree
with... I'm just helping someone out of a jam.
Storing this data in the Request.Items collection is not suitable to solve this problem as my colleague requires a static due to some poor design decisions.
Many Thanks

As I understand it during Async operations the ASP.NET thread originally running the operation is returned to the thread pool and a different thread may be used to finish the request after the Async operation has completed.
That is correct. But let's talk about async on ASP.NET for just a minute.
async requires .NET 4.5. Furthermore, ASP.NET 4.5 introduces a "quirks mode" on the server side, and you have to turn the SynchronizationContext quirk off. You can do this by either setting httpRuntime.targetFramework to 4.5 or using an appSettings with aspnet:UseTaskFriendlySynchronizationContext value of true.
If your web.config does not have one of those entries, then the behavior of async is undefined. See this post for more details. I recommend using the targetFramework setting and fixing any problems that come up.
Does this affect the HttpContext.Current.Items collection? Is an item that was in the Items collection guaranteed to be there when the Request resumes?
The AspNetSynchronizationContext preserves the current request context across await points. This includes HttpContext.Current (which includes Items, User, etc).
Another possibility is CallContext.Logical[Get|Set]Data, which also flows across await points. This is useful if you don't want a code dependency on HttpContext, but has slightly more overhead.
I gave a talk at ThatConference a couple weeks ago on async on the server side; you may find the slides helpful, particularly the ones dealing with Context and Thread-Local State.

Cutting a long story short, it normally should. Unless you are using ConfigureAwait(false) which can have a side effect with continuation not flowing the context.
Alternatively try adding this setting in your app.
<appSettings>
<add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
</appSettings>
UPDATE
NOTE!!
Initially I put false. But it must be true so that context flows.

Using HttpContext in Async Task

I have the following mvc action.
public async Task<JsonResult> DoSomeLongRunningOperation()
{
return await Task.Run(() =>
{
//Do a lot of long running stuff
//The underlying framework uses the HttpContext.Current.User.Identity.Name so the user is passed on the messagebus.
}
}
In the task the HttpContext gets null. We did a lot of tricking, but nothing assures us of the HttpContext being available always in our new thread.
Is there a solution to use HttpContext within out async tasks?
In our IocContainer we have registered the following object which passes the username to the framework.
public class HttpContextUserIdentityName : ICredentials
{
public string Name
{
get { return HttpContext.Current.User.Identity.Name; }
}
}
This code is called in a lot of places before persisting to the database.
We need either another way of getting the username of the user initiated the webrequest or fix the issue with the HttpContext being null.
Because the persisting to the database happens in the Task I can't access the HttpContext before entering the task.
I also can't think of a safe way to temporary persist the username so I can implement another ICredentials service object.

You almost never want to use Task.Run in an ASP.NET method.
I think the cleanest solution (but the most work) is to implement async-compatible interfaces at your other layers:
public async Task<JsonResult> DoSomeLongRunningOperation()
{
//Do a lot of long running stuff
var intermediateResult = await DoLongRunningStuff();
return await DetermineFinalResult(intermediateResult);
}

You should get whatever information you need from the current context before you start the new thread. In this case, add something like:
string username = HttpContext.Current.User.Username;
before Task.Run and then use that inside of the other thread.
On a side note, as it stands, there's no reason to await the task. You can just return the task directly and not mark the method as Async.
If you need to access the Response object, which will presumably to utilize the results of the long running operation and thus can't be before Task.Run you should do so after the Task.Run (but ensure that the task is awaited). If you end up doing this then you can't do what I suggested in my previous paragraph.

I would try passing in the reference to the HttpContext as the state object, because that should create a new instance of that object on the stack for the thread that executes the work. Instead of using Task.Run, use
return await Task.Factory.StartNew((ctx) =>
{
var context = (HttpContext)ctx;
//Do stuff
}, httpContextObject);
Task.Run and Task.Factory.StartNew return immediately, so asp.net continues on in the event lifecycle in the worker thread that is handling the request while your thread is operating on the object that has already been disposed.