谈谈.NET Core下如何利用 AsyncLocal 实现共享变量

前言

在Web 应用程序中，我们经常会遇到这样的场景，如用户信息，租户信息本次的请求过程中都是固定的，我们希望是这种信息在本次请求内，一次赋值，到处使用。本文就来探讨一下，如何在.NET Core 下去利用AsyncLocal 实现全局共享变量。

简介

我们如果需要整个程序共享一个变量，我们仅需将该变量放在某个静态类的静态变量上即可（不满足我们的需求，静态变量上，整个程序都是固定值）。我们在Web 应用程序中，每个Web 请求服务器都为其分配了一个独立线程，如何实现用户，租户等信息隔离在这些独立线程中。这就是今天要说的线程本地存储。针对线程本地存储 .NET 给我们提供了两个类 ThreadLocal 和 AsyncLocal。我们可以通过查看以下例子清晰的看到两者的区别：

[TestClass]
public class TastLocal {
    private static ThreadLocal<string> threadLocal = new ThreadLocal<string>();
    private static AsyncLocal<string> asyncLocal = new AsyncLocal<string>();
    [TestMethod]
    public void Test() {
        threadLocal.Value = "threadLocal";
        asyncLocal.Value = "asyncLocal";
        var threadId = Thread.CurrentThread.ManagedThreadId;
        Task.Factory.StartNew(() => {
            var threadId = Thread.CurrentThread.ManagedThreadId;
            Debug.WriteLine($"StartNew:threadId:{ threadId}; threadLocal:{threadLocal.Value}");
            Debug.WriteLine($"StartNew:threadId:{ threadId}; asyncLocal:{asyncLocal.Value}");
        });
        CurrThread();
    }
    public void CurrThread() {
        var threadId = Thread.CurrentThread.ManagedThreadId;
        Debug.WriteLine($"CurrThread:threadId:{threadId};threadLocal:{threadLocal.Value}");
        Debug.WriteLine($"CurrThread:threadId:{threadId};asyncLocal:{asyncLocal.Value}");
    }
}

输出结果：

CurrThread:threadId:4;threadLocal:threadLocal
StartNew:threadId:11; threadLocal:
CurrThread:threadId:4;asyncLocal:asyncLocal
StartNew:threadId:11; asyncLocal:asyncLocal

从上面结果中可以看出 ThreadLocal 和 AsyncLocal 都能实现基于线程的本地存储。但是当线程切换后，只有 AsyncLocal 还能够保留原来的值。在Web 开发中，我们会有很多异步场景，在这些场景下，可能会出现线程的切换。所以我们使用AsyncLocal 去实现在Web 应用程序下的共享变量。

AsyncLocal 解读

1. 官方文档
2. 源码地址

源码查看：

public sealed class AsyncLocal<T> : IAsyncLocal
{
    private readonly Action<AsyncLocalValueChangedArgs<T>>? m_valueChangedHandler;

    //
    // 无参构造函数
    //
    public AsyncLocal()
    {
    }

    //
    // 构造一个带有委托的AsyncLocal<T>，该委托在当前值更改时被调用
    // 在任何线程上
    //
    public AsyncLocal(Action<AsyncLocalValueChangedArgs<T>>? valueChangedHandler)
    {
        m_valueChangedHandler = valueChangedHandler;
    }

    [MaybeNull]
    public T Value
    {
        get
        {
            object? obj = ExecutionContext.GetLocalValue(this);
            return (obj == null) ? default : (T)obj;
        }
        set => ExecutionContext.SetLocalValue(this, value, m_valueChangedHandler != null);
    }

    void IAsyncLocal.OnValueChanged(object? previousValueObj, object? currentValueObj, bool contextChanged)
    {
        Debug.Assert(m_valueChangedHandler != null);
        T previousValue = previousValueObj == null ? default! : (T)previousValueObj;
        T currentValue = currentValueObj == null ? default! : (T)currentValueObj;
        m_valueChangedHandler(new AsyncLocalValueChangedArgs<T>(previousValue, currentValue, contextChanged));
    }
}

//
// 接口，允许ExecutionContext中的非泛型代码调用泛型AsyncLocal<T>类型
//
internal interface IAsyncLocal
{
    void OnValueChanged(object? previousValue, object? currentValue, bool contextChanged);
}

public readonly struct AsyncLocalValueChangedArgs<T>
{
    public T? PreviousValue { get; }
    public T? CurrentValue { get; }

    //
    // If the value changed because we changed to a different ExecutionContext, this is true.  If it changed
    // because someone set the Value property, this is false.
    //
    public bool ThreadContextChanged { get; }

    internal AsyncLocalValueChangedArgs(T? previousValue, T? currentValue, bool contextChanged)
    {
        PreviousValue = previousValue!;
        CurrentValue = currentValue!;
        ThreadContextChanged = contextChanged;
    }
}

