如何在DelegatingHandler中访问Durable Entity并解决DI注入问题？

在Durable Functions编排任务中，需要对外部API请求实现全局限流，尝试通过Durable Entity跟踪特定时间范围内的剩余调用次数，但在AuthenticationHandler（自定义DelegatingHandler）中注入DurableTaskClient时应用崩溃，无法正确访问Durable Entity。

提供的代码 #

AuthenticationHandler 代码

public class AuthenticationHandler : DelegatingHandler
{
    private readonly IKeyVaultService _keyVaultService;
    private readonly ILogger<AuthenticationHandler> _logger;
    private readonly DurableTaskClient _durableClient;

    public AuthenticationHandler(IKeyVaultService keyVaultService, ILogger<AuthenticationHandler> logger, DurableTaskClient durableClient)
    {
        _keyVaultService = keyVaultService ?? throw new ArgumentNullException(nameof(keyVaultService));
        _logger = logger;
        _durableClient = durableClient;
    }
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {       
        string token = await _keyVaultService.GetAccessTokenAsync();
        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);

        var entityId = new EntityInstanceId(nameof(RateLimiterEntity), "RateLimiter");
        EntityMetadata<RateLimiterEntity>? entity = await _durableClient.Entities.GetEntityAsync<RateLimiterEntity>(entityId, cancellation: cancellationToken);

        _logger.LogInformation("Remaining requests: {rem}", entity.State.RemainingRequests.ToString());

        if (entity != null && entity.State.RemainingRequests <= 25)
        {
            var delay = entity.State.ResetTime - DateTime.UtcNow;
            if (delay > TimeSpan.Zero)
            {
                _logger.LogWarning("Rate limit reached. Delaying for {Delay} seconds", delay.TotalSeconds);
                await Task.Delay(delay, cancellationToken);
            }
            await _durableClient.Entities.SignalEntityAsync(entityId, nameof(RateLimiterEntity.Reset), cancellationToken);
        }
        await _durableClient.Entities.SignalEntityAsync(entityId, nameof(RateLimiterEntity.Decrement), cancellationToken);
    
        var response = await base.SendAsync(request, cancellationToken);
        return response;
    }
}

RateLimiterEntity 代码（.NET 8孤立模式）

[DurableTask(nameof(RateLimiterEntity))]
public class RateLimiterEntity
{
    public int RemainingRequests { get; set; } = 100;
    public DateTime ResetTime { get; set; } = DateTime.UtcNow.AddSeconds(60);

    public void Reset()
    {
        RemainingRequests = 100;
        ResetTime = DateTime.UtcNow.AddSeconds(60);
    }

    public void Decrement()
    {
        RemainingRequests--;
    }

    [Function(nameof(RateLimiterEntity))]
    public static Task Run([EntityTrigger] TaskEntityDispatcher dispatcher)
        => dispatcher.DispatchAsync<RateLimiterEntity>();
}

核心问题：如何正确注册DurableTaskClient并在DelegatingHandler中访问该实体？

1. 正确注册服务（Program.cs） #

在.NET 8孤立模式下，必须显式注册DurableTaskClient和自定义DelegatingHandler，同时配置HttpClient关联该Handler。修改Program.cs的服务配置逻辑：

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        // 注册Durable Task客户端，绑定Azure存储配置
        services.AddDurableTaskClient(clientBuilder =>
        {
            clientBuilder.UseAzureStorage(Environment.GetEnvironmentVariable("AzureWebJobsStorage"));
        });

        // 注册自定义AuthenticationHandler
        services.AddTransient<AuthenticationHandler>();

        // 配置HttpClient并添加限流Handler
        services.AddHttpClient("ExternalApiClient")
            .AddHttpMessageHandler<AuthenticationHandler>();

        // 注册KeyVault服务（根据你的实现调整）
        services.AddScoped<IKeyVaultService, KeyVaultService>();
    })
    .Build();

host.Run();

2. 修正Handler中的实体访问逻辑 #

GetEntityAsync可能返回null（实体未初始化），且直接读取状态后修改存在并发风险，需调整逻辑确保实体状态正确初始化，所有状态变更通过信号（Signal）发送：

protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{       
    string token = await _keyVaultService.GetAccessTokenAsync();
    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);

    var entityId = new EntityInstanceId(nameof(RateLimiterEntity), "RateLimiter");

    // 初始化实体（如果不存在）
    var entity = await _durableClient.Entities.GetEntityAsync<RateLimiterEntity>(entityId, cancellation: cancellationToken);
    if (entity == null)
    {
        await _durableClient.Entities.SignalEntityAsync(entityId, nameof(RateLimiterEntity.Reset), cancellationToken);
        entity = await _durableClient.Entities.GetEntityAsync<RateLimiterEntity>(entityId, cancellation: cancellationToken);
    }

    _logger.LogInformation("Remaining requests: {rem}", entity.State.RemainingRequests);

    // 处理限流逻辑
    if (entity.State.RemainingRequests <= 25)
    {
        var delay = entity.State.ResetTime - DateTime.UtcNow;
        if (delay > TimeSpan.Zero)
        {
            _logger.LogWarning("Rate limit reached. Delaying for {Delay} seconds", delay.TotalSeconds);
            await Task.Delay(delay, cancellationToken);
            // 延迟后重置计数
            await _durableClient.Entities.SignalEntityAsync(entityId, nameof(RateLimiterEntity.Reset), cancellationToken);
        }
    }

    // 递减请求计数
    await _durableClient.Entities.SignalEntityAsync(entityId, nameof(RateLimiterEntity.Decrement), cancellationToken);
    
    var response = await base.SendAsync(request, cancellationToken);
    return response;
}

3. 关键注意事项 #

• 并发安全：Durable Entity是单线程处理命令的，所有状态修改必须通过SignalEntityAsync发送指令，禁止直接修改本地读取的状态，避免并发更新导致的数据不一致。
• 实体初始化：首次访问实体时可能未创建，需先发送Reset信号完成初始化，否则访问entity.State会引发异常。
• 依赖注入验证：确保DurableTaskClient、IKeyVaultService等服务都已正确注册，避免注入失败导致应用崩溃。

内容的提问来源于stack exchange，提问作者Dorian-B