ASP：在服务器端触发操作并返回

Question

I need to do some upload post-processing.

The user upload a .zip with a lot of images (usually between 100 and 500). I need to process that zip file to extract,modify and store images.

My problem is that this postprocessing is a bit long so the user get an HTTP error 500 because the time allocated to process the request is over.

The actual process :

1. User upload the zip file
2. Once the server has the file, process every images
3. wait and return an HTTP 200

I would like to have an asynchronous postprocessing :

1. User upload the zip file
2. Once the server has the file, trigger the postprocessing asynchronous method
3. don't wait and return an HTTP 200

Is it possible ? Is there a better way of doing it ?

NB: No code because that's just a question about architecture

Answer 1

Just execute your post-processing in another thread.

You may use

Task.Factory.StartNew(() => 
{ 
   /*your post-processing code*/ 
   //...etc
   /*when finished, notify user that post-processing has finished, maybe by email*/
});

It is much better not to make all requests create new threads. This will be very bad in case with a lot of concurrent users. But instead to create a single "worker" thread that executes "queued tasks" one by one.

So you can:

private class ProcessItem
{
   //define each task data
}

private static readonly ConcurrentQueue<ProcessItem> queue 
                                      = new ConcurrentQueue<ProcessItem>();

private static Thread worker = new Thread(() =>
{
    while (true) // infinite
    {
        ProcessItem item;
        if (!queue.TryDequeue(out item))
        { //no availble items, wait for an item
           Monitor.Enter(queue); Monitor.Wait(queue); Monitor.Exit(queue);
          continue; //I have been notified, repeat check
        }
        //now process item
    }
});

static YourClass()
{
     worker.Start(); //start the worker at class first-load
}

To add items to queue:

queue.Enqueue(item);
Monitor.Enter(queue); 
Monitor.PulseAll(queue); //notifiy the waiting threads
Monitor.Exit(queue);

Answer 2

I dont think the best way would be to use Async, because you will possibly be borrowing a thread from the main threadpool, which means your server has less threads available to process other requests. Now imagine within in 1-2 secs you have 20 zip files uploaded you will be borrowing 20 or even more threads from the threadpool to process your zip file, which means in that time your server can process 20 fewer requests.

The best way for doing this would be to kick off a completely seperate Background process/App. For example you might.

1. User upload the zip file.
2. Save the file to some location and return from your web app with 200 ok.
3. Your web app writes and instruction ie some sort of record to adataabase or a file somewhere, containing the instructions on how to process the zip file.
4. some sort of app for example an exe can read these instructions and process the zip file and write the status back to database or something.

Hope this makes sense.