Parallelizing synchronous tasks while retaining the HttpContext.Current in ASP.NET
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. :)
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.
HttpContext.Currentfor 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:
public async Task<IHttpActionResult> DoIt(IEnumerable<int> inputs)
var context = HttpContext.Current;
var tasks = inputs.Select(i =>
HttpContext.Current = context;
var results = Some3rdPartyTool.CallEndpointSynchronously(MyRestEndpointConfig[i]);
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 (
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
Note that there are several bad practices in this solution:
- Accessing the same
HttpContextinstance simultaneously from multiple threads.
- Blocking on asynchronous code (done by
AsyncContext.Runand also presumably
In conclusion, I again recommend updating/rewriting/replacing
Some3rdPartyTool. But this pile of hacks might work.
The cross-thread usage of HttpContext.Current property and related things
There are four things working together to cause the behavior you are asking about:
- HttpContext is an instance object whose reference can be found in
- Thread is also an instance object whose reference can be found in
Thread.CurrentThreadis static but references a different
Threadobject in every thread
HttpContext.Currentactually points to
Conclusions we can draw from the above givens:
HttpContextis an instance object and not static we need its reference to access it
HttpContext.Currentactually points to a property on
Thread.CurrentThreadto a different object will likely change
Thread.CurrentThread' changes when switching threads,
HttpContext.Currentalso changes when switching threads (in this case
Bringing this all together, what causes
HttpContext.Current to not work in a new Thread? The
Thread.CurrentThread reference change, which happens when switching threads, changes the
HttpContext.Current reference, which prevents us from getting to the HttpContext instance we want.
To reiterate, the only magic thing going on here is
Thread.CurrentThread referencing a different object in every Thread. HttpContext works just like any other instance object. Since threads in the same AppDomain can reference the same objects, all we have to do is pass a reference for HttpContext to our new thread. There is no context info to load or anything like that. (there are some fairly serious potential gotchas with passing around HttpContext to other threads but nothing to prevent you from doing it).
A few final side notes I came across while researching:
In some cases a Thread's ExecutionContext is 'flowed' (copied) from one Thread to another. Why then is HttpContext not 'flowed' to our new Thread? Because HttpContext doesn't implement the ILogicalThreadAffinative interface. A class stored in the ExecutionContext is only flowed if it implements ILogicalThreadAffinative.
How does ASP.NET move HttpContext from Thread to Thread (Thread-Agility) if it isn't flowed? I'm not entirely sure, but it looks like it might pass it in
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