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webapi 2 - how to properly invoke long running method async/in new thread, and return response to client

I am developing a web-api that takes data from client, and saves it for later use. Now i have an external system that needs to know of all events, so i want to setup a notification component in my web-api.
What i do is, after data is saved, i execute a SendNotification(message) method in my new component. Meanwhile i don't want my client to wait or even know that we're sending notifications, so i want to return a 201 Created / 200 OK response as fast as possible to my clients.
Yes this is a fire-and-forget scenario. I want the notification component to handle all exception cases (if notification fails, the client of the api doesn't really care at all).
I have tried using async/await, but this does not work in the web-api, since when the request-thread terminates, the async operation does so aswell.
So i took a look at Task.Run().
My controller looks like so:
public IHttpActionResult PostData([FromBody] Data data) {
_dataService.saveData(data);
//This could fail, and retry strategy takes time.
Task.Run(() => _notificationHandler.SendNotification(new Message(data)));
return CreatedAtRoute<object>(...);
}
And the method in my NotificationHandler
public void SendNotification(Message message) {
//..send stuff to a notification server somewhere, syncronously.
}
I am relatively new in the C# world, and i don't know if there is a more elegant(or proper) way of doing this. Are there any pitfalls with using this method?

It really depends how long. Have you looked into the possibility of QueueBackgroundWorkItem as detailed here. If you want to implement a very fast fire and forget you also might want to consider a queue to pop these messages onto so you can return from the controller immediately. You'd then have to have something which polls the queue and sends out the notifications i.e. Scheduled Task, Windows service etc. IIRC, if IIS recycles during a task, the process is killed whereas with QueueBackgroundWorkItem there is a grace period for which ASP.Net will let the work item finish it's job.

I would take a look on Hangfire. It is fairly easy to setup, it should be able to run within your ASP.NET process and is easy to migrate to a standalone process in case your IIS load suddenly increases.
I experimented with Hangfire a while ago but in standalone mode. It has enough docs and easy to understand API.

How to create a custom thread pool

I'm using C# with .NET Framework 4.5. I'm writing a server application that should be able to connect to arbitrarily many clients at once. So I have one thread that will listen for connections, and then it will send the connection to a background thread to go into a loop waiting for messages. Since the number of supportable client connections should be very high, spawning a new thread for every connection won't work. Instead what I need is a thread pool. However, I don't want to use the system thread pool because these threads will essentially be blocked in a call to Socket.Select indefinitely, or at least for the life of the connections they host.
So I think I need a custom ThreadPool that I can explicitly round-robin the connections over to. How to achieve this in C#?

There's no point in using threads for this - that's just wasting resources.
Instead, you want to use asynchronous I/O. Now, ignoring all the complexities involved with networking, the basic idea is something like this:
async Task Loop()
{
using(var client = new TcpClient())
{
await client.ConnectAsync(IPAddress.Loopback, 6536).ConfigureAwait(false);
var stream = client.GetStream();
byte[] outBuf = new byte[4096];
// The "message loop"
while (true)
{
// Asynchronously wait for the message
var read = await stream.ReadAsync(outBuf, 0, outBuf.Length);
// Handle the message, asynchronously send replies, whatever...
}
}
}
You can run this method for each of the connection you're making (without using await - very important). The thread handling that particular socket will be released on every await - the continuation of that await will be posted on a thread-pool thread.
The result being, the amount of threads used at any given time will tend to self-balance with the available CPU cores etc., while you can easily service thousands of connections at a time.
Do not use the exact code I posted. You need to add tons of error-handling code etc. Handling networking safely and reliably is very tricky.

Establish remote SSL connection after or before local user connection for SSL wrapper?

I'm trying to make a stunnel clone in C# just for fun. The main loop goes something like this (ignore the catch-everything-and-do-nothing try-catches just for now)
ServicePointManager.ServerCertificateValidationCallback = Validator;
TcpListener a = new TcpListener (9999);
a.Start ();
while (true) {
Console.Error.WriteLine ("Spinning...");
try {
TcpClient remote = new TcpClient ("XXX.XX.XXX.XXX", 2376);
SslStream ssl = new SslStream(remote.GetStream(), false, new RemoteCertificateValidationCallback(Validator));
ssl.AuthenticateAsClient("mirai.ca");
TcpClient user = a.AcceptTcpClient ();
new Thread (new ThreadStart(() => {
Thread.CurrentThread.IsBackground = true;
try{
forward(user.GetStream(), ssl); //forward is a blocking function I wrote
}catch{}
})).Start ();
} catch {
Thread.Sleep (1000);
}
}
I found that if I do the remote SSL connection, as I did, before waiting for the user, then when the user connects the SSL is already set up (this is for tunneling HTTP so latency is pretty important). On the other hand, my server closes long-inactive connections, so if no new connection happens in, say, 5 minutes, everything locks up.
What is the best way?
Also, I observe my program generating as much as 200 threads, which of course means that context-switching overhead is pretty big and sometimes results in the whole thing just blocking for seconds, even with just one user tunneling through the program. My forward function goes, in a gist, like
new Thread(new ThreadStart(()=>in.CopyTo(out))).Start();
out.CopyTo(in);
of course with lots of error handling to prevent broken connections from holding up forever. This seems to stall a lot though. I can't figure how to use asynchronous methods like BeginRead which should help according to google.

