Polly Resilience 上下文为空

Question

我正在为我的 .NET Core Web Api 实现弹性，并且我需要一种使用

ResilienceContext

传递值的方法，但当我调试时，

args.Context.Properties

始终为空。我不知道我错过了什么。

我正在使用

Microsoft.Extensions.Http.Resilience

使用

Polly

这是我的代码：

public static class ResilienceKeys
{
    public static readonly ResiliencePropertyKey<string> LocationId = new("location-id");
}

// WordPressService.cs

public class WordPressService : IWordPressService
{
    private readonly HttpClient _httpClient;
    private readonly ResiliencePipelineProvider<string> _pipelineProvider;

    public WordPressService(HttpClient httpClient, ResiliencePipelineProvider<string> pipelineProvider)
    {
        _httpClient = httpClient;
        _httpClient.BaseAddress = new Uri("https://api.example.com/");

        _pipelineProvider = pipelineProvider;
    }

    public async Task<string> GetBannersByLocationAsync(int locationId, CancellationToken cancellationToken)
    {
        var pipeline = _pipelineProvider.GetPipeline<HttpResponseMessage>("my-pipeline");

        var context = ResilienceContextPool.Shared.Get(cancellationToken);

        context.Properties.Set(ResilienceKeys.LocationId, locationId.ToString());

        var response = await pipeline.ExecuteAsync<HttpResponseMessage, ResilienceContext>(
            async (context, cancellationToken) =>
            {
                // Simulate an error for testing purposes
                await Task.Delay(100, cancellationToken);
                var response = new HttpResponseMessage(HttpStatusCode.InternalServerError);
                return response;
            },
            context,
            cancellationToken);

        return await response.Content.ReadAsStringAsync(cancellationToken);
    }
}

我还使用依赖注入来添加 ResiliencePipeline，并使用

RetryStrategy

和

FallbackStrategy

从数据库获取内容，以防 WordPress 服务器没有响应

// Program.cs

builder.Services.AddResiliencePipeline<string, HttpResponseMessage>("my-pipeline", (pipelineBuilder, context) =>
{
    var predicateBuilder = new PredicateBuilder<HttpResponseMessage>()
        .Handle<HttpRequestException>()
        .HandleResult(r => r.StatusCode >= HttpStatusCode.InternalServerError);

    pipelineBuilder
    .AddFallback(new FallbackStrategyOptions<HttpResponseMessage>()
    {
        ShouldHandle = predicateBuilder,
        FallbackAction = async (args) =>
        {
            if (args.Context.Properties.TryGetValue(ResilienceKeys.LocationId, out var data))
            {
                Console.WriteLine("FallbackAction, Location Id: {0}", data);
            }

            var fallbackResponse = new HttpResponseMessage()
            {
                StatusCode = HttpStatusCode.OK,
                Content = new StringContent("Fallback content")
            };

            return Outcome.FromResult(fallbackResponse);
        }
    })
    .AddRetry(new RetryStrategyOptions<HttpResponseMessage>()
    {
        ShouldHandle = predicateBuilder,
        Delay = TimeSpan.FromSeconds(2),
        MaxRetryAttempts = 2,
        BackoffType = DelayBackoffType.Exponential,
        UseJitter = true,
    });
});

Answer 1

简短的回答是您使用了错误的

ExecuteAsync

重载。你应该使用这个：

var response = await pipeline.ExecuteAsync<HttpResponseMessage>(
    async ctx =>
    {
        // Simulate an error for testing purposes
        await Task.Delay(100, ctx.CancellationToken);
        return new HttpResponseMessage(HttpStatusCode.InternalServerError);
    },
    context);

---

超载，你所使用的，没有利用

Context

。如果你看到方法签名，那就很清楚了。

public async ValueTask<TResult> ExecuteAsync<TResult, TState>(
    Func<TState, CancellationToken, ValueTask<TResult>> callback,
    TState state,
    CancellationToken cancellationToken = default)

您已将

Context

作为

state

对象传递。 这里我们记录了

Context

和

State

之间有什么区别。

目前不存在预期会出现

CancellationToken

和

Context

的此类过载。 但你根本不需要。

每当您从

Context

请求

ResilienceContextPool

时，传递的

CancellationToken

就会附加到检索到的

Context

。如果您查看建议解决方案的代码片段，您可以发现

CancellationToken

是通过

Context

(

ctx.CancellationToken

) 访问的。

因此，您应该使用以下重载：

public async ValueTask<TResult> ExecuteAsync<TResult>(
    Func<ResilienceContext, ValueTask<TResult>> callback,
    ResilienceContext context)

---

作为旁注，请不要忘记归还借用的弹性上下文到池中：

ResilienceContextPool.Shared.Return(context);