Polly断路器后退不工作

Question

我有以下政策：

var retryPolicy = Policy.Handle<Exception>(e => (e is HttpRequestException || e.InnerException is HttpRequestException)).WaitAndRetry(
                retryCount: maxRetryCount,
                sleepDurationProvider: attempt => TimeSpan.FromSeconds(Math.Pow(2, attempt)),
                onRetry: (exception, calculatedWaitDuration, retryCount, context) =>
                {
                    Log.Error($"Retry => Count: {retryCount}, Wait duration: {calculatedWaitDuration}, Policy Wrap: {context.PolicyWrapKey}, Policy: {context.PolicyKey}, Endpoint: {context.OperationKey}, Exception: {exception}.");
                });

var circuitBreaker = Policy.Handle<Exception>(e => (e is HttpRequestException || e.InnerException is HttpRequestException)).CircuitBreaker(maxExceptionsBeforeBreaking, TimeSpan.FromSeconds(circuitBreakDurationSeconds), onBreak, onReset);

var sharedBulkhead = Policy.Bulkhead(maxParallelizations, maxQueuingActions, onBulkheadRejected);

var fallbackForCircuitBreaker = Policy<bool>
             .Handle<BrokenCircuitException>()
             .Fallback(
                 fallbackValue: false,
                 onFallback: (b, context) =>
                 {
                     Log.Error($"Operation attempted on broken circuit => Policy Wrap: {context.PolicyWrapKey}, Policy: {context.PolicyKey}, Endpoint: {context.OperationKey}");
                 }
             );

            var fallbackForAnyException = Policy<bool>
                .Handle<Exception>()
                .Fallback(
                    fallbackAction: (context) => { return false; },
                    onFallback: (e, context) =>
                    {
                        Log.Error($"An unexpected error occured => Policy Wrap: {context.PolicyWrapKey}, Policy: {context.PolicyKey}, Endpoint: {context.OperationKey}");
                    }
                );

var resilienceStrategy = Policy.Wrap(retryPolicy, circuitBreaker, sharedBulkhead);
            var policyWrap = fallbackForAnyException.Wrap(fallbackForCircuitBreaker.Wrap(resilienceStrategy));

public bool CallApi(ChangeMapModel changeMessage)
    {
        var httpClient = new HttpClient();
        var endPoint = changeMessage.EndPoint;
        var headers = endPoint.Headers;
        if (headers != null)
        {
            foreach (var header in headers)
            {
                if (header.Contains(':'))
                {
                    var splitHeader = header.Split(':');
                    httpClient.DefaultRequestHeaders.Add(splitHeader[0], splitHeader[1]); 
                }
            } 
        }

        var res = httpClient.PostAsync(endPoint.Uri, null);
        var response = res.Result;
        response.EnsureSuccessStatusCode();
        return true;
    }

我是这样执行政策的：

policyWrap.Execute((context) => CallApi(changeMessage), new Context(endPoint));

问题是，当一个动作在开路上执行时，我在断路器回调中没有得到命中。

我希望通过策略放置一个API调用，要处理的异常类型为HttpRequestException。政策定义有问题吗？为什么不叫断路器后备？

Answer 1

我创建了以下最小、完整、可验证的示例来帮助探索问题：

注意：不一定是成品；只是发布代码的一些次要mods和额外的注释，以帮助探索问题。

using Polly;
using Polly.CircuitBreaker;
using System;
using System.Net.Http;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        int maxRetryCount = 6;
        double circuitBreakDurationSeconds = 0.2 /* experiment with effect of shorter or longer here, eg: change to = 1, and the fallbackForCircuitBreaker is correctly invoked */ ;
        int maxExceptionsBeforeBreaking = 4; /* experiment with effect of fewer here, eg change to = 1, and the fallbackForCircuitBreaker is correctly invoked */
        int maxParallelizations = 2;
        int maxQueuingActions = 2;

        var retryPolicy = Policy.Handle<Exception>(e => (e is HttpRequestException || (/*!(e is BrokenCircuitException) &&*/ e.InnerException is HttpRequestException))) // experiment with introducing the extra (!(e is BrokenCircuitException) && ) clause here, if necessary/desired, depending on goal
            .WaitAndRetry(
                retryCount: maxRetryCount,
                sleepDurationProvider: attempt => TimeSpan.FromMilliseconds(50 * Math.Pow(2, attempt)),
                onRetry: (ex, calculatedWaitDuration, retryCount, context) =>
                {
                    Console.WriteLine(String.Format("Retry => Count: {0}, Wait duration: {1}, Policy Wrap: {2}, Policy: {3}, Endpoint: {4}, Exception: {5}", retryCount, calculatedWaitDuration, context.PolicyWrapKey, context.PolicyKey, context.OperationKey, ex.Message));
                });

