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Semaphore slim to handle throttling per time period

I have a requirement from a client, to call their API, however, due to the throttling limit, we can only make 100 API calls in a minute. I am using SemaphoreSlim to handle that, Here is my code.
async Task<List<IRestResponse>> GetAllResponses(List<string> locationApiCalls)
{
var semaphoreSlim = new SemaphoreSlim(initialCount: 100, maxCount: 100);
var failedResponses = new ConcurrentBag<IReadOnlyCollection<IRestResponse>>();
var passedResponses = new ConcurrentBag<IReadOnlyCollection<IRestResponse>>();
var tasks = locationApiCalls.Select(async locationApiCall =>
{
await semaphoreSlim.WaitAsync();
try
{
var response = await RestApi.GetResponseAsync(locationApi);
if (response.IsSuccessful)
{
passedResponses.Add((IReadOnlyCollection<IRestResponse>)response);
}
else
{
failedResponses.Add((IReadOnlyCollection<IRestResponse>)response);
}
}
finally
{
semaphoreSlim.Release();
}
});
await Task.WhenAll(tasks);
var passedResponsesList = passedResponses.SelectMany(x => x).ToList();
}
However this line
var passedResponsesList = passedResponses.SelectMany(x => x).ToList();
never gets executed and I see Lots of failedResponses as well, I guess I have to add Task.Delay (for 1 minute) somewhere in the code as well.

You need to keep track of the time when each of the previous 100 requests was executed. In the sample implementation below, the ConcurrentQueue<TimeSpan> records the relative completion time of each of these previous 100 requests. By dequeuing the first (and hence earliest) time from this queue, you can check how much time has passed since 100 requests ago. If it's been less than a minute, then the next request needs to wait for the remainder of the minute before it can be executed.
async Task<List<IRestResponse>> GetAllResponses(List<string> locationApiCalls)
{
var semaphoreSlim = new SemaphoreSlim(initialCount: 100, maxCount: 100);
var total = 0;
var stopwatch = Stopwatch.StartNew();
var completionTimes = new ConcurrentQueue<TimeSpan>();
var failedResponses = new ConcurrentBag<IReadOnlyCollection<IRestResponse>>();
var passedResponses = new ConcurrentBag<IReadOnlyCollection<IRestResponse>>();
var tasks = locationApiCalls.Select(async locationApiCall =>
{
await semaphoreSlim.WaitAsync();
if (Interlocked.Increment(ref total) > 100)
{
completionTimes.TryDequeue(out var earliest);
var elapsed = stopwatch.Elapsed - earliest;
var delay = TimeSpan.FromSeconds(60) - elapsed;
if (delay > TimeSpan.Zero)
await Task.Delay(delay);
}
try
{
var response = await RestApi.GetResponseAsync(locationApi);
if (response.IsSuccessful)
{
passedResponses.Add((IReadOnlyCollection<IRestResponse>)response);
}
else
{
failedResponses.Add((IReadOnlyCollection<IRestResponse>)response);
}
}
finally
{
completionTimes.Enqueue(stopwatch.Elapsed);
semaphoreSlim.Release();
}
});
await Task.WhenAll(tasks);
var passedResponsesList = passedResponses.SelectMany(x => x).ToList();
}
If you're calling this method from the UI thread of a WinForms or WPF application, remember to add ConfigureAwait(false) to its await statements.

How do I run a method both parallel and sequentially in C#?

I have a C# console app. In this app, I have a method that I will call DoWorkAsync. For the context of this question, this method looks like this:
private async Task<string> DoWorkAsync()
{
System.Threading.Thread.Sleep(5000);
var random = new Random();
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var length = random.Next(10, 101);
await Task.CompletedTask;
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
I call DoWorkAsync from another method that determines a) how many times this will get ran and b) if each call will be ran in parallel or sequentially. That method looks like this:
private async Task<Task<string>[]> DoWork(int iterations, bool runInParallel)
{
var tasks = new List<Task<string>>();
for (var i=0; i<iterations; i++)
{
if (runInParallel)
{
var task = Task.Run(() => DoWorkAsync());
tasks.Add(task);
}
else
{
await DoWorkAsync();
}
}
return tasks.ToArray();
}
After all of the tasks are completed, I want to display the results. To do this, I have code that looks like this:
var random = new Random();
var tasks = await DoWork(random.Next(10, 101);
Task.WaitAll(tasks);
foreach (var task in tasks)
{
Console.WriteLine(task.Result);
}
This code works as expected if the code runs in parallel (i.e. runInParallel is true). However, when runInParallel is false (i.e. I want to run the Tasks sequentially) the Task array doesn't get populated. So, the caller doesn't have any results to work with. I don't know how to fix it though. I'm not sure how to add the method call as a Task that will run sequentially. I understand that the idea behind Tasks is to run in parallel. However, I have this need to toggle between parallel and sequential.
Thank you!

