I have problem in a windows service which is using TPL DataFlow to manage a queue (database) and redirects work to a grid computing service. And at one point BufferBlock stops releasing tasks, and I am not sure why. I think it's because some exceptions happen during execution of some tasks, but they get suppressed and it's difficult to understand at which point BufferBlock stops accepting new tasks.
I tried to simplify it in the working example below. It doesn't have any exception handling and I and wondering how to properly handle exceptions in TPL. I found something similar here TPL Dataflow, guarantee completion only when ALL source data blocks completed. In this example I have 100 requests, and process data in batches with 10 requests. Emulating some exception which happens if ID % 9 == 0 If I don't catch this exception, it works a bit and then stops accepting new requests. If I handle and return Result.Failure it works fine I believe, but I'm not sure if it's a proper way to have it in production environment.
I'm new to TPL, forget me if I didn't explain more clearly my question. GitHub Project
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;
using System.Timers;
using CSharpFunctionalExtensions;
namespace TestTPL
{
public class ServicePipeline
{
public const int batches = 100;
private int currentBatch = 0;
public ServicePipeline(int maxRequestsInParallel)
{
MaxRequestsInParallel = maxRequestsInParallel;
}
public int MaxRequestsInParallel { get; }
public BufferBlock<MyData> QueueBlock { get; private set; }
public List<TransformBlock<MyData, Result>> ExecutionBlocks
{ get; private set; }
public ActionBlock<Result> ResultBlock { get; private set; }
private void Init()
{
QueueBlock = new BufferBlock<MyData>(new DataflowBlockOptions()
{ BoundedCapacity = MaxRequestsInParallel });
ExecutionBlocks = new List<TransformBlock<MyData, Result>>();
ResultBlock = new ActionBlock<Result>(_ => _.OnFailure(
() => Console.WriteLine($"Error: {_.Error}")));
for (int blockIndex = 0; blockIndex < MaxRequestsInParallel;
blockIndex++)
{
var executionBlock = new TransformBlock<MyData, Result>((d) =>
{
return ExecuteAsync(d);
}, new ExecutionDataflowBlockOptions() { BoundedCapacity = 1 });
executionBlock.LinkTo(ResultBlock, new DataflowLinkOptions()
{ PropagateCompletion = true });
QueueBlock.LinkTo(executionBlock, new DataflowLinkOptions()
{ PropagateCompletion = true });
ExecutionBlocks.Add(executionBlock);
}
}
public static Result ExecuteAsync(MyData myData)
{
//try
//{
WebClient web = new WebClient();
TaskCompletionSource<Result> res = new TaskCompletionSource<Result>();
Task task = Task<Result>.Run(() => web.DownloadStringAsync(
new Uri("http://localhost:49182/Slow.ashx")));
task.Wait();
Console.WriteLine($"Data = {myData}");
if (myData != null && myData.Id % 9 == 0)
throw new Exception("Test");
return Result.Ok();
//}
//catch (Exception ex)
//{
// return Result.Failure($"Exception: {ex.Message}");
//}
}
public async void Start()
{
Init();
while (currentBatch < batches)
{
Thread.Sleep(1000);
await SubmitNextRequests();
}
Console.WriteLine($"Completed: {batches}");
}
private async Task<int> SubmitNextRequests()
{
var emptySlots = MaxRequestsInParallel - QueueBlock.Count;
Console.WriteLine($"Empty slots: {emptySlots}" +
$", left = {batches - currentBatch}");
if (emptySlots > 0)
{
var dataRequests = await GetNextRequests(emptySlots);
foreach (var data in dataRequests)
{
await QueueBlock.SendAsync(data);
}
}
return emptySlots;
}
private async Task<List<MyData>> GetNextRequests(int request)
{
MyData[] myDatas = new MyData[request];
Task<List<MyData>> task = Task<List<MyData>>.Run(() =>
{
for (int i = 0; i < request; i++)
{
myDatas[i++] = new MyData(currentBatch);
currentBatch++;
}
return new List<MyData>(myDatas);
});
return await task;
}
}
public class MyData
{
public int Id { get; set; }
public MyData(int id) => Id = id;
public override string ToString() { return Id.ToString(); }
}
}
EDIT: 10/30/2019 It works as expected when the exception is handled and called explicitly Result.Failure($"Exception: {ex.Message}");
public static Result ExecuteAsync(MyData myData)
{
try
{
WebClient web = new WebClient();
TaskCompletionSource<Result> res = new TaskCompletionSource<Result>();
Task task = Task<Result>.Run(() => Thread.Sleep(2000));
task.Wait();
Console.WriteLine($"Data = {myData}");
if (myData != null && myData.Id % 9 == 0)
throw new Exception("Test");
return Result.Ok();
}
catch (Exception ex)
{
return Result.Failure($"Exception: {ex.Message}");
}
}