For any kind of proxy server (including an stunnel clone), opening the backend connection after you accept the frontend connection is clearly much simpler to implement.
If you pre-open backend connections in anticipation of receiving frontend connections, you can certainly save an RTT (which is good for latency), but you have to deal with the issue you hinted at: the backend will close idle connections. At any time that you receive a frontend connections, you run the risk that the backend connection that you are about to associate with this frontend connection and which has been opened some time ago is too old to use and may be closed by the backend. You will have to manage a pool of currently open backend connections and periodically close and refresh them when they become idle for too long. There is even a race condition where if the backend decided the connection has been idle too long and decides to close it but the proxy server receives a new frontend connection at the same time, the frontend may decide to forward a request through the backend connection while the backend is closing this connection. That means that you must be able to know a priori how long backend connections can be idle for before the backend will close them (you must know what the timeout values that are configured on the backend are set to) so you can give them up just before the backend will decide they are too old.
So in summary: pre-opening backend connections will save an RTT versus opening them only on demand, but it is a lot of work, including subtle connection pool management that it quite tough to implement bug-free. Up to you to judge if the extra complexity is worth it.
By the way, concerning your comment about handling several hundred simultaneous connections, I recommend implementing such an I/O-bound program as a proxy server based around an event loop instead of based around threads. Basically, you use non-blocking sockets and process events in a single thread (e.g. "this socket has new data waiting to be forwarded to the other side") instead of spawning a thread for each connection (which can get expensive both in thread creation and context switches). In order to scale such an event-based model to multiple CPU cores, you can start a small number of parallel threads of processes (more or less one per CPU core) which each handle many hundreds (or thousands) of simultaneous connections.

How to implement an IIS-like threadpool on a worker-server

EDIT I realised my question was not stated clearly enough and have edited it heavily.
This is a bit of an open ended question so apologies in advance.
In a nutshell, I want to implement IIS-style asynchronous request processing in an Azure worker role.
It may be very simple or it may be insanely hard - I am looking for pointers to where to research.
While my implementation will use Azure Workers and Service Bus Queues, the general principle is applicable to any scenario where a worker process is listening for incoming requests and then servicing them.
What IIS does
In IIS there is a fixed-size threadpool. If you deal with all request synchronously then the maximum number of requests you can deal with in parallel == maxthreads. However, if you have to do slow external I/O to serve requests then this is highly inefficient because you can end up with the server being idle, yet have all threads tied up waiting for external I/O to complete.
From MSDN:
On the Web server, the .NET Framework maintains a pool of threads that are used to service ASP.NET requests. When a request arrives, a thread from the pool is dispatched to process that request. If the request is processed synchronously, the thread that processes the request is blocked while the request is being processed, and that thread cannot service another request.
This might not be a problem, because the thread pool can be made large enough to accommodate many blocked threads. However, the number of threads in the thread pool is limited. In large applications that process multiple simultaneous long-running requests, all available threads might be blocked. This condition is known as thread starvation. When this condition is reached, the Web server queues requests. If the request queue becomes full, the Web server rejects requests with an HTTP 503 status (Server Too Busy).
In order to overcome this issue, IIS has some clever logic that allows you to deal with requests asynchronously:
When an asynchronous action is invoked, the following steps occur:
The Web server gets a thread from the thread pool (the worker thread) and schedules it to handle an incoming request. This worker thread initiates an asynchronous operation.
The worker thread is returned to the thread pool to service another Web request.
When the asynchronous operation is complete, it notifies ASP.NET.
The Web server gets a worker thread from the thread pool (which might be a different thread from the thread that started the asynchronous operation) to process the remainder of the request, including rendering the response.
The important point here is when the asynchronous request returns, the return action is scheduled to run on one of the same pool of threads that serves the initial incoming requests. This means that the system is limiting how much work it is doing concurrently and this is what I would like to replicate.
What I want to do
I want to create a Worker role which will listen for incoming work requests on Azure Service Bus Queues and also potentially on TCP sockets. Like IIS I want to have a maxium threadpool size and I want to limit how much actual work the worker is doing in parallel; If the worker is busy serving existing requests - whether new incoming ones or the callbacks from previous async calls - I don't want to pick up any new incoming requests until some threads have been freed up.
It is not a problem to limit how many jobs I start concurrently - that is easy to control; It is limiting how many I am actually working on concurrently.
Let's assume a threadpool of 100 threads.
I get 100 requests to send an email come in and each email takes 5 seconds to send to the SMTP server. If I limit my server to only process 100 requests at the same time then my server will be unable to do anything else for 5 seconds, while the CPU is completely idle. So, I don't really mind starting to send 1,000 or 10,000 emails at the same time, because 99% of the "request process time" will be spent waiting for external I/O and my server will still be very quiet.
So, that particular scenario I could deal with by just keeping on accepting incoming requests with no limit (or only limit the start of the request until I fire off the async call; as soon as the BeginSend is called, I'll return and start serving another request).
Now, imagine instead that I have a type of request that goes to the database to read some data, does some heavy calculation on it and then writes that back to the database. There are two database requests there that should be made asynchronous but 90% of the request processing time will be spent on my worker. So, if I follow the same logic as above and keep start async calls and just letting the return do whatever it needs to get a thread to continue on then I will end up with a server that is very overloaded.
Somehow, what IIS does is make sure that when an async call returns it uses the same fixed-size thread pool. This means that if I fire off a lot of async calls and they then return and start using my threads, IIS will not accept new requests until those returns have finished. And that is perfect because it ensures a sensible load on the server, especially when I have multiple load-balanced servers and a queue system that the servers pick work from.
I have this sneaky suspicion that this might be very simple to do, there is just something basic I am missing. Or maybe it is insanely hard.