//
// Interface used to store an IAsyncLocal => object mapping in ExecutionContext.
// Implementations are specialized based on the number of elements in the immutable
// map in order to minimize memory consumption and look-up times.
//
internal interface IAsyncLocalValueMap
{
    bool TryGetValue(IAsyncLocal key, out object? value);
    IAsyncLocalValueMap Set(IAsyncLocal key, object? value, bool treatNullValueAsNonexistent);
}

我们知道在.NET 里面，每个线程都关联着执行上下文。我们可以通 Thread.CurrentThread.ExecutionContext 属性进行访问 或者通过 ExecutionContext.Capture() 获取。

从上面我们可以看出 AsyncLocal 的 Value 存取是通过 ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 进行操作的，我们可以继续从 ExecutionContext 里面取出部分代码查看（源码地址），为了更深入地理解 AsyncLocal 我们可以查看一下源码，看看内部实现原理。

internal static readonly ExecutionContext Default = new ExecutionContext();
private static volatile ExecutionContext? s_defaultFlowSuppressed;

private readonly IAsyncLocalValueMap? m_localValues;
private readonly IAsyncLocal[]? m_localChangeNotifications;
private readonly bool m_isFlowSuppressed;
private readonly bool m_isDefault;

private ExecutionContext()
{
    m_isDefault = true;
}

private ExecutionContext(
    IAsyncLocalValueMap localValues,
    IAsyncLocal[]? localChangeNotifications,
    bool isFlowSuppressed)
{
    m_localValues = localValues;
    m_localChangeNotifications = localChangeNotifications;
    m_isFlowSuppressed = isFlowSuppressed;
}

public void GetObjectData(SerializationInfo info, StreamingContext context)
{
    throw new PlatformNotSupportedException();
}

public static ExecutionContext? Capture()
{
    ExecutionContext? executionContext = Thread.CurrentThread._executionContext;
    if (executionContext == null)
    {
        executionContext = Default;
    }
    else if (executionContext.m_isFlowSuppressed)
    {
        executionContext = null;
    }

    return executionContext;
}

internal static object? GetLocalValue(IAsyncLocal local)
{
ExecutionContext? current = Thread.CurrentThread._executionContext;
if (current == null)
{
    return null;
}

Debug.Assert(!current.IsDefault);
Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");
current.m_localValues.TryGetValue(local, out object? value);
return value;
}

internal static void SetLocalValue(IAsyncLocal local, object? newValue, bool needChangeNotifications)
{
ExecutionContext? current = Thread.CurrentThread._executionContext;

object? previousValue = null;
bool hadPreviousValue = false;
if (current != null)
{
    Debug.Assert(!current.IsDefault);
    Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");

    hadPreviousValue = current.m_localValues.TryGetValue(local, out previousValue);
}

if (previousValue == newValue)
{
    return;
}

// Regarding 'treatNullValueAsNonexistent: !needChangeNotifications' below:
// - When change notifications are not necessary for this IAsyncLocal, there is no observable difference between
//   storing a null value and removing the IAsyncLocal from 'm_localValues'
// - When change notifications are necessary for this IAsyncLocal, the IAsyncLocal's absence in 'm_localValues'
//   indicates that this is the first value change for the IAsyncLocal and it needs to be registered for change
//   notifications. So in this case, a null value must be stored in 'm_localValues' to indicate that the IAsyncLocal
//   is already registered for change notifications.
IAsyncLocal[]? newChangeNotifications = null;
IAsyncLocalValueMap newValues;
bool isFlowSuppressed = false;
if (current != null)
{
    Debug.Assert(!current.IsDefault);
    Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context");

    isFlowSuppressed = current.m_isFlowSuppressed;
    newValues = current.m_localValues.Set(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications);
    newChangeNotifications = current.m_localChangeNotifications;
}
else
{
    // First AsyncLocal
    newValues = AsyncLocalValueMap.Create(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications);
}

//
// Either copy the change notification array, or create a new one, depending on whether we need to add a new item.
//
if (needChangeNotifications)
{
    if (hadPreviousValue)
    {
        Debug.Assert(newChangeNotifications != null);
        Debug.Assert(Array.IndexOf(newChangeNotifications, local) >= 0);
    }
    else if (newChangeNotifications == null)
    {
        newChangeNotifications = new IAsyncLocal[1] { local };
    }
    else
    {
        int newNotificationIndex = newChangeNotifications.Length;
        Array.Resize(ref newChangeNotifications, newNotificationIndex + 1);
        newChangeNotifications[newNotificationIndex] = local;
    }
}

