Cancel parallel process

Question

I have a MainProcess.cs and ChildProcess.cs. Mainprocess is used to gather all required info and submit all data to parallel foreach loop. Child process will call to seperate API.

MainProcess.cs

//MainProcess.cs

string resourceId = GenerateUID();
Parallel.ForEach(Accounts, parallelOptions, async (account) =>
{                
    await childProcess.ProcessMethod(resourceId, account);

});

ChildProcess.cs

//ChildProcess.cs

public async Task ProcessMethod(string resourceId, string account){

    var Req = {}; //Required info set in here.

    using (HttpClient client = new HttpClient())
    {

        var result = await client.PostAsJsonAsync($"{baseUrl}api/services/app/Process/Run", Req);
        if (result.IsSuccessStatusCode)
        {
            //Save returned data to DB against the resourceId. (Figure 1)
        }
    }
}

Data save a table like below.

Figure 1

There is MainProcess class and it has a method to run tasks parallelly. There is a ChildProcess class and it has "ProcessMethod". Inside "ProcessMethod" I call to separate API to do some work. Above mention code and procedure is working 100% correctly.

But my issue is, at a point I need to cancel processing. Lets use below "figure 2" to make this easier to understand.

Figure 2

According to above figure, total process will finish approximate in 10 seconds. At the 4th second, I need to cancel processing. (Call 1 and Call 5 already returned the response) Call 2, Call 3, Call 4 are sent the request, but still not received the response.

What I need to do is, cancel all the processes at a given time. Simply It's like, I typed a Url in the browser and hit enter. before appearing the web page I close the browser. So I don't care about the responce.

I try to do this with cancellation token. But where I stuck is, I couldn't cancel api calls for specific resourceId.

Then I modified the code of ChildProcess.cs.

added instance varibale

CancellationTokenSource tokenSource = new CancellationTokenSource();

then change

var result = await client.PostAsJsonAsync($"{baseUrl}api/services/app/Process/Run", Req, tokenSource.Token);

I know, it should have to use below code for cancel process.

tokenSource.Cancel();

But the problem is how to cancel specific resourceId's process from a different function? (Cancel specific resourceId's process, because ProcessMethod can be called concurrently. If it is processing 2nd resourceId's process, It should not effect to 2nd process. 1st resourceId batch should have to close/cancel)

Any expert can help me on this?

The `Parallel.ForEach` [is not async-friendly](https://stackoverflow.com/questions/15136542/parallel-foreach-with-asynchronous-lambda). — Theodor Zoulias, Nov 23 '20 at 19:41

score 3 · Answer 1 · answered Nov 23 '20 at 15:04

Above mention code and procedure is working 100% correctly.

It's really not. async cannot be used with Parallel. Instead of parallelism (multiple threads), you should use asynchronous concurrency (multiple operations without a thread for each):

//MainProcess.cs
string resourceId = GenerateUID();
var tasks = Accounts.Select(account => childProcess.ProcessMethod(resourceId, account)).ToList();
await Task.WhenAll(tasks);

But the problem is how to cancel specific resourceId's process from a different function? (Cancel specific resourceId's process, because ProcessMethod can be called concurrently. If it is processing 2nd resourceId's process, It should not effect to 2nd process. 1st resourceId batch should have to close/cancel)

You don't need to do any special logic around this. You can use the same cancellation token for all of them because each operation will ignore cancellation requests after it has completed.

The only code you would need to add is to ignore any cancellation exceptions.

Something like this:

public async Task ProcessMethod(string resourceId, string account)
{
  try
  {
    var Req = {}; //Required info set in here.
    using (HttpClient client = new HttpClient())
    {
      var result = await client.PostAsJsonAsync($"{baseUrl}api/services/app/Process/Run", Req, tokenSource.Token);
      if (result.IsSuccessStatusCode)
      {
        //Save returned data to DB against the resourceId. (Figure 1)
      }
    }
  }
  catch (OperationCanceledException)
  {
  }
}

Thanks very much for giving me a reply. Apology for not giving quick response for your worth answer. I modified your code a bit. It's because resourceId should return before completing whole job. var tasks = Accounts.Select(async(account) => await childProcess.ProcessMethod(resourceId, account)).ToList(); Task.WhenAll(tasks.ToArray()); Then I make Thread.Sleep(1000); and then call to cancelProcess(); Its implementations is, public void cancelProcess() { tokenSource.Cancel(); } According to the code, it should not return values after executing 1st second. But still running jobs in background. — weeraa, Nov 26 '20 at 08:18

Cancel parallel process

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