C#MySql驱动程序-异步操作 [英] C# MySql Driver - Async Operations

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问题描述

最近我已经开始使用针对C#的MySQL驱动程序 https://github.com/mysql/mysql-connector-net

Lately i have began working with the MySQL driver for C# https://github.com/mysql/mysql-connector-net

使用异步/等待，我尝试在并行任务中运行简单的选择查询

Working with async/await i tried to run simple select queries in parallel tasks

这基本上就是代码的样子:

This is basically how the code looks:

    private async Task<List<string>> RunQueryA()
    {
        List<string> lst = new List<string>();

        using (MySqlConnection conn = new MySqlConnection(someConnectionString))
        using (MySqlCommand cmd = conn.CreateCommand())
        {
            await conn.OpenAsync();
            cmd.CommandText = "select someField from someTable ...";

            using (var reader = await cmd.ExecuteReaderAsync())
            {
                // ...
            }
        }

        return lst;
    }

    private async Task<List<string>> RunQueryB()
    {
        List<string> lst = new List<string>();

        using (MySqlConnection conn = new MySqlConnection(someConnectionString))
        using (MySqlCommand cmd = conn.CreateCommand())
        {
            await conn.OpenAsync();
            cmd.CommandText = "select someField2 from someTable2 ...";

            using (var reader = await cmd.ExecuteReaderAsync())
            {
                // ...
            }
        }

        return lst;
    }

    public async Task Run()
    {
        await Task.WhenAll(RunQueryA(), RunQueryB());
    }

我期望两个查询能够并行运行，我看到的是RunQueryA()开始运行，只有完成后RunQueryB才能开始.

What i expected was for the both queries to run in parallel, what i saw was that RunQueryA() began to run and only once it was done RunQueryB could begin.

自然，这表明查询中使用的一种或多种方法正在阻止. 为了找出答案，我下载了最新的MySQL驱动程序源代码(从其github存储库中)并寻找了异步方法的实现.

Naturally it would suggest that one or more of the methods that were used in the query are blocking. To find out, i downloaded the latest MySQL driver source code (from their github repo) and looked for the implementation of the async methods.

例如，我查看了ExecuteReaderAsync的实现，它使我进入了基类System.Data.Common.DbCommand，它是BCL的一部分

I looked for instance at the implementation of ExecuteReaderAsync and it led me to the base class System.Data.Common.DbCommand which is part of the BCL

我在.NET参考源中查找了该类 https://referencesource.microsoft.com/# System.Data/System/Data/Common/DBCommand.cs,1875e74763fd9ef2

I looked up that class in the .NET Reference source https://referencesource.microsoft.com/#System.Data/System/Data/Common/DBCommand.cs,1875e74763fd9ef2

我所看到的真的让我感到困惑:

And what i saw really confused me:

public Task<DbDataReader> ExecuteReaderAsync() {
            return ExecuteReaderAsync(CommandBehavior.Default, CancellationToken.None);
        }

        public Task<DbDataReader> ExecuteReaderAsync(CancellationToken cancellationToken) {
            return ExecuteReaderAsync(CommandBehavior.Default, cancellationToken);
        }

        public Task<DbDataReader> ExecuteReaderAsync(CommandBehavior behavior) {
            return ExecuteReaderAsync(behavior, CancellationToken.None);
        }

        public Task<DbDataReader> ExecuteReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken) {
            return ExecuteDbDataReaderAsync(behavior, cancellationToken);
        }

        protected virtual Task<DbDataReader> ExecuteDbDataReaderAsync(CommandBehavior behavior, CancellationToken cancellationToken) {
            if (cancellationToken.IsCancellationRequested) {
                return ADP.CreatedTaskWithCancellation<DbDataReader>();
            }
            else {
                CancellationTokenRegistration registration = new CancellationTokenRegistration();
                if (cancellationToken.CanBeCanceled) {
                    registration = cancellationToken.Register(CancelIgnoreFailure);
                }

                try {
                    return Task.FromResult<DbDataReader>(ExecuteReader(behavior));
                }
                catch (Exception e) {
                    registration.Dispose();
                    return ADP.CreatedTaskWithException<DbDataReader>(e);
                }
            }
        }

一切都归结为这一行:

return Task.FromResult<DbDataReader>(ExecuteReader(behavior));

在这一行中，ExecuteReader将同步运行并阻塞调用线程.

In this line, ExecuteReader would run synchronously and block the calling thread.

ExecuteReader调用抽象方法

ExecuteReader calls an abstract method

abstract protected DbDataReader ExecuteDbDataReader(CommandBehavior behavior);

在MySQL驱动程序中被覆盖:

which is overridden inside the MySQL driver:

protected override DbDataReader ExecuteDbDataReader(CommandBehavior behavior)
    {
      return ExecuteReader(behavior);
    }

MySQL内部的实现基本上调用了ExecuteReader的同步版本...

The implementation inside the MySQL basically calls the synchronous version of ExecuteReader...

因此，简而言之，ExecuteReaderAsync()同步运行ExecuteReader()并阻塞调用线程.

So in short, ExecuteReaderAsync() runs ExecuteReader() synchronously and blocks the calling thread.

如果我弄错了，请纠正我，但事实确实如此.

Please correct me if i am mistaken but it really seems to be the case.

在BCL的DbCommand类或MySQL驱动程序实现中，我无法确切地指出是谁该引起的.

I can't exactly pinpoint who is to blame here, the DbCommand class of the BCL or the MySQL driver implementation...

一方面，MySQL驱动程序应该考虑到它，另一方面，由于DbCommand提供ExecuteDbDataReaderAsync的基本实现，因此它至少应在工作线程中启动ExecuteReader的同步版本(更不用说使用实际的异步I/O了)，这样它就不会阻塞.

On one hand, the MySQL driver should've taken it into account, On the other hand, since DbCommand provides a base implementation of ExecuteDbDataReaderAsync it should at least launch the synchronous version of ExecuteReader in a worker thread (let alone using actual asynchronous I/O) so it would not block.

对此有何看法?

我该怎么办? 我可以自己将ExecuteReaderAsync作为任务启动，但是我不喜欢这种解决方案.

What can i do as a work around? I could just launch ExecuteReaderAsync as a task by myself but i don't like this solution.

您有什么建议?

谢谢，阿里克

C#MySql驱动程序-异步操作 [英] C# MySql Driver - Async Operations

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