为什么块按此顺序运行？

时间：2016-08-09 10:07:35

标签： c# .net task-parallel-library tpl-dataflow dataflow

这是一个简短的代码示例，可以快速向您介绍我的问题：

using System;
using System.Linq;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace DataflowTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var firstBlock = new TransformBlock<int, int>(x => x, new ExecutionDataflowBlockOptions
            {
                MaxDegreeOfParallelism = 4
            });
            var secondBlock = new TransformBlock<int,string>(async x =>
            {
                if (x == 12)
                {
                    await Task.Delay(5000);
                    return $"{DateTime.Now}: Message is {x} (This is delayed message!) ";
                }

                return $"{DateTime.Now}: Message is {x}";
            }, new ExecutionDataflowBlockOptions
            {
                MaxDegreeOfParallelism = 4
            });
            var thirdBlock = new ActionBlock<string>(s => Console.WriteLine(s), new ExecutionDataflowBlockOptions
            {
                MaxDegreeOfParallelism = 4
            });

            firstBlock.LinkTo(secondBlock);
            secondBlock.LinkTo(thirdBlock);

            var populateTask = Task.Run(async () =>
            {
                foreach (var x in Enumerable.Range(1, 15))
                {
                    await firstBlock.SendAsync(x);
                }
            });

            populateTask.Wait();
            secondBlock.Completion.Wait();
        }
    }
}

输出结果为：

09.08.2016 15:03:08: Message is 1
09.08.2016 15:03:08: Message is 5
09.08.2016 15:03:08: Message is 6
09.08.2016 15:03:08: Message is 7
09.08.2016 15:03:08: Message is 8
09.08.2016 15:03:08: Message is 9
09.08.2016 15:03:08: Message is 10
09.08.2016 15:03:08: Message is 11
09.08.2016 15:03:08: Message is 3
09.08.2016 15:03:08: Message is 2
09.08.2016 15:03:08: Message is 4
09.08.2016 15:03:13: Message is 12 (This is delayed message!)
09.08.2016 15:03:08: Message is 15
09.08.2016 15:03:08: Message is 13
09.08.2016 15:03:08: Message is 14

为什么这个订单以及如何更改网络以获得下面的输出？

09.08.2016 15:03:08: Message is 1
09.08.2016 15:03:08: Message is 5
09.08.2016 15:03:08: Message is 6
09.08.2016 15:03:08: Message is 7
09.08.2016 15:03:08: Message is 8
09.08.2016 15:03:08: Message is 9
09.08.2016 15:03:08: Message is 10
09.08.2016 15:03:08: Message is 11
09.08.2016 15:03:08: Message is 3
09.08.2016 15:03:08: Message is 2
09.08.2016 15:03:08: Message is 4
09.08.2016 15:03:08: Message is 15
09.08.2016 15:03:08: Message is 13
09.08.2016 15:03:08: Message is 14
09.08.2016 15:03:13: Message is 12 (This is delayed message!)

所以我想知道为什么所有其他块（或这里的任务）都在等待延迟块？

更新

既然你们让我更详细地解释我的问题，我就把这个样本做得更接近我正在研究的真正的管道。假设应用程序下载一些数据并根据返回的响应计算哈希值。

using System;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace DataflowTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var firstBlock = new TransformBlock<int, string>(x => x.ToString(), new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4 });

            var secondBlock = new TransformBlock<string, Tuple<string, string>>(async x =>
            {
                using (var httpClient = new HttpClient())
                {
                    if (x == "4") await Task.Delay(5000);

                    var result = await httpClient.GetStringAsync($"http://scooterlabs.com/echo/{x}");
                    return new Tuple<string, string>(x, result);
                }
            }, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4 });

            var thirdBlock = new TransformBlock<Tuple<string, string>, Tuple<string, byte[]>>(x =>
             {
                 using (var algorithm = SHA256.Create())
                 {
                     var bytes = Encoding.UTF8.GetBytes(x.Item2);
                     var hash = algorithm.ComputeHash(bytes);

                     return new Tuple<string, byte[]>(x.Item1, hash);
                 }
             }, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4 });

            var fourthBlock = new ActionBlock<Tuple<string, byte[]>>(x =>
            {
                var output = $"{DateTime.Now}: Hash for element #{x.Item1}: {GetHashAsString(x.Item2)}";

