I need to send a request to multiple servers and am trying to use tasks to run each connection asynchronously. I have a function that is configured to make the connections:
internal static Task<EventRecordEx> GetEventRecordFromServer(string server, string activityID)
I have tried the following but it runs synchronously...
var taskList = new List<Task<EventRecordEx>>();
foreach (string server in server_list)
{
taskList.Add(GetEventRecordFromServer(server, id));
}
await Task.Factory.ContinueWhenAll(taskList.ToArray(), completedTasks =>
{
foreach (var task in completedTasks)
{
// do something with the results
}
});
What am I doing wrong?
In my understanding when you use .ContinueWhenAll, you'll have a hard time debugging for the exceptions when one of the tasks fail as it will return an AggregateException, I'd suggest running the task individually, then use .ConfigureAwait(false) to make sure that it runs in a nun UI thread like so:
foreach(Task task in taskList.ToArray()){
await task.ConfigureAwait(false);
// Do something.
}
Related
A class has async method MonitorAsync(), which starts a long-running parallel operation. I have a collection of these monitors; these are all kicked off as follows:
internal async Task RunAsync()
{
var tasks = monitors.Select((p) => p.Value.MonitorAsync());
await Task.WhenAll(tasks);
}
If a monitor falls over, I need to know (basically I will run it up again). I've looked into ContinueWith and so on but when running a bunch of async tasks in parallel, how can I ensure I definitely know when one ends?
For context, RunAsync is basically the core of my application.
If a monitor falls over, I need to know (basically I will run it up again).
The easiest way to do this is to define this logic in a separate method:
internal async Task RunAsync()
{
var tasks = monitors.Select(p => MonitorAndRestart(p));
await Task.WhenAll(tasks);
async Task MonitorAndRestart(P p)
{
while (true)
{
try { await p.Value.MonitorAsync(); }
catch { ... }
p.Restart();
}
}
}
If you want to know when one ends (and that does not affect the others), ContinueWith() could be the way.
Alternatively, how about WaitAny in a loop?
while(anyTaskUnfinished){
await Task.WaitAny(tasks);
}
//Stuff you do after WhenAll() comes here
I am uncertain if you have to remove already finished Tasks. Or if it waits for any newly finishing.
You can try this:
If you do not want to call the Task.Wait method to wait for a task's completion, you can also retrieve the AggregateException exception from the task's Exception property
internal async Task RunAsync()
{
var tasks = monitors.Select((p) => p.Value.MonitorAsync());
try
{
await Task.WhenAll(tasks);
}
catch (Exception)
{
foreach (var task in tasks.Where(x => x.IsFaulted))
foreach (var exception in task.Exception.InnerExceptions)
{
// Do Something
}
}
}
Reference: Exception handling (Task Parallel Library)
I have an API which needs to be run in a loop for Mass processing.
Current single API is:
public async Task<ActionResult<CombinedAddressResponse>> GetCombinedAddress(AddressRequestDto request)
We are not allowed to touch/modify the original single API. However can be run in bulk, using foreach statement. What is the best way to run this asychronously without locks?
Current Solution below is just providing a list, would this be it?
public async Task<ActionResult<List<CombinedAddressResponse>>> GetCombinedAddress(List<AddressRequestDto> requests)
{
var combinedAddressResponses = new List<CombinedAddressResponse>();
foreach(AddressRequestDto request in requests)
{
var newCombinedAddress = (await GetCombinedAddress(request)).Value;
combinedAddressResponses.Add(newCombinedAddress);
}
return combinedAddressResponses;
}
Update:
In debugger, it has to go to combinedAddressResponse.Result.Value
combinedAddressResponse.Value = null
and Also strangely, writing combinedAddressResponse.Result.Value gives error below "Action Result does not contain a definition for for 'Value' and no accessible extension method
I'm writing this code off the top of my head without an IDE or sleep, so please comment if I'm missing something or there's a better way.
