private AsyncPolicyWrap DefineAndRetrieveResiliencyStrategy()

in Hands-on lab/lab-files/src/TollBooth/TollBooth/FindLicensePlateText.cs [186:235]
38 lines of code
3 McCabe index (conditional complexity)

        private AsyncPolicyWrap<HttpResponseMessage> DefineAndRetrieveResiliencyStrategy()
        {
            // Retry when these status codes are encountered.
            HttpStatusCode[] httpStatusCodesWorthRetrying = {
               HttpStatusCode.InternalServerError, // 500
               HttpStatusCode.BadGateway, // 502
               HttpStatusCode.GatewayTimeout // 504
            };

            // Immediately fail (fail fast) when these status codes are encountered.
            HttpStatusCode[] httpStatusCodesToImmediatelyFail = {
               HttpStatusCode.BadRequest, // 400
               HttpStatusCode.Unauthorized, // 401
               HttpStatusCode.Forbidden // 403
            };

            // Define our waitAndRetry policy: retry n times with an exponential backoff in case the Computer Vision API throttles us for too