Polly CircuitBreaker后备不起作用

Question

I have the following policies:

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;
    }

I execute the policy like so:

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

The problem is that I am not getting a hit in the CircuitBreaker callback when an action is executed on open circuit.

I want an API call to be placed through the policy with the exception type to be handled being HttpRequestException . Is there something wrong with the policy definitions? Why isn't the circuit breaker fallback called?

Answer 1

I created the following minimum, complete, verifiable example to help explore the problem:

Note: not necessarily a finished product; just some minor mods to the posted code and extra annotation, to help explore the question.

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;
        }
    }
}

---

More than one factor could be causing the fallbackForCircuitBreaker not to be invoked:

1. The circuitBreakDurationSeconds may be set shorter than the overall time taken by the various tries and waits between retries.

If so, the circuit may revert to half-open state. In half-open state or closed state , an exception which causes the circuit to break is rethrown as-is. BrokenCircuitException is only thrown when an call is prevented from being attempted by a (fully) open circuit.

Thus, if your circuit has reverted to half-open by the time retries exhaust, the exception thrown back on to the wrapping fallback policies will be an HttpRequestException , not BrokenCircuitException .

2. .Handle<Exception>(e => (e is HttpRequestException || e.InnerException is HttpRequestException)) clauses may catch a CircuitBreakerException s having InnerException is HttpRequestException

A CircuitBreakerException contains the exception which caused the circuit to break as its InnerException . So an overly greedy/looser check of e.InnerException is HttpRequestException could also catch a CircuitBreakerException having InnerException is HttpRequestException . This may or may not be desired depending on your goal.

I believe this not to be happening for the original posted code because of the particular way it is constructed. Blocking on the Task returned by HttpClient.DoSomethingAsync(...) causes already an AggregateException->HttpRequestException , meaning a resulting CircuitBreakerException nests the HttpRequestException two-deep:

CircuitBreakerException -> AggregateException -> HttpRequestException

So this doesn't fall in to the one -deep check in the posted code. However, be aware that a CircuitBreakerException contains the exception which caused the circuit to break as its InnerException . This could cause a handle clause checking only e.InnerException is HttpRequestException to unwantedly (unexpectedly, if it is not your goal) retry for a CircuitBreakerException , if either:

(a) the code is changed to async / await , which will remove the AggregateException and so cause the nesting to be only one-deep

(b) the code is changed to Polly's .HandleInner<HttpRequestException>() syntax , which is recursively-greedy, and so would catch a nested-two-deep CircuitBreakerException->AggregateException->HttpRequestException .

---

Suggestions /* commented out */ // with additional explanation in the above code suggest how the code-as-posted may be adjusted so that fallbackForCircuitBreaker invokes as expected.

---

Two other thoughts:

3. Consider changing to async / await throughout if possible.

Blocking on HttpClient.DoSomethingAsync() by calling .Result can impact performance, or risk deadlocks if mixed with other async code, and is bringing in the whole AggregateException -with- InnerException pain.

4. Consider disposal and lifetime of HttpClient instances.

(Kept these points 3 and 4 intentionally brief as widely discussed elsewhere.)