        var circuitBreaker = Policy.Handle<Exception>(e => (e is HttpRequestException || e.InnerException is HttpRequestException))
            .CircuitBreaker(maxExceptionsBeforeBreaking,
                TimeSpan.FromSeconds(circuitBreakDurationSeconds),
                onBreak: (ex, breakDuration) => {
                    Console.WriteLine(String.Format("Circuit breaking for {0} ms due to {1}", breakDuration.TotalMilliseconds, ex.Message));
                },
                onReset: () => {
                    Console.WriteLine("Circuit closed again.");
                },
                onHalfOpen: () => { Console.WriteLine("Half open."); });

        var sharedBulkhead = Policy.Bulkhead(maxParallelizations, maxQueuingActions);

        var fallbackForCircuitBreaker = Policy<bool>
             .Handle<BrokenCircuitException>()
            /* .OrInner<BrokenCircuitException>() */ // Consider this if necessary.
            /* .Or<Exception>(e => circuitBreaker.State != CircuitState.Closed) */ // This check will also detect the circuit in anything but healthy state, regardless of the final exception thrown.
             .Fallback(
                 fallbackValue: false,
                 onFallback: (b, context) =>
                 {
                     Console.WriteLine(String.Format("Operation attempted on broken circuit => Policy Wrap: {0}, Policy: {1}, Endpoint: {2}", context.PolicyWrapKey, context.PolicyKey, context.OperationKey));
                 }
             );

        var fallbackForAnyException = Policy<bool>
                .Handle<Exception>()
                .Fallback<bool>(
                    fallbackAction: (context) => { return false; },
                    onFallback: (e, context) =>
                    {
                        Console.WriteLine(String.Format("An unexpected error occured => Policy Wrap: {0}, Policy: {1}, Endpoint: {2}, Exception: {3}", context.PolicyWrapKey, context.PolicyKey, context.OperationKey, e.Exception.Message));
                    }
                );

        var resilienceStrategy = Policy.Wrap(retryPolicy, circuitBreaker, sharedBulkhead);
        var policyWrap = fallbackForAnyException.Wrap(fallbackForCircuitBreaker.Wrap(resilienceStrategy));

        bool outcome = policyWrap.Execute((context) => CallApi("http://www.doesnotexistattimeofwriting.com/"), new Context("some endpoint info"));
    }

    public static bool CallApi(string uri)
    {
        using (var httpClient = new HttpClient() { Timeout = TimeSpan.FromSeconds(1) }) // Consider HttpClient lifetimes and disposal; this pattern is for minimum change from original posted code, not a recommendation.
        {
            Task<HttpResponseMessage> res = httpClient.GetAsync(uri);
            var response = res.Result; // Consider async/await rather than blocking on the returned Task.
            response.EnsureSuccessStatusCode();
            return true;
        }
    }
}

不止一个因素可能会导致Fallbackforc断路器无法调用：

1. 该电路断开持续时间秒可以设置为短于各种尝试和重试之间的等待所花费的总时间。

如果是这样，电路可能会恢复到半开路状态。在半断开状态或闭合状态下，导致电路断开的异常会按原样重新启动BrokenCircuiteException仅在（完全）断路阻止尝试调用时才会引发。

因此，如果您的电路在重试耗尽时间时恢复到半开状态，则返回到包装回退策略的异常将是Http刚需异常，而不是BrokenCircuitExc0019。

CircuitBreakerException包含导致电路中断的异常，作为其InnerException。因此，对e.InnerException is HttpRequestException进行过于贪婪/松散的检查也可能捕获CircuitBreakerException具有InnerException is HttpRequestException。根据你的目标，这可能是你想要的，也可能不是。

我相信这不会发生在最初发布的代码中，因为它是以特定的方式构造的。阻止HttpClient返回的任务。DoSomethingAsync（…） 已导致聚合异常-

CircuitBreakerException -> AggregateException -> HttpRequestException

所以这不属于发布代码中的一个深度检查。但是，请注意，CircuitBreakerException包含导致电路中断的异常作为其内部异常。这可能会导致handle子句只检查e。InnerException是HttpRequestException到 不自觉地 （如果不是您的目标，则意外地）重试 断路器异常，如果：

（a）代码更改为async/wait，这将删除Aggregate Exc0019，因此导致嵌套只有一个深度

（b） 代码更改为Polly的。汉德莱纳

建议/*注释掉*///并在上面的代码中附加解释，建议如何调整发布的代码，以便Fallbackforce断路器按预期调用。

还有两个想法：

在HttpClient上阻塞。DoSomethingAsync（）调用。结果可能会影响性能，如果与其他异步代码混合，则可能会导致死锁，并带来整个聚合异常——以及内部异常痛苦。

（故意将第3点和第4点保持简短，正如其他地方广泛讨论的那样。）