But effectively I think you want to run all your requests at once (not sequentially) doing something like this:
public async Task<ActionResult<List<CombinedAddressResponse>>> GetCombinedAddress(List<AddressRequestDto> requests)
{
var combinedAddressResponses = new List<CombinedAddressResponse>(requests.Count);
var tasks = new List<Task<ActionResult<CombinedAddressResponse>>(requests.Count);
foreach (var request in requests)
{
tasks.Add(Task.Run(async () => await GetCombinedAddress(request));
}
//This waits for all the tasks to complete
await tasks.WhenAll(tasks.ToArray());
combinedAddressResponses.AddRange(tasks.Select(x => x.Result.Value));
return combinedAddressResponses;
}
looking for a way to speed things up and run in parallel thanks
What you need is "asynchronous concurrency". I use the term "concurrency" to mean "doing more than one thing at a time", and "parallel" to mean "doing more than one thing at a time using threads". Since you're on ASP.NET, you don't want to use additional threads; you'd want to use a form of concurrency that works asynchronously (which uses fewer threads). So, Parallel and Task.Run should not be parts of your solution.
The way to do asynchronous concurrency is to build a collection of tasks, and then use await Task.WhenAll. E.g.:
public async Task<ActionResult<IReadOnlyList<CombinedAddressResponse>>> GetCombinedAddress(List<AddressRequestDto> requests)
{
// Build the collection of tasks by doing an asynchronous operation for each request.
var tasks = requests.Select(async request =>
{
var combinedAddressResponse = await GetCombinedAdress(request);
return combinedAddressResponse.Value;
}).ToList();
// Wait for all the tasks to complete and get the results.
var results = await Task.WhenAll(tasks);
return results;
}
I have an IEnumerable<Task>, where each Task will call the same endpoint. However, the endpoint can only handle so many calls per second. How can I put, say, a half second delay between each call?
I have tried adding Task.Delay(), but of course awaiting them simply means that the app waits a half second before sending all the calls at once.
Here is a code snippet:
var resultTasks = orders
.Select(async task =>
{
var result = new VendorTaskResult();
try
{
result.Response = await result.CallVendorAsync();
}
catch(Exception ex)
{
result.Exception = ex;
}
return result;
} );
var results = Task.WhenAll(resultTasks);
I feel like I should do something like
Task.WhenAll(resultTasks.EmitOverTime(500));
... but how exactly do I do that?
What you describe in your question is in other words rate limiting. You'd like to apply rate limiting policy to your client, because the API you use enforces such a policy on the server to protect itself from abuse.
While you could implement rate limiting yourself, I'd recommend you to go with some well established solution. Rate Limiter from Davis Desmaisons was the one that I picked at random and I instantly liked it. It had solid documentation, superior coverage and was easy to use. It is also available as NuGet package.
Check out the simple snippet below that demonstrates running semi-overlapping tasks in sequence while defering the task start by half a second after the immediately preceding task started. Each task lasts at least 750 ms.
using ComposableAsync;
using RateLimiter;
using System;
using System.Threading.Tasks;
namespace RateLimiterTest
{
class Program
{
static void Main(string[] args)
{
Log("Starting tasks ...");
var constraint = TimeLimiter.GetFromMaxCountByInterval(1, TimeSpan.FromSeconds(0.5));
var tasks = new[]
{
DoWorkAsync("Task1", constraint),
DoWorkAsync("Task2", constraint),
DoWorkAsync("Task3", constraint),
DoWorkAsync("Task4", constraint)
};
Task.WaitAll(tasks);
Log("All tasks finished.");
Console.ReadLine();
}
static void Log(string message)
{
Console.WriteLine(DateTime.Now.ToString("HH:mm:ss.fff ") + message);
}
static async Task DoWorkAsync(string name, IDispatcher constraint)
{
await constraint;
Log(name + " started");
await Task.Delay(750);
Log(name + " finished");
}
}
}
Sample output:
10:03:27.121 Starting tasks ...
10:03:27.154 Task1 started
10:03:27.658 Task2 started
10:03:27.911 Task1 finished
10:03:28.160 Task3 started
10:03:28.410 Task2 finished
10:03:28.680 Task4 started
10:03:28.913 Task3 finished
10:03:29.443 Task4 finished
10:03:29.443 All tasks finished.