                Console.WriteLine(output);
            }, new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4 });

            firstBlock.LinkTo(secondBlock);
            secondBlock.LinkTo(thirdBlock);
            thirdBlock.LinkTo(fourthBlock);

            var populateTasks = Enumerable.Range(1, 10).Select(x => firstBlock.SendAsync(x));
            Task.WhenAll(populateTasks).ContinueWith(x => firstBlock.Complete()).Wait();

            fourthBlock.Completion.Wait();
        }

        private static string GetHashAsString(byte[] bytes)
        {
            var sb = new StringBuilder();
            int i;
            for (i = 0; i < bytes.Length; i++)
            {
                sb.AppendFormat("{0:X2}", bytes[i]);
                if (i % 4 == 3) sb.Append(" ");
            }

            return sb.ToString();
        }
    }
}

让我们来看看请求的顺序：

requests

这绝对有道理。所有请求都是尽快提出的。缓慢的第四个请求在列表的末尾。

现在让我们看看我们有什么输出：

09.08.2016 20:44:53: Hash for element #3: 4D0AB933 EE521204 CA784F3E 248EC698 F9E4D5F3 8F23A78F 3A00E069 29E73E32
09.08.2016 20:44:53: Hash for element #2: 4D0AB933 EE521204 CA784F3E 248EC698 F9E4D5F3 8F23A78F 3A00E069 29E73E32
09.08.2016 20:44:53: Hash for element #1: 4D0AB933 EE521204 CA784F3E 248EC698 F9E4D5F3 8F23A78F 3A00E069 29E73E32
09.08.2016 20:44:58: Hash for element #6: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3
09.08.2016 20:44:58: Hash for element #8: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3
09.08.2016 20:44:58: Hash for element #9: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3
09.08.2016 20:44:58: Hash for element #10: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3
09.08.2016 20:44:58: Hash for element #4: 44A63CBF 8E27D0DD AFE5A761 AADA4E49 AA52FE8E E3D7DC82 AFEAAF1D 72A9BC7F
09.08.2016 20:44:58: Hash for element #5: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3
09.08.2016 20:44:58: Hash for element #7: FC86E4F8 A83036BA 365BC7EE F9371778 59A11186 ED12A43C 3885D686 5004E6B3

你可以看到第三个响应来之后立即计算出第三个之后的所有哈希值。

因此，基于这两个事实，我们可以说所有下载的页面都在等待缓慢的第四个请求完成。最好不要在下载数据后立即等待第四个请求和计算哈希值。有什么方法可以实现这个目标吗？

3 个答案:

答案 0 :(得分：5)

好的，通过@SirRufo的引用，我开始考虑实现我自己的TransformBlock以满足我的需求并处理传入的项目，而不考虑订购。通过这种方式，它不会破坏网络，在部分下载中建立块之间的差距，这将是一种优雅的方式。

所以我开始研究什么以及如何做到这一点。要查看TransformBlock本身的来源似乎是一个很好的起点，所以我在Github上打开TransformBlock来源并开始分析它。

从课程开始就发现了这个有趣的事情： //如果采用并行性，我们将需要支持无序完成的重新排序消息。

// However, a developer can override this with EnsureOrdered == false.
if (dataflowBlockOptions.SupportsParallelExecution && dataflowBlockOptions.EnsureOrdered)
{
    _reorderingBuffer = new ReorderingBuffer<TOutput>(this, (owningSource, message) => ((TransformBlock<TInput, TOutput>)owningSource)._source.AddMessage(message));
}

看起来像我们想要的确切事情！让我们在Github上的EnsureOrdered课程中看到这个DataflowBlockOptions选项：

/// <summary>Gets or sets whether ordered processing should be enforced on a block's handling of messages.</summary>
/// <remarks>
/// By default, dataflow blocks enforce ordering on the processing of messages. This means that a
/// block like <see cref="TransformBlock{TInput, TOutput}"/> will ensure that messages are output in the same
/// order they were input, even if parallelism is employed by the block and the processing of a message N finishes 
/// after the processing of a subsequent message N+1 (the block will reorder the results to maintain the input
/// ordering prior to making those results available to a consumer).  Some blocks may allow this to be relaxed,
/// however.  Setting <see cref="EnsureOrdered"/> to false tells a block that it may relax this ordering if
/// it's able to do so.  This can be beneficial if the immediacy of a processed result being made available
/// is more important than the input-to-output ordering being maintained.
/// </remarks>
public bool EnsureOrdered
{
    get { return _ensureOrdered; }
    set { _ensureOrdered = value; }
}