Creating a threadpool should be considered as independent of Windows Azure. Since a Worker Role instance is effectively Windows 2008 Server R2 (or SP2), there's nothing really different. You'd just need to set things up from your OnStart() or Run().
One thing you wanted to do was use queue length as a determining factor when scaling to more/less worker instances. Note that Service Bus Queues don't advertise queue length, where Windows Azure Queues (based on Storage, vs. Service Bus) do. With Windows Azure Queues, you'll need to poll synchronously for messages (whereas Service Bus Queues have long-polling operations). Probably a good idea to review the differences between Service Bus Queues and Windows Azure Queues, here.

Have you considered having a dedicated WCF instance (not WAS or IIS hosted) to buffer the long running requests? It will have its own dedicated app pool, with a separate Max value setting from IIS that won't contend with your ASP.NET HTTP requests. (HTTP requests are served by
Then use IIS Async methods to call WCF with the constrained app pool.

I've used the SmartThreadPool project in the past as a per-instance pool and, if I'm reading you correctly, it should have all the callback and worker-limiting functionality you need. My company actually has it running currently on Azure for the exact purpose you describe of reading message bus requests asynchronously.

I have been digging around in this and found that it is indeed relatively easy.
http://www.albahari.com/threading/ has got some good information and I actually ended up buying the book which that website is essentially promoting.
What I found out is that;
Your application has a ThreadPool available to it by default
You can limit the number of threads available in the ThreadPool
When you use QueueUserWorkItem or Task.Factory.StartNew the job you start run on a Thread in the ThreadPool
When you use one of the asynchronous IO calls in the framework (Begin... methods or WebcClient.DownloadStringAsync etc) the the callbacks will also run on a Thread from the ThreadPool (what happens with the IO request itself is outside the scope of this discussion).
So far, so good. The problem is that I can keep calling Task.Factory.StartNew as much as I like and the ThreadPool will simply queue up the work until there are free threads to service them. So, in the case of an Azure Worker, I could easily empty the Queue even though my worker is busy servicing existing requests (and callbacks from existing requests). That is the core of my problem. What I want is to not take anything out of the queue until I actually have some free threads to service the request.
This is a very simple example of how this could be achieved. In essence, I am using an AutoResetEvent to make sure that I don't start another task from the queue until the previous task has actually started. Granted, I do actually take stuff out of the queue before there is a free thread, but on balance this should avoid crazy overloads of the worker and allow me to spin up more workers to share the load.
ThreadPool.SetMaxThreads(5, 1000); // Limit to 5 concurrent threads
ThreadPool.SetMinThreads(5, 10); // Ensure we spin up all threads
var jobStart = new AutoResetEvent(true);
// The "listen" loop
while (true)
{
var job = this.jobQueue.Dequeue();
jobStart.WaitOne(); // Wait until the previous job has actually been started
Task.Factory.StartNew(
() =>
{
jobStart.Set(); // Will happen when the threadpool allocates this job to a thread
this.Download(job);
});
}
This can - and probably should - be made a lot more sophisticated, including having timeouts, putting the work item back in the queue if a thread can't be allocated within a reasonable time and so on.
An alternative would be to use ThreadPool.GetAvailableThreads to check if there are free threads before starting to listen to the queue but that feels rather more error prone.