If you change the constraint to allow maximum two tasks per second (var constraint = TimeLimiter.GetFromMaxCountByInterval(2, TimeSpan.FromSeconds(1));), which is not the same as one per half a second, then the output could be like:
10:06:03.237 Starting tasks ...
10:06:03.264 Task1 started
10:06:03.268 Task2 started
10:06:04.026 Task2 finished
10:06:04.031 Task1 finished
10:06:04.275 Task3 started
10:06:04.276 Task4 started
10:06:05.032 Task4 finished
10:06:05.032 Task3 finished
10:06:05.033 All tasks finished.
Note that the current version of Rate Limiter targets .NETFramework 4.7.2+ or .NETStandard 2.0+.
This is just a thought, but another approach could be to create a queue and add another thread that runs polling the queue for calls that need to go out to your endpoint.
Have you considered just turning that into a foreach-loop with a Task.Delay call? You seem to want to explicitly call them sequentially, it won't hurt if that is obvious from your code.
var results = new List<YourResultType>;
foreach(var order in orders){
var result = new VendorTaskResult();
try
{
result.Response = await result.CallVendorAsync();
results.Add(result.Response);
}
catch(Exception ex)
{
result.Exception = ex;
}
}
Instead of selecting from orders you could loop over them, and inside the loop put the result into a list and then call Task.WhenAll.
Would look something like:
var resultTasks = new List<VendorTaskResult>(orders.Count);
orders.ToList().ForEach( item => {
var result = new VendorTaskResult();
try
{
result.Response = await result.CallVendorAsync();
}
catch(Exception ex)
{
result.Exception = ex;
}
resultTasks.Add(result);
Thread.Sleep(x);
});
var results = Task.WhenAll(resultTasks);
If you want to control the number of requests executed simultaneously, you have to use a semaphore.
I have something very similar, and it works fine with me. Please note that I call ToArray() after the Linq query finishes, that triggers the tasks:
using (HttpClient client = new HttpClient()) {
IEnumerable<Task<string>> _downloads = _group
.Select(job => {
await Task.Delay(300);
return client.GetStringAsync(<url with variable job>);
});
Task<string>[] _downloadTasks = _downloads.ToArray();
_pages = await Task.WhenAll(_downloadTasks);
}
Now please note that this will create n nunmber of tasks, all in parallel, and the Task.Delay literally does nothing. If you want to call the pages synchronously (as it sounds by putting a delay between the calls), then this code may be better:
using (HttpClient client = new HttpClient()) {
foreach (string job in _group) {
await Task.Delay(300);
_pages.Add(await client.GetStringAsync(<url with variable job>));
}
}
The download of the pages is still asynchronous (while downloading other tasks are done), but each call to download the page is synchronous, ensuring that you can wait for one to finish in order to call the next one.
The code can be easily changed to call the pages asynchronously in chunks, like every 10 pages, wait 300ms, like in this sample:
IEnumerable<string[]> toParse = myData
.Select((v, i) => new { v.code, group = i / 20 })
.GroupBy(x => x.group)
.Select(g => g.Select(x => x.code).ToArray());
using (HttpClient client = new HttpClient()) {
foreach (string[] _group in toParse) {
string[] _pages = null;
IEnumerable<Task<string>> _downloads = _group
.Select(job => {
return client.GetStringAsync(<url with job>);
});
Task<string>[] _downloadTasks = _downloads.ToArray();
_pages = await Task.WhenAll(_downloadTasks);
await Task.Delay(5000);
}
}
All this does is group your pages in chunks of 20, iterate through the chunks, download all pages of the chunk asynchronously, wait 5 seconds, move on to the next chunk.