Thread.CurrentThread._executionContext =
    (!isFlowSuppressed && AsyncLocalValueMap.IsEmpty(newValues)) ?
    null : // No values, return to Default context
    new ExecutionContext(newValues, newChangeNotifications, isFlowSuppressed);

if (needChangeNotifications)
{
    local.OnValueChanged(previousValue, newValue, contextChanged: false);
}
}

从上面可以看出，ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 都是通过对 m_localValues 字段进行操作的。

m_localValues 的类型是 IAsyncLocalValueMap ,IAsyncLocalValueMap 的实现 和 AsyncLocal.cs 在一起，感兴趣的可以进一步查看 IAsyncLocalValueMap 是如何创建，如何查找的。

可以看到，里面最重要的就是ExecutionContext 的流动，线程发生变化时ExecutionContext 会在前一个线程中被默认捕获，流向下一个线程，它所保存的数据也就随之流动。在所有会发生线程切换的地方，基础类库(BCL) 都为我们封装好了对执行上下文的捕获 (如开始的例子，可以看到 AsyncLocal 的数据不会随着线程的切换而丢失)，这也是为什么 AsyncLocal 能实现 线程切换后，还能正常获取数据，不丢失。

总结

1. AsyncLocal 本身不保存数据，数据保存在 ExecutionContext 实例。

2. ExecutionContext 的实例会随着线程切换流向下一线程（也可以禁止流动和恢复流动），保证了线程切换时，数据能正常访问。

在.NET Core 中的使用示例

1. 先创建一个上下文对象

点击查看代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace NetAsyncLocalExamples.Context
{
    /// <summary>
    /// 请求上下文  租户ID
    /// </summary>
    public class RequestContext
    {
        /// <summary>
        /// 获取请求上下文
        /// </summary>
        public static RequestContext Current => _asyncLocal.Value;
        private readonly static AsyncLocal<RequestContext> _asyncLocal = new AsyncLocal<RequestContext>();

        /// <summary>
        /// 将请求上下文设置到线程全局区域
        /// </summary>
        /// <param name="userContext"></param>
        public static IDisposable SetContext(RequestContext userContext)
        {
            _asyncLocal.Value = userContext;
            return new RequestContextDisposable();
        }

        /// <summary>
        /// 清除上下文
        /// </summary>
        public static void ClearContext()
        {
            _asyncLocal.Value = null;
        }

        /// <summary>
        /// 租户ID
        /// </summary>
        public string TenantId { get; set; }

    }
}

namespace NetAsyncLocalExamples.Context
{
    /// <summary>
    /// 用于释放对象
    /// </summary>
    internal class RequestContextDisposable : IDisposable
    {
        internal RequestContextDisposable() { }
        public void Dispose()
        {
            RequestContext.ClearContext();
        }
    }
}

1. 创建请求上下文中间件

点击查看代码

using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using NetAsyncLocalExamples.Context;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace NetAsyncLocalExamples.Middlewares
{
    /// <summary>
    /// 请求上下文
    /// </summary>
    public class RequestContextMiddleware : IMiddleware
    {

        protected readonly IServiceProvider ServiceProvider;
        private readonly ILogger<RequestContextMiddleware> Logger;
        public RequestContextMiddleware(IServiceProvider serviceProvider, ILogger<RequestContextMiddleware> logger)
        {

            ServiceProvider = serviceProvider;
            Logger = logger;
        }
        public virtual async Task InvokeAsync(HttpContext context, RequestDelegate next)
        {
            var requestContext = new RequestContext();
            using (RequestContext.SetContext(requestContext))
            {
                requestContext.TenantId = $"租户ID:{DateTime.Now.ToString("yyyyMMddHHmmsss")}";
                await next(context);
            }
        }

    }
}

1. 注册中间件

点击查看代码

public void ConfigureServices(IServiceCollection services)
{
	services.AddTransient<RequestContextMiddleware>();
	services.AddRazorPages();
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
        // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthorization();

    //增加上下文
    app.UseMiddleware<RequestContextMiddleware>();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapRazorPages();
    });
}

1. 一次赋值，到处使用

点击查看代码

namespace NetAsyncLocalExamples.Pages
{
    public class IndexModel : PageModel
    {
        private readonly ILogger<IndexModel> _logger;

        public IndexModel(ILogger<IndexModel> logger)
        {
            _logger = logger;
            _logger.LogInformation($"测试获取全局变量1：{RequestContext.Current.TenantId}");
        }

        public void OnGet()
        {
            _logger.LogInformation($"测试获取全局变量2：{RequestContext.Current.TenantId}");
        }
    }
}