Somehow, what IIS does is make sure that when an async call returns
it uses the same fixed-size thread pool.
This is not true: When your code runs in response to an HTTP-Request you decide on what threads the continuation function executes. Usually, this is the thread pool. And the thread pool is an appdomain-wide resource that is shared among all requests.
I think IIS does less "magic" than you think it does. All it does is to limit the number of parallel HTTP-requests and the backlog size. You decide what happens once you have been given control by ASP.NET.
If your code is not protected against overloading the server, you will overload the server even on IIS.

From what I understand you want to constrain the number of threads used for processing a certain type of message at the same time.
One approach would be to simply wrap the message processor, invoked on a new thread with something like
try
{
Interlocked.Increment(ref count)
Process(message);
}
finally
{
Interlocked.Decrement(ref count)
}
Before invoking the wrapper, simply check if the ‘count’ is less than your threshold count; and stop polling/handling more messages till the count is sufficiently lower.
EDIT Added more information based on comment
Frans, not sure why you see the infrastructure and business code being coupled. Once you place your business process to be serviced as a task on a new thread to run asynchronously, you need not worry about performing additional IO bound calls asynchronously. This is a simpler model to program in.
Here is what I am thinking.
// semi - pseudo-code
// Infrastructure – reads messages from the queue
// (independent thread, could be a triggered by a timer)
while(count < maxCount && (message = Queue.GetMessage()) != null)
{
Interlocked.Increment(ref count);
// process message asynchronously on a new thread
Task.Factory.StartNew(() => ProcessWrapper(message));
}
// glue / semi-infrastructure - deals with message deletion and exceptions
void ProcessWrapper(Message message)
{
try
{
Process(message);
Queue.DeleteMessage(message);
}
catch(Exception ex)
{
// Handle exception here.
// Log, write to poison message queue etc ...
}
finally
{
Interlocked.Decrement(ref count)
}
}
// business process
void Process(Message message)
{
// actual work done here
;
}

Sequential access to asynchronous sockets

I have a server that has several clients C1...Cn to each of which there is a TCP connection established. There are less than 10,000 clients.
The message protocol is request/response based, where the server sends a request to a client and then the client sends a response.
The server has several threads, T1...Tm, and each of these may send requests to any of the clients. I want to make sure that only one of these threads can send a request to a specific client at any one time, while the other threads wanting to send a request to the same client will have to wait.
I do not want to block threads from sending requests to different clients at the same time.
E.g. If T1 is sending a request to C3, another thread T2 should not be able to send anything to C3 until T1 has received its response.
I was thinking of using a simple lock statement on the socket:
lock (c3Socket)
{
// Send request to C3
// Get response from C3
}
I am using asynchronous sockets, so I may have to use Monitor instead:
Monitor.Enter(c3Socket); // Before calling .BeginReceive()
And
Monitor.Exit(c3Socket); // In .EndReceive
I am worried about stuff going wrong and not letting go of the monitor and therefore blocking all access to a client. I'm thinking that my heartbeat thread could use Monitor.TryEnter() with a timeout and throw out sockets that it cannot get the monitor for.
Would it make sense for me to make the Begin and End calls synchronous in order to be able to use the lock() statement? I know that I would be sacrificing concurrency for simplicity in this case, but it may be worth it.
Am I overlooking anything here? Any input appreciated.

My answer here would be a state machine per socket. The states would be free and busy:
If socket is free, the sender thread would mark it busy and start sending to client and waiting for response.
You might want to setup a timeout on that wait just in case a client gets stuck somehow.
If the state is busy - the thread sleeps, waiting for signal.
When that client-related timeout expires - close the socket, the client is dead.
When a response is successfully received/parsed, mark the socket free again and signal/wakeup the waiting threads.
Only lock around socket state inquiry and manipulation, not the actual network IO. That means a lock per socket, plus some sort of wait primitive like a conditional variables (sorry, don't remember what's really available in .NET)
Hope this helps.

You certainly can't use the locking approach that you've described. Since your system is primarily asynchronous, you can't know what thread operations will be running on. This means that you may call Exit on the wrong thread (and have a SynchronizationLockException thrown), or some other thread may call Enter and succeed even though that client is "in use", just because it happened to get the same thread that Enter was originally called on.
I'd agree with Nikolai that you need to hold some additional state alongside each socket to determine whether it is currently in use or not. You woud of course need locking to update this shared state.