the Task array doesn't get populated.
So populate it:
else
{
var task = DoWorkAsync();
tasks.Add(task);
await task;
}
P.S.
Also your DoWorkAsync looks kinda wrong to me, why Thread.Sleep and not await Task.Delay (it is more correct way to simulate asynchronous execution, also you won't need await Task.CompletedTask this way). And if you expect DoWorkAsync to be CPU bound just make it like:
private Task<string> DoWorkAsync()
{
return Task.Run(() =>
{
// your cpu bound work
return "string";
});
}
After that you can do something like this (for both async/cpu bound work):
private async Task<string[]> DoWork(int iterations, bool runInParallel)
{
if(runInParallel)
{
var tasks = Enumerable.Range(0, iterations)
.Select(i => DoWorkAsync());
return await Task.WhenAll(tasks);
}
else
{
var result = new string[iterations];
for (var i = 0; i < iterations; i++)
{
result[i] = await DoWorkAsync();
}
return result;
}
}

Why is DoWorkAsync an async method?
It isn't currently doing anything asynchronous.
It seems that you are trying to utilise multiple threads to improve the performance of expensive CPU-bound work, so you would be better to make use of Parallel.For, which is designed for this purpose:
private string DoWork()
{
System.Threading.Thread.Sleep(5000);
var random = new Random();
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var length = random.Next(10, 101);
return new string(Enumerable.Repeat(chars, length)
.Select(s => s[random.Next(s.Length)]).ToArray());
}
private string[] DoWork(int iterations, bool runInParallel)
{
var results = new string[iterations];
if (runInParallel)
{
Parallel.For(0, iterations - 1, i => results[i] = DoWork());
}
else
{
for (int i = 0; i < iterations; i++) results[i] = DoWork();
}
return results;
}
Then:
var random = new Random();
var serial = DoWork(random.Next(10, 101));
var parallel = DoWork(random.Next(10, 101), true);

I think you'd be better off doing the following:
Create a function that creates a (cold) list of tasks (or an array Task<string>[] for instance). No need to run them. Let's call this GetTasks()
var jobs = GetTasks();
Then, if you want to run them "sequentially", just do
var results = new List<string>();
foreach (var job in jobs)
{
var result = await job;
results.Add(result);
}
return results;
If you want to run them in parallel :
foreach (var job in jobs)
{
job.Start();
}
await results = Task.WhenAll(jobs);
Another note,
All this in itself should be a Task<string[]>, the Task<Task<... smells like a problem.

TaskFactory, Starting a new Task when one ends

I have found many methods of using the TaskFactory but I could not find anything about starting more tasks and watching when one ends and starting another one.
I always want to have 10 tasks working.
I want something like this
int nTotalTasks=10;
int nCurrentTask=0;
Task<bool>[] tasks=new Task<bool>[nThreadsNum];
for (int i=0; i<1000; i++)
{
string param1="test";
string param2="test";
if (nCurrentTask<10) // if there are less than 10 tasks then start another one
tasks[nCurrentThread++] = Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
});
// How can I stop the for loop until a new task is finished and start a new one?
}

Check out the Task.WaitAny method:
Waits for any of the provided Task objects to complete execution.
Example from the documentation:
var t1 = Task.Factory.StartNew(() => DoOperation1());
var t2 = Task.Factory.StartNew(() => DoOperation2());
Task.WaitAny(t1, t2)

I would use a combination of Microsoft's Reactive Framework (NuGet "Rx-Main") and TPL for this. It becomes very simple.
Here's the code:
int nTotalTasks=10;
string param1="test";
string param2="test";
IDisposable subscription =
Observable
.Range(0, 1000)
.Select(i => Observable.FromAsync(() => Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
})))
.Merge(nTotalTasks)
.ToArray()
.Subscribe((bool[] results) =>
{
/* Do something with the results. */
});
The key part here is the .Merge(nTotalTasks) which limits the number of concurrent tasks.
If you need to stop the processing part way thru just call subscription.Dispose() and everything gets cleaned up for you.
If you want to process each result as they are produced you can change the code from the .Merge(...) like this:
.Merge(nTotalTasks)
.Subscribe((bool result) =>
{
/* Do something with each result. */
});

This should be all you need, not complete, but all you need to do is wait on the first to complete and then run the second.
Task.WaitAny(task to wait on);
Task.Factory.StartNew()

Have you seen the BlockingCollection class? It allows you to have multiple threads running in parallel and you can wait from results from one task to execute another. See more information here.

