I have created a task which creates an XML string. The task can last multiple seconds. When the task isn't finished after 5 seconds, I want to cancel the Task 'smootly' and continue with writing the rest of the XML. So I built in cancellation inside my task. But although I see the following message in the Log:
ProcessInformationTask timed out
I also see this line in my log
Adding the process information took 10001 ms
I wonder why this can happen because I want to cancel the Task after 5 seconds (if not finished). So I expect the task to last 5 seconds max. How can I solve this? Probably the cancelation isn't setup properly?
Code where I call my task
string additionalInformation = null;
var contextInfo = new StringBuilder();
var xmlWriterSettings = new XmlWriterSettings()
{
OmitXmlDeclaration = true,
ConformanceLevel = ConformanceLevel.Fragment
};
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
using (XmlWriter xmlWriter = XmlWriter.Create(contextInfo, xmlWriterSettings))
{
try
{
xmlWriter.WriteStartElement("AdditionalInformation");
//Write xml (not long running)
var watch = System.Diagnostics.Stopwatch.StartNew();
string processInformation = AddProcessesInformation(xmlWriterSettings);
watch.Stop();
var elapsedMs = watch.ElapsedMilliseconds;
Log.Info("Adding the process information took : " + elapsedMs + " ms");
if (!string.IsNullOrEmpty(processInformation))
{
xmlWriter.WriteRaw(processInformation);
}
//Write xml (not long running)
xmlWriter.WriteEndElement();
additionalInformation = contextInfo.ToString();
}
catch (Exception e)
{
Log.Info("An exception occured during writing the additional information: " + e.Message);
return false;
}
return true;
}
Task method
private static string AddProcessesInformation(XmlWriterSettings xmlWriterSettings)
{
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
var contextInfo = new StringBuilder();
var processInformationTask = Task<string>.Factory.StartNew(() =>
{
if (token.IsCancellationRequested)
{
Log.Info("Cancellation request for the ProcessInformationTask");
}
Thread.Sleep(10000);
return "Ran without problems'";
}, token);
if (!processInformationTask.Wait(5000, token))
{
Log.Info("ProcessInformationTask timed out");
tokenSource.Cancel();
}
return processInformationTask?.Result;
}
I think you should check if cancellation is requested multiple Times, after each step in your method. Here is example using for loop:
private static string AddProcessesInformation(XmlWriterSettings xmlWriterSettings)
{
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
var contextInfo = new StringBuilder();
var processInformationTask = Task<string>.Factory.StartNew(() =>
{
for(int i = 0; i < 10; i++)
{
if (token.IsCancellationRequested)
{
Console.WriteLine("Cancellation request for the ProcessInformationTask");
return string.Empty;
}
Thread.Sleep(1000);
}
return "Ran without problems'";
}, token);
if (!processInformationTask.Wait(5000, token))
{
Console.WriteLine("ProcessInformationTask timed out");
tokenSource.Cancel();
}
return processInformationTask?.Result;
}
If inside task method you call other methods, you can pass cancellation token to them and use method token.ThrowIfCancellationRequested() inside.
var processInformationTask = Task<string>.Factory.StartNew(() =>
{
try
{
Method1(token);
Thread.Sleep(1000);
Method2(token);
Thread.Sleep(1000);
}
catch(OperationCanceledException ex)
{
return string.Empty;
}
return "Ran without problems'";
}, token);
private static void Method1(CancellationToken token)
{
token.ThrowIfCancellationRequested();
// do more work
}
Related
This question is kind of an extension to this one, I have updated my code below to reflect a task that runs continuously updating web clients using through a Websocket connection, the tasks checks for stock prices for a provided company (symbol) received in the Recv() method, when the user changes the company the Recv() method should cancel the running task (for the older company) and start a new one for the newly provided company/symbol.
The problem I'm having is that the line where await Task.Run(() => StockPricingAsync(jsonResult), priceToken.Token); in the Recv method gets executed doesn't cancel the existing task although there is a priceToken.Cancel just right before. Any idea how to get the running task cancelled and start the same one immediately?
I have the priceToken variable declared globally so it can be accessed from within the running task, but this does not work.
I also tried to assign the task to variable and make that variable = null but that did not cancel the task as well.
I'm also open to change the approach completely if I have to, all I need is a running task that provides pricing information for a passed parameter which gets changed by the client while the connection is still open.