它看起来非常好所以我立即切换到IDE来设置它。不幸的是，没有这样的设置：

No EnsureOrdered

我一直在搜索并找到note：

  

4.5.25-β-23019

     

包已重命名为System.Threading.Tasks.Dataflow

当我用Google搜索并找到package时，称为System.Threading.Tasks.Dataflow！所以我卸载了Microsoft.Tpl.Dataflow软件包，并通过发出：{/ p>安装了System.Threading.Tasks.Dataflow 

Install-Package System.Threading.Tasks.Dataflow

还有EnsureOrdered选项。我更新了代码，并将EnsureOrdered设置为false：

using System;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

namespace DataflowTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var options = new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 4, EnsureOrdered = false };
            var firstBlock = new TransformBlock<int, string>(x => x.ToString(), options);

            var secondBlock = new TransformBlock<string, Tuple<string, string>>(async x =>
            {
                using (var httpClient = new HttpClient())
                {
                    if (x == "4") await Task.Delay(5000);

                    var result = await httpClient.GetStringAsync($"http://scooterlabs.com/echo/{x}");
                    return new Tuple<string, string>(x, result);
                }
            }, options);

            var thirdBlock = new TransformBlock<Tuple<string, string>, Tuple<string, byte[]>>(x =>
             {
                 using (var algorithm = SHA256.Create())
                 {
                     var bytes = Encoding.UTF8.GetBytes(x.Item2);
                     var hash = algorithm.ComputeHash(bytes);

                     return new Tuple<string, byte[]>(x.Item1, hash);
                 }
             }, options);

            var fourthBlock = new ActionBlock<Tuple<string, byte[]>>(x =>
            {
                var output = $"{DateTime.Now}: Hash for element #{x.Item1}: {GetHashAsString(x.Item2)}";

                Console.WriteLine(output);
            }, options);

            firstBlock.LinkTo(secondBlock);
            secondBlock.LinkTo(thirdBlock);
            thirdBlock.LinkTo(fourthBlock);

            var populateTasks = Enumerable.Range(1, 10).Select(x => firstBlock.SendAsync(x));
            Task.WhenAll(populateTasks).ContinueWith(x => firstBlock.Complete()).Wait();

            fourthBlock.Completion.Wait();
        }

        private static string GetHashAsString(byte[] bytes)
        {
            var sb = new StringBuilder();
            int i;
            for (i = 0; i < bytes.Length; i++)
            {
                sb.AppendFormat("{0:X2}", bytes[i]);
                if (i % 4 == 3) sb.Append(" ");
            }

            return sb.ToString();
        }
    }
}

结果输出正是我想要的：

10.08.2016 11:03:23: Hash for element #3: 8BA8A86D F25E058E 180F7AA9 1EE996B0 8D721C84 AEE8AA19 0A3F7C44 9FFED481
10.08.2016 11:03:23: Hash for element #1: 8BA8A86D F25E058E 180F7AA9 1EE996B0 8D721C84 AEE8AA19 0A3F7C44 9FFED481
10.08.2016 11:03:23: Hash for element #2: 8BA8A86D F25E058E 180F7AA9 1EE996B0 8D721C84 AEE8AA19 0A3F7C44 9FFED481
10.08.2016 11:03:23: Hash for element #10: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:23: Hash for element #8: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:23: Hash for element #9: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:23: Hash for element #5: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:23: Hash for element #7: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:23: Hash for element #6: C8C87B26 B17A6329 3F6213CD 4F9AFE0D 4F90127B AAE49EB0 7D8C15DF 74F020B1
10.08.2016 11:03:27: Hash for element #4: FD25E52B FCD8DE81 BD38E11B 13C20B96 09473283 F25346B2 04593B70 E4357BDA

答案 1 :(得分：3)

这是设计和documented

  

因为每个预定义的源数据流块类型都保证消息按接收顺序传播出来，所以......

