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TClientSocket in Delphi persistent connection

I have an application written in Delphi using TClientSocket that is sending data to another application written in C#. For many reasons, the C# application is slow to respond, blocking my Delphi application and not respecting the time-out I have set.
My Delphi application reads responses like this:
Sock.Socket.ReceiveText
This causes the application to wait for a response. But if I do this instead, the application waits and respects the time-out:
while not receiveData do
begin
if Sock.Socket.ReceiveLength > 0 then
begin
receiveData := True;
end;
Inc(Cont);
Sleep(100);
if (Cont > 10) then
raise Exception.Create('Timeout');
end;
My Delphi app sends two requests. The first one times out, but C# is still processing it. My Delphi app then sends the second request, and this time C# sends the response for the first request.
Will the second request receive data for the first request? Or, when I timeout in Delphi, will they cross information?

Once your Delphi code times out, it forgets about the first request, but your C# code does not know that. Since you are not dropping the connection, the second request will indeed receive the response data for the first request. By implementing timeout logic and then ignoring the cause of the timeout, you are getting your two apps out of sync with each other. So, either use a longer timeout (or no timeout at all), or else drop the connection if a timeout occurs.
As for your Delphi app freezing, that should only happen if you are using the TClientSocket component in blocking mode and performing your reading in the context of the main UI thread. You should not be using blocking mode in the main UI thread. Either:
Use TClientSocket in non-blocking mode, do all of your reading in the OnRead event only, and do not read more than ReceiveLength indicates.
Use TClientSocket in blocking mode, and do all of your reading in a worker thread, and then signal the main UI thread only when there is data available for it to process (better would be to process the data in the worker thread, and only sync with the main thread when making UI updates).

Async calls in Web Api

In asp.net Web API how can I call long running blocking actions asynchronously?
I have a web api controller action that needs to make a relatively (10 seconds, as an example) long running DB call. This seems like a candidate for an asynchronous method. I can offload the long running task onto a new thread, and unblock my asp.net request thread to handle some other requests, so my simplified controller action would look like this:
public async Task<IHttpActionResult> Get()
{
IEnumerable<Thing> things = await Task.Run(() => DoLongDbCall());
return Ok(things);
}
I have encountered a couple of blogs (this one, for example) that suggest this may not be an optimal way to achieve this in asp.net. The author suggests using Task.FromResult() and doing the DB call synchronously, but i cant see how this helps; My request thread is still going to be blocked waiting for the DB call to return.

First, consider what happens with a synchronous call:
public IHttpActionResult Get()
{
IEnumerable<Thing> things = DoLongDbCall();
return Ok(things);
}
A request comes in, and ASP.NET grabs a thread pool thread to handle the request. This thread invokes the Get method, which does the work. One thread pool thread is used during the entire request.
Now, let's walk through what happens in the current code (using Task.Run):
public async Task<IHttpActionResult> Get()
{
IEnumerable<Thing> things = await Task.Run(() => DoLongDbCall());
return Ok(things);
}
A request comes in, and ASP.NET grabs a thread pool thread to handle the request. This thread invokes the Get method, which then grabs another thread pool thread to do the work and returns the original thread pool thread back to the thread pool. One thread pool thread is used during the entire request (and two thread pool threads are used for a very brief period of time).
So, the Task.Run code forces an extra thread transition without providing any benefit (the entire point of using async on the server side is to free up threads). This is why I recommend against using Task.Run (or any other way to run work on the thread pool) on ASP.NET.
A proper asynchronous solution would be to use asynchronous DB calls:
public async Task<IHttpActionResult> Get()
{
IEnumerable<Thing> things = await DoLongDbCallAsync();
return Ok(things);
}
A request comes in, and ASP.NET grabs a thread pool thread to handle the request. This thread invokes the Get method, which then starts the asynchronous operation and returns the thread pool thread back to the thread pool. Later, when the db call completes, ASP.NET grabs a thread pool thread to finish the request. For most of the request, no thread pool threads are used (one thread pool thread is used for a brief period of time at the beginning and end of the request).

my request thread is still going to be blocked waiting for the DB call to return.
That's not completely true:
Your request will still wait for the long operation to end, but your thread will be free to process other operations.
You must keep in mind that IIS has a reduced number of available threads (especially if running under non server systems) and freeing one that is not needed is always a good thing.
Moreover, if you have shortage of threads, and if you use Task.Run, the asynchronous operation will wait for an available thread, and if you don't release your current one, you'll end up with an horrible deadlock.

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
;
}

Passing exception or message from thread/task to main thread

We are dealing with processing and uploading of large files in windows service (.net 4.0). Process and upoload steps can take minutes to complete. Admin has the ability to mark job as cancelled, directly in database but to clear in memory queue it requires service restart. Goal is to abandon that job and pick next job in queue without service restart. Here's what I want to do:
In main entry point, start two tasks:
Task processTask = Task.Factory.StartNew(ProcessJob);
Task monitorTask = Task.Factory.StartNew(MonitorDB);
ProcessJob would call multiple long running steps like ProcessFile, UploadFile. We are checking for job status between the steps but job may be stuck in one of these long running steps.
If monitorTask detects job status change in DB, it should communicate that to main thread (through exception or message), so that main thread can quit, removing itself from the processing queue and allow next job in queue to start. Without Wait cannot get to exception but cannot wait because need to run these tasks in parallel.
At this time we are not concerned with the fact that some synch step in ProcessJob may still be going on and it may eventually complete. We would handle that in code.
So far in all of my applications, I have used Task.ContinueWith for success and failure, but never had to communicate back to main thread.

You could communicate between the monitor thread and the main thread through a BlockingCollection<T>.
Here's a simple example I wrote.
BlockingCollection<string> queue =
new BlockingCollection<string>();
// monitor thread.
Task.Factory.StartNew(() =>
{
while (true)
{
Thread.Sleep(1000);
queue.Add("event occured.");
}
});
// main thread.
while (true)
{
// blocks when no messages are in queue.
string message = queue.Take();
// kill process thread here.
}

You can use a CancellationToken for this. If you look at the Task class, there is an overload where you can pass in a CancellationToken. Store a reference to this token in your service and simply have your upload/process routine periodically check if the token has been cancelled like so:
if (yourToken.IsCancellationRequested)
break;

thread in asp.net/c#

I want to create a thread on user_login Event or Form_load Event.
In this thread i want to call a class function to execute some sql statement, and the user do not have to wait for the result or background process to be finished, he is just directed to his desired page let say my profile or what ever.
I am using ASP.NET, C#.

For what it is worth, this is a Bad Idea. There are a variety of ways that that IIS could terminate that background thread (IIS restart, app pool restart, etc., all of which are normal expected behavior of IIS), and the result would be that your DB transaction gets silently rolled back.
If these queries need to be reliably executed, you should either execute them in the request or send them to a windows service or other long-lived process. This doesn't mean that the user can't get feedback on the progress- the IIS request itself could be executed via an AJAX call.

Here are some examples of asynchronous calls in C#.
http://support.microsoft.com/kb/315582
The choice of pattern depends on your needs. Note that in sample 5 you can provide null as the callback if you don't want any code executed when the action is done.

You could do this by calling your method in a thread using Thread.Start. IsBackground is false by default, which should prevent your application from stopping.
http://msdn.microsoft.com/en-us/library/7a2f3ay4.aspx
But, since most of your time will probably be spent in your database call. Why not just execute it asynchronously without a callback? On a SqlCommand that would be BeginExecuteNonQuery.