CancellationTokenSource priceToken = new CancellationTokenSource();
public async Task GetDataAsync(WebSocket ws)
{
CancellationToken token = new();
try
{
var sendTask = Task.Run(() => Send(ws, token));
var recvTask = Task.Run(() => Recv(ws, token));
do
{
await Task.Delay(1000);
} while (ws.State == WebSocketState.Open);
}
finally
{
await ws.CloseAsync(WebSocketCloseStatus.Empty, "", token);
}
}
async Task Recv(WebSocket ws, CancellationToken token)
{
Console.WriteLine("Recv task started...");
var buffer = WebSocket.CreateClientBuffer(1024, 1024);
WebSocketReceiveResult taskResult;
while (ws.State == WebSocketState.Open)
{
string jsonResult = "";
do
{
taskResult = await ws.ReceiveAsync(buffer, token);
jsonResult += Encoding.UTF8.GetString(buffer.Array, 0, taskResult.Count);
} while (!taskResult.EndOfMessage);
if (!string.IsNullOrEmpty(jsonResult))
{
Console.WriteLine("Queueing {0}", jsonResult);
priceToken.Cancel()
priceToken = new CancellationTokenSource();
await Task.Run(() => StockPricingAsync(jsonResult), priceToken.Token);
}
}
Console.WriteLine("Recv task exiting...");
}
async static Task Send(WebSocket ws, CancellationToken token)
{
Console.WriteLine("Send task started...");
do
{
string sendMsg = sendQueue.Take();
Console.WriteLine("Sending {0}", sendMsg);
var sendMsgBytes = Encoding.UTF8.GetBytes(sendMsg);
ArraySegment<byte> segmentBuffer = new ArraySegment<byte>(sendMsgBytes, 0, sendMsgBytes.Length);
await ws.SendAsync(segmentBuffer, WebSocketMessageType.Text, true, token);
} while (ws.State == WebSocketState.Open);
Console.WriteLine("Send task exiting...");
}
public async Task StockPricingAsync(string symbol)
{
var lastUpdated = DateTime.MinValue;
double previousPrice = 0;
new StockService().GetStockPricing(symbol, true);
while (!priceToken.Token.IsCancellationRequested)
{
var price = new StockService().GetStockPricing(symbol, false);
if (price != null && lastUpdated != price.LastPriceDate)
{
lastUpdated = price.LastPriceDate;
if (price.LastPrice > previousPrice)
price.Tick = Stock.Tick.UpTick;
else if (price.LastPrice < previousPrice)
price.Tick = Stock.Tick.DownTick;
else
price.Tick = Stock.Tick.NoChange;
previousPrice = price.LastPrice;
var json = JsonConvert.SerializeObject(new ServerData(new KeyValuePair<string, object>("StockPricing", price), _eventMessage), _jsonSerializerSettings);
sendQueue.Add(json);
}
await Task.Delay(3000, this.priceToken.Token);
}
if (this.priceToken.Token.IsCancellationRequested)
this.priceToken.Token.ThrowIfCancellationRequested();
}
In the WPF .net core app there is the following:
An Observable Collection of items (itemObservCollection).
A static readonly HttpClient _httpclient
XML Responses
I am making a URL call to the api on each item in the observable collection (0 to 1000 items in collection). The return is XML. The XML is parsed using XElement. The property values in the observable collection are updated from the XML.
Task.Run is used to run the operation off the UI thread. Parallel.Foreach is used to make the calls in Parallel.
I feel I have made the solution overly complicated. Is there a way to simplify this? UpdateItems() is called from a button click.
private async Task UpdateItems()
{
try
{
await Task.Run(() => Parallel.ForEach(itemObservCollection, new ParallelOptions { MaxDegreeOfParallelism = 12 }, async item =>
{
try
{
var apiRequestString = $"http://localhost:6060/" + item.Name;
HttpResponseMessage httpResponseMessage = await _httpclient.GetAsync(apiRequestString);
var httpResponseStream = await httpResponseMessage.Content.ReadAsStreamAsync();
StringBuilder sb = new StringBuilder(1024);
XElement doc = XElement.Load(httpResponseStream);
foreach (var elem in doc.Descendants())
{
if (elem.Name == "ItemDetails")
{
var itemUpdate = itemObservCollection.FirstOrDefault(updateItem => updateItem.Name == item.Name);
if (itemUpdate != null)
{
itemUpdate.Price = decimal.Parse(elem.Attribute("Price").Value);
itemUpdate.Quantity = int.Parse(elem.Attribute("Quantity").Value);
}
}
}
}
catch (Exception ex)
{
LoggerTextBlock.Text = ('\n' + ex.ToString());
}
}));
}
catch (Exception ex)
{
LoggerTextBlock.Text = ('\n' + ex.ToString());
}
}
You could create an array of tasks and await them all using Task.WhenAll.
The following sample code kicks off a task per item in the ObservableCollection<int> and then wait asynchronously for all tasks to finish:
ObservableCollection<int> itemObservCollection =
new ObservableCollection<int>(Enumerable.Range(1, 10));
async Task SendAsync()
{
//query the HTTP API here...
await Task.Delay(1000);
}
await Task.WhenAll(itemObservCollection.Select(x => SendAsync()).ToArray());
If you want to limit the number of concurrent requests, you could either iterate through a subset of the source collecton to send requests in batches or use a SemaphoreSlim to limit the number of actual concurrent requests:
Task[] tasks = new Task[itemObservCollection.Count];
using (SemaphoreSlim semaphoreSlim = new SemaphoreSlim(12))
{
for (int i = 0; i < itemObservCollection.Count; ++i)
{
async Task SendAsync()
{
//query the HTTP API here...
try
{
await Task.Delay(5000);
}
finally
{
semaphoreSlim.Release();
}
}
await semaphoreSlim.WaitAsync();
tasks[i] = SendAsync();
}
await Task.WhenAll(tasks);
}
I have the following test code to simulate the use of a semaphore and throttling task execution. Is there a way to not continue to create new tasks if one of the running tasks throws an exception like the below. I don't need the existing tasks to stop running, I just want no new tasks to start after the exception is encountered.
Currently the tasks will all start in this scenario below. I want it to stop after a few tasks are ran because of the exceptions being thrown.
var testStrings = new List<string>();
for (var i = 0; i < 5000; i++)
{
testStrings.Add($"string-{i}");
}
using (var semaphore = new SemaphoreSlim(10))
{
var tasks = testStrings.Select(async testString =>
{
await semaphore.WaitAsync();
try
{
Console.WriteLine($"{testString}-Start");
await Task.Delay(2000);
throw new Exception("test");
}
finally
{
semaphore.Release();
}
});
await Task.WhenAll(tasks);
}
As per Sinatr's comments, I think this may work for me by adding a cancellation token to be monitored.
var testStrings = new List<string>();
for (var i = 0; i < 5000; i++)
{
testStrings.Add($"string-{i}");
}
var cancellationTokenSource = new CancellationTokenSource();
var cancellationToken = cancellationTokenSource.Token;
using (var semaphore = new SemaphoreSlim(10))
{
var tasks = testStrings.Select(async testString =>
{
await semaphore.WaitAsync();
try
{
if (!cancellationToken.IsCancellationRequested)
{
Console.WriteLine($"{testString}-Start");
await Task.Delay(2000);
throw new Exception("test");
}
}
catch (Exception ex)
{
if (!cancellationTokenSource.IsCancellationRequested)
cancellationTokenSource.Cancel();
throw;
}
finally
{
semaphore.Release();
}
});
await Task.WhenAll(tasks);
}
I have a process that reads a message from an Azure Service Bus Queue and converts that message to a Video to be Encoded by Azure Media Services. I noticed that if the process is kicked off very quickly in a row, the same video was being encoded right after another. Here is my code that adds the Video to the Queue
public class VideoManager
{
string _connectionString = ConfigurationManager.AppSettings["Microsoft.ServiceBus.ConnectionString"];
string _queueName = ConfigurationManager.AppSettings["ServiceBusQueueName"];
QueueClient _client;
public VideoManager()
{
var conStringBuilder = new ServiceBusConnectionStringBuilder(_connectionString)
{
OperationTimeout = TimeSpan.FromMinutes(120)
};
var messagingFactory = MessagingFactory.CreateFromConnectionString(conStringBuilder.ToString());
_client = messagingFactory.CreateQueueClient(_queueName);
}
public void Approve(Video video)
{
// Set video to approved.