<强>证明： 

var ts = Environment.TickCount;

var firstBlock = new TransformBlock<int, int>(
    x => x,
    new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = 4,
    } );

var secondBlock = new TransformBlock<int, string>(
    x =>
    {
        var start = Environment.TickCount;

        if ( x == 3 )
        {
            Thread.Sleep( 5000 );
            return $"Start {start-ts} Finished {Environment.TickCount - ts}: Message is {x} (This is delayed message!) ";
        }

        return $"Start {start - ts} Finished {Environment.TickCount - ts}: Message is {x}";
    },
    new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = 4,
        // limit the internal queue to 10 items
        BoundedCapacity = 10,
    } );

var thirdBlock = new ActionBlock<string>(
    s =>
    {
        Console.WriteLine( s );
    },
    new ExecutionDataflowBlockOptions
    {
        // limit to a single task to watch the order
        MaxDegreeOfParallelism = 1,
    } );

firstBlock.LinkTo( secondBlock, new DataflowLinkOptions { PropagateCompletion = true, } );
secondBlock.LinkTo( thirdBlock, new DataflowLinkOptions { PropagateCompletion = true, } );

foreach ( var x in Enumerable.Range( 1, 15 ) )
{
    // to ensure order of items
    firstBlock.Post( x );
}

firstBlock.Complete();
thirdBlock.Completion.Wait();

<强>输出： 

Start 31 Finished 31: Message is 1
Start 31 Finished 31: Message is 2
Start 31 Finished 5031: Message is 3 (This is delayed message!)
Start 31 Finished 31: Message is 4
Start 31 Finished 31: Message is 5
Start 31 Finished 31: Message is 6
Start 31 Finished 31: Message is 7
Start 31 Finished 31: Message is 8
Start 31 Finished 31: Message is 9
Start 31 Finished 31: Message is 10
Start 31 Finished 31: Message is 11
Start 31 Finished 31: Message is 12
Start 5031 Finished 5031: Message is 13
Start 5031 Finished 5031: Message is 14
Start 5031 Finished 5031: Message is 15

解决方案1 ​​

不要将DataFlow用于下载部分，因为订单保证会阻止您正在寻找的处理。

var ts = Environment.TickCount;

var thirdBlock = new ActionBlock<string>(
    s =>
    {
        Console.WriteLine( s );
    },
    new ExecutionDataflowBlockOptions
    {
        // limit to a single task to watch the order
        MaxDegreeOfParallelism = 4,
    } );

Parallel.ForEach(
    Enumerable.Range( 1, 15 ),
    new ParallelOptions { MaxDegreeOfParallelism = 4, },
    x =>
    {
        var start = Environment.TickCount;
        string result;

        if ( x == 12 )
        {
            Thread.Sleep( 5000 );
            result = $"Start {start - ts} Finished {Environment.TickCount - ts}: Message is {x} (This is delayed message!) ";
        }
        else
            result = $"Start {start - ts} Finished {Environment.TickCount - ts}: Message is {x}";
        thirdBlock.Post( result );
    } );

thirdBlock.Complete();
thirdBlock.Completion.Wait();

<强>输出： 

Start 32 Finished 32: Message is 2
Start 32 Finished 32: Message is 6
Start 32 Finished 32: Message is 5
Start 32 Finished 32: Message is 8
Start 32 Finished 32: Message is 9
Start 32 Finished 32: Message is 10
Start 32 Finished 32: Message is 11
Start 32 Finished 32: Message is 7
Start 32 Finished 32: Message is 13
Start 32 Finished 32: Message is 14
Start 32 Finished 32: Message is 15
Start 32 Finished 32: Message is 3
Start 32 Finished 32: Message is 4
Start 32 Finished 32: Message is 1
Start 32 Finished 5032: Message is 12 (This is delayed message!)

解决方案2

当然，您可以在自定义类中实施IPropagatorBlock<TInput,TOutput> 不保证项目的顺序。

答案 2 :(得分：0)

查看时间戳，第二个块的输出正如您所期望的那样工作 - 延迟的TransformBlock正在所有其他TransformBlock之后运行。它似乎是ActionBlock中的Console.WriteLine，它没有按照你期望的顺序调用。

您的代码secondBlock.Completion.Wait();是否不正确 - 是否应为thirdBlock.Completion.Wait();才能获得您期望的结果？

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