I hope that is what you were waiting for :)
The proposed method EmitOverTime is doable, but only by blocking the current thread:
public static IEnumerable<Task<TResult>> EmitOverTime<TResult>(
this IEnumerable<Task<TResult>> tasks, int delay)
{
foreach (var item in tasks)
{
Thread.Sleep(delay); // Delay by blocking
yield return item;
}
}
Usage:
var results = await Task.WhenAll(resultTasks.EmitOverTime(500));
Probably better is to create a variant of Task.WhenAll that accepts a delay argument, and delays asyncronously:
public static async Task<TResult[]> WhenAllWithDelay<TResult>(
IEnumerable<Task<TResult>> tasks, int delay)
{
var tasksList = new List<Task<TResult>>();
foreach (var task in tasks)
{
await Task.Delay(delay).ConfigureAwait(false);
tasksList.Add(task);
}
return await Task.WhenAll(tasksList).ConfigureAwait(false);
}
Usage:
var results = await WhenAllWithDelay(resultTasks, 500);
This design implies that the enumerable of tasks should be enumerated only once. It is easy to forget this during development, and start enumerating it again, spawning a new set of tasks. For this reason I propose to make it an OnlyOnce enumerable, as it is shown in this question.
Update: I should mention why the above methods work, and under what premise. The premise is that the supplied IEnumerable<Task<TResult>> is deferred, in other words non-materialized. At the method's start there are no tasks created yet. The tasks are created one after the other during the enumeration of the enumerable, and the trick is that the enumeration is slow and controlled. The delay inside the loop ensures that the tasks are not created all at once. They are created hot (in other words already started), so at the time the last task has been created some of the first tasks may have already been completed. The materialized list of half-running/half-completed tasks is then passed to Task.WhenAll, that waits for all to complete asynchronously.
I'm writing a Windows service and am looking for a way to execute a number of foreach loops in parallel, where each loop makes a call to an asynchronous (TAP) method. I initially tried the following code, which doesn't work because Parallel.ForEach and async/await are not compatible. Does anyone know whether there is an alternate approach that can achieve this?
Parallel.ForEach(messagesByFromNumber, async messageGroup =>
{
foreach (var message in messageGroup)
{
await message.SendAsync();
}
});
For the sake of clarity, due to the way that SendAsync() operates, each copy of the foreach loop must execute serially; in other words, the foreach loop cannot become concurrent / parallel.
There's no need to use Parallel.Foreach if your goal is to run these concurrently. Simply go over all your groups, create a task for each group that does a foreach of SendAsync, get all the tasks, and await them all at once with Task.WhenAll:
var tasks = messagesByFromNumber.Select(async messageGroup =>
{
foreach (var message in messageGroup)
{
await message.SendAsync();
}
});
await Task.WhenAll(tasks)
You can make it more cleat with AsyncEnumerator NuGet Package:
using System.Collections.Async;
await messagesByFromNumber.ParallelForEachAsync(async messageGroup =>
{
foreach (var message in messageGroup)
{
await message.SendAsync();
}
}, maxDegreeOfParallelism: 10);
I am using SignalR. When conversation is done I am trying to add all recipients in conversations and notiy the online ones.
All code is good but it notifies only the first recipient. so I suppose broadcasting is waiting only for the first time
public void NotifyConversation(ConversationModel model, string name)
{
var groupId = model.ID.ToString();
var recipients = model.Recipients;
var allconnections = new List<string>();
foreach (var recipient in recipients)
{
var connections = _manager.GetConnections(recipient.Name).Where(x => x != null);
allconnections.AddRange(connections);
}
var tasks = allconnections
.Select(connection =>
Task.Run(() => { Context.Groups.Add(connection, groupId); })).ToArray();
Task.WaitAll(tasks);
Context.Clients.Group(groupId).broadcastConversation(model);
}
Add() is asynchronous: it returns a Task. Task.Run() understands this, but you need to return that Task to it (notice that the lambda is no longer in a block):
Task.Run(() => Context.Groups.Add(connection, groupId))
This is similar to calling Wait() on the returned Task, except that it's better, because it doesn't block a thread.
Groups.Add is asynchronous. You're creating a task to run the Groups.Add but it completes almost instantly because the inner part of the task does not wait until the groups.add has completed.
Therefore you need to wait until the groups.Add has completed inside your Task.Run.
Something like:
var tasks = allconnections
.Select(connection =>
Task.Run(() => { Context.Groups.Add(connection, groupId).Wait(); })).ToArray();