video.ApprovalStatus = ApprovalStatus.Approved;
var message = new BrokeredMessage(new VideoMessage(video, VideoMessage.MessageTypes.Approve, string.Empty));
message.MessageId = video.RowKey;
_client.Send(message);
}
}
And the process that reads from the Queue
class Program
{
static QueueClient client;
static void Main(string[] args)
{
VideoManager videoManager = new VideoManager();
var connectionString = ConfigurationManager.AppSettings["Microsoft.ServiceBus.ConnectionString"];
var conStringBuilder = new ServiceBusConnectionStringBuilder(connectionString)
{
OperationTimeout = TimeSpan.FromMinutes(120)
};
var messagingFactory = MessagingFactory.CreateFromConnectionString(conStringBuilder.ToString());
client = messagingFactory.CreateQueueClient(ConfigurationManager.AppSettings["ServiceBusQueueName"]);
Console.WriteLine("Starting: Broadcast Center Continuous Video Processing Job");
OnMessageOptions options = new OnMessageOptions
{
MaxConcurrentCalls = 25,
AutoComplete = false
};
client.OnMessageAsync(async message =>
{
bool shouldAbandon = false;
try
{
await HandleMessage(message);
}
catch (Exception ex)
{
shouldAbandon = true;
Console.WriteLine(ex.Message);
}
if (shouldAbandon)
{
await message.AbandonAsync();
}
}, options);
while (true) { }
}
async static Task<int> HandleMessage(BrokeredMessage message)
{
VideoMessage videoMessage = message.GetBody<VideoMessage>();
Console.WriteLine(String.Format("Message body: {0}", videoMessage.Video.Title));
Console.WriteLine(String.Format("Message id: {0}", message.MessageId));
VideoProcessingService vp = new VideoProcessingService(videoMessage.Video);
Task task;
switch (videoMessage.MessageType)
{
case VideoMessage.MessageTypes.CreateThumbnail:
task = new Task(() => vp.ProcessThumbnail(videoMessage.TimeStamp));
task.Start();
while (!task.IsCompleted)
{
await Task.Delay(15000);
message.RenewLock();
}
await task;
Console.WriteLine(task.Status.ToString());
Console.WriteLine("Processing Complete");
Console.WriteLine("Awaiting Message");
break;
case VideoMessage.MessageTypes.Approve:
task = new Task(() => vp.Approve());
task.Start();
while (!task.IsCompleted)
{
await Task.Delay(15000);
message.RenewLock();
}
await task;
Console.WriteLine(task.Status.ToString());
Console.WriteLine("Processing Complete");
Console.WriteLine("Awaiting Message");
break;
default:
break;
}
return 0;
}
}
What I see in the Console Window is the following if I kick off the process 3 times in a row
Message id: 76aca19a-0698-449b-bf58-a24876fc4314
Message id: 76aca19a-0698-449b-bf58-a24876fc4314
Message id: 76aca19a-0698-449b-bf58-a24876fc4314
I thought maybe I did not have the settings correct, but they are there
I am really at a loss here, as I would expect this to be fairly out of the box behavior. Does duplicate detection only work if the message has been completed, so I can't use OnMessageAsync()?
The issue is not the completion (as it was in the code), but the fact that you have in essence multiple consumers (25 concurrent callbacks) and it seems like the LockDuration is elapsing faster than the processing takes. As a result of that, message re-appears and re-processed. As a result of that you see the same message ID logged more than once.
Possible solutions are (as I've outlined in a comment above):
Let OnMessage API manage timeout extension for you (example)
Manually renew the lock as you've done using BrokeredMessage.RenewLock
There is a line of code missing from your HandleMessage code.
async static Task<int> HandleMessage(BrokeredMessage message)
{
VideoMessage videoMessage = message.GetBody<VideoMessage>();
message.CompleteAsync(); // This line...
Console.WriteLine(String.Format("Message id: {0}", message.MessageId));
// Processes Message
}
So yes you have to mark the message with either, Complete, Defer etc..
Also see this Answer, also found this which may be useful in how duplicate detection works
There're Task.WaitAll method which waits for all tasks and Task.WaitAny method which waits for one task. How to wait for any N tasks?
Use case: search result pages are downloaded, each result needs a separate task to download and process it. If I use WaitAll to wait for the results of the subtasks before getting next search result page, I will not use all available resources (one long task will delay the rest). Not waiting at all can cause thousands of tasks to be queued which isn't the best idea either.
So, how to wait for a subset of tasks to be completed? Or, alternatively, how to wait for the task scheduler queue to have only N tasks?
This looks like an excellent problem for TPL Dataflow, which will allow you to control parallelism and buffering to process at maximum speed.
Here's some (untested) code to show you what I mean:
static void Process()
{
var searchReader =
new TransformManyBlock<SearchResult, SearchResult>(async uri =>
{
// return a list of search results at uri.
return new[]
{
new SearchResult
{
IsResult = true,
Uri = "http://foo.com"
},
new SearchResult
{
// return the next search result page here.
IsResult = false,
Uri = "http://google.com/next"
}
};
}, new ExecutionDataflowBlockOptions
{
BoundedCapacity = 8, // restrict buffer size.
MaxDegreeOfParallelism = 4 // control parallelism.
});
// link "next" pages back to the searchReader.
searchReader.LinkTo(searchReader, x => !x.IsResult);
var resultActor = new ActionBlock<SearchResult>(async uri =>
{
// do something with the search result.
}, new ExecutionDataflowBlockOptions
{
BoundedCapacity = 64,
MaxDegreeOfParallelism = 16
});
// link search results into resultActor.
searchReader.LinkTo(resultActor, x => x.IsResult);
// put in the first piece of input.
searchReader.Post(new SearchResult { Uri = "http://google/first" });
}
struct SearchResult
{
public bool IsResult { get; set; }
public string Uri { get; set; }
}
I think you should independently limit the number of parallel download tasks and the number of concurrent result processing tasks. I would do it using two SemaphoreSlim objects, like below. This version doesn't use the synchronous SemaphoreSlim.Wait (thanks #svick for making the point). It was only slightly tested, the exception handling can be improved; substitute your own DownloadNextPageAsync and ProcessResults:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
namespace Console_21666797
{
partial class Program
{
// the actual download method
// async Task<string> DownloadNextPageAsync(string url) { ... }
// the actual process methods
// void ProcessResults(string data) { ... }
// download and process all pages
async Task DownloadAndProcessAllAsync(
string startUrl, int maxDownloads, int maxProcesses)
{
// max parallel downloads
var downloadSemaphore = new SemaphoreSlim(maxDownloads);
// max parallel processing tasks
var processSemaphore = new SemaphoreSlim(maxProcesses);
var tasks = new HashSet<Task>();
var complete = false;
var protect = new Object(); // protect tasks
var page = 0;
// do the page
Func<string, Task> doPageAsync = async (url) =>
{
bool downloadSemaphoreAcquired = true;
try
{
// download the page
var data = await DownloadNextPageAsync(
url).ConfigureAwait(false);
if (String.IsNullOrEmpty(data))
{
Volatile.Write(ref complete, true);
}
else
{
// enable the next download to happen
downloadSemaphore.Release();
downloadSemaphoreAcquired = false;
// process this download
await processSemaphore.WaitAsync();
try
{
await Task.Run(() => ProcessResults(data));
}
finally
{
processSemaphore.Release();
}
}
}
catch (Exception)
{
Volatile.Write(ref complete, true);
throw;
}
finally
{
if (downloadSemaphoreAcquired)
downloadSemaphore.Release();
}
};
// do the page and save the task
Func<string, Task> queuePageAsync = async (url) =>
{
var task = doPageAsync(url);
lock (protect)
tasks.Add(task);
await task;
lock (protect)
tasks.Remove(task);
};
// process pages in a loop until complete is true
while (!Volatile.Read(ref complete))
{
page++;
// acquire download semaphore synchrnously
await downloadSemaphore.WaitAsync().ConfigureAwait(false);
// do the page
var task = queuePageAsync(startUrl + "?page=" + page);
}
// await completion of the pending tasks
Task[] pendingTasks;
lock (protect)
pendingTasks = tasks.ToArray();
await Task.WhenAll(pendingTasks);
}
static void Main(string[] args)
{
new Program().DownloadAndProcessAllAsync("http://google.com", 10, 5).Wait();
Console.ReadLine();
}
}
}
Something like this should work. There might be some edge cases, but all in all it should ensure a minimum of completions.
public static async Task WhenN(IEnumerable<Task> tasks, int n, CancellationTokenSource cts = null)
{
var pending = new HashSet<Task>(tasks);
if (n > pending.Count)
{
n = pending.Count;
// or throw
}
var completed = 0;
while (completed != n)
{
var completedTask = await Task.WhenAny(pending);
pending.Remove(completedTask);
completed++;
}
if (cts != null)
{
cts.Cancel();
}
}
Usage:
static void Main(string[] args)
{
var tasks = new List<Task>();
var completed = 0;
var cts = new CancellationTokenSource();
for (int i = 0; i < 100; i++)
{
tasks.Add(Task.Run(async () =>
{
await Task.Delay(temp * 100, cts.Token);
Console.WriteLine("Completed task {0}", i);
completed++;
}, cts.Token));
}
Extensions.WhenN(tasks, 30, cts).Wait();
Console.WriteLine(completed);
Console.ReadLine();
}
Task[] runningTasks = MyTasksFactory.StartTasks();
while(runningTasks.Any())
{
int finished = Task.WaitAny(runningTasks);
Task.Factory.StareNew(()=> {Consume(runningTasks[Finished].Result);})
runningTasks.RemoveAt(finished);
}