The answer depends on whether the tasks to be scheduled are CPU or I/O bound.
For CPU-intensive work I would use Parallel.For() API setting the number of thread/tasks through MaxDegreeOfParallelism property of ParallelOptions
For I/O bound work the number of concurrently executing tasks can be significantly larger than the number of available CPUs, so the strategy is to rely on async methods as much as possible, which reduces the total number of threads waiting for completion.
How can I stop the for loop until a new task is finished and start a
new one?
The loop can be throttled by using await:
static void Main(string[] args)
{
var task = DoWorkAsync();
task.Wait();
// handle results
// task.Result;
Console.WriteLine("Done.");
}
async static Task<bool> DoWorkAsync()
{
const int NUMBER_OF_SLOTS = 10;
string param1="test";
string param2="test";
var results = new bool[NUMBER_OF_SLOTS];
AsyncWorkScheduler ws = new AsyncWorkScheduler(NUMBER_OF_SLOTS);
for (int i = 0; i < 1000; ++i)
{
await ws.ScheduleAsync((slotNumber) => DoWorkAsync(i, slotNumber, param1, param2, results));
}
ws.Complete();
await ws.Completion;
}
async static Task DoWorkAsync(int index, int slotNumber, string param1, string param2, bool[] results)
{
results[slotNumber] = results[slotNumber} && await Task.Factory.StartNew<bool>(() =>
{
MyClass cls = new MyClass();
bool bRet = cls.Method1(param1, param2, i); // takes up to 2 minutes to finish
return bRet;
}));
}
A helper class AsyncWorkScheduler uses TPL.DataFlow components as well as Task.WhenAll():
class AsyncWorkScheduler
{
public AsyncWorkScheduler(int numberOfSlots)
{
m_slots = new Task[numberOfSlots];
m_availableSlots = new BufferBlock<int>();
m_errors = new List<Exception>();
m_tcs = new TaskCompletionSource<bool>();
m_completionPending = 0;
// Initial state: all slots are available
for(int i = 0; i < m_slots.Length; ++i)
{
m_slots[i] = Task.FromResult(false);
m_availableSlots.Post(i);
}
}
public async Task ScheduleAsync(Func<int, Task> action)
{
if (Volatile.Read(ref m_completionPending) != 0)
{
throw new InvalidOperationException("Unable to schedule new items.");
}
// Acquire a slot
int slotNumber = await m_availableSlots.ReceiveAsync().ConfigureAwait(false);
// Schedule a new task for a given slot
var task = action(slotNumber);
// Store a continuation on the task to handle completion events
m_slots[slotNumber] = task.ContinueWith(t => HandleCompletedTask(t, slotNumber), TaskContinuationOptions.ExecuteSynchronously);
}
public async void Complete()
{
if (Interlocked.CompareExchange(ref m_completionPending, 1, 0) != 0)
{
return;
}
// Signal the queue's completion
m_availableSlots.Complete();
await Task.WhenAll(m_slots).ConfigureAwait(false);
// Set completion
if (m_errors.Count != 0)
{
m_tcs.TrySetException(m_errors);
}
else
{
m_tcs.TrySetResult(true);
}
}
public Task Completion
{
get
{
return m_tcs.Task;
}
}
void SetFailed(Exception error)
{
lock(m_errors)
{
m_errors.Add(error);
}
}
void HandleCompletedTask(Task task, int slotNumber)
{
if (task.IsFaulted || task.IsCanceled)
{
SetFailed(task.Exception);
return;
}
if (Volatile.Read(ref m_completionPending) == 1)
{
return;
}
// Release a slot
m_availableSlots.Post(slotNumber);
}
int m_completionPending;
List<Exception> m_errors;
BufferBlock<int> m_availableSlots;
TaskCompletionSource<bool> m_tcs;
Task[] m_slots;
}

List of Tasks C#

var tasks = new List<Task>();
for (int i = 0; i < pageCount; i++)
{
var task = Task.Run(() =>
{
worker.GetHouses(currentPage);
});
tasks.Add(task);
currentPage++;
}
Task.WaitAll(tasks.ToArray());
There is something i don't understand.
Whenever i use:
var tasks = new[]
{
Task.Run(() => {worker.GetHouses(1);}),
Task.Run(() => {worker.GetHouses(2);}),
Task.Run(() => {worker.GetHouses(3);})
};
And i loop trough that array, i get results perfectly fine. (when using Task.WaitAll(tasks)
When i use:
var tasks = new List<Task>();
my Task.WaitAll(tasks.toArray()) doesn't seem to work, my tasks "Status" stays on "RanToCompletion"
What did i do wrong?

You have a synchronization problem with the currentPage variable. Also create tasks with result.
Solution:
var tasks = new List<Task<List<House>>>();
for (int i = 0; i < pageCount; i++)
{
var currentPageCopy = currentPage;
var task = Task.Run(() =>
{
return worker.GetHouses(currentPageCopy);
});
tasks.Add(task);
currentPage++;
}
Task.WaitAll(tasks.ToArray());
The problem with your code is that all GetHouses invocations will be called with currentPage + pageCount - 1 as the last value will be used for all method calls...

There's been little issue with task types.
In your sample you were using System.Threading.Tasks.Task, which does not have the result - it's intended just to do some job, like void method.
In your code here:
var tasks = new[]
{
Task.Run(() => {worker.GetHouses(1);}),
Task.Run(() => {worker.GetHouses(2);}),
Task.Run(() => {worker.GetHouses(3);})
};
no type were specified explicitly, so it turned out to be System.Threading.Tasks.Task<List<House>>, but first piece of code you specified the System.Threading.Tasks.Task explicitly:
var tasks = new List<Task>();
What you need to use is System.Threading.Tasks.Task<TResult>:
var tasks = new List<Task<List<House>>>();// <- task type specified explicitly
for (int i = 0; i < pageCount; i++)
{
var task = Task.Factory.StartNew<List<House>>(() =>// <- task type specified explicitly , though it's mandatory here
{
return worker.GetHouses(currentPage);
});
tasks.Add(task);
currentPage++;
}
In similar situations I tend to define types explicitly, so that code becomes clearer to read and as you can see, even to work.

Why TPL Dataflow block.LinkTo does not give any output?

I am quite new to the topic TPL Dataflow. In the book Concurrency in C# I tested the following example. I can't figure out why there's no output which should be 2*2-2=2;
static void Main(string[] args)
{
//Task tt = test();
Task tt = test1();
Console.ReadLine();
}
static async Task test1()
{
try
{
var multiplyBlock = new TransformBlock<int, int>(item =>
{
if (item == 1)
throw new InvalidOperationException("Blech.");
return item * 2;
});
var subtractBlock = new TransformBlock<int, int>(item => item - 2);
multiplyBlock.LinkTo(subtractBlock,
new DataflowLinkOptions { PropagateCompletion = true });
multiplyBlock.Post(2);
await subtractBlock.Completion;
int temp = subtractBlock.Receive();
Console.WriteLine(temp);
}
catch (AggregateException e)
{
// The exception is caught here.
foreach (var v in e.InnerExceptions)
{
Console.WriteLine(v.Message);
}
}
}
Update1: I tried another example. Still I did not use Block.Complete() but I thought when the first block's completed, the result is passed into the second block automatically.
private static async Task test3()
{
TransformManyBlock<int, int> tmb = new TransformManyBlock<int, int>((i) => { return new int[] {i, i + 1}; });
ActionBlock<int> ab = new ActionBlock<int>((i) => Console.WriteLine(i));
tmb.LinkTo(ab);
for (int i = 0; i < 4; i++)
{
tmb.Post(i);
}
//tmb.Complete();
await ab.Completion;
Console.WriteLine("Finished post");
}

This part of the code:
await subtractBlock.Completion;
int temp = subtractBlock.Receive();
is first (asynchronously) waiting for the subtraction block to complete, and then attempting to retrieve an output from the block.
There are two problems: the source block is never completed, and the code is attempting to retrieve output from a completed block. Once a block has completed, it will not produce any more data.
(I assume you're referring to the example in recipe 4.2, which will post 1, causing the exception, which completes the block in a faulted state).
So, you can fix this test by completing the source block (and the completion will propagate along the link to the subtractBlock automatically), and by reading the output before (asynchronously) waiting for subtractBlock to complete:
multiplyBlock.Complete();
int temp = subtractBlock.Receive();
await subtractBlock.Completion;