TPL DataFlow 处理异常的正确方法

Question

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

Image Empty Slots

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}");
        }
    }

Answer 1

When linking two block, there is an option to propagate completion forward, but not backward. This becomes a problem when the BoundedCapacity option is used, and an error occurs, because it can block the feeder of the pipeline and cause a dead-lock. It is quite easy to propagate completion manually though. Here is a method that you can use.

async void OnErrorComplete(IDataflowBlock block1, IDataflowBlock block2)
{
    await Task.WhenAny(block1.Completion); // Safe awaiting
    if (block1.Completion.IsFaulted) block2.Complete();
}

It waits asynchronously for block1 to complete, and if it has failed it completes immediately the block2 . Completing the upstream block is usually enough, but you can also propagate the specific exception if you want:

async void OnErrorPropagate(IDataflowBlock block1, IDataflowBlock block2)
{
    await Task.WhenAny(block1.Completion); // Safe awaiting
    if (block1.Completion.IsFaulted)
        block2.Fault(block1.Completion.Exception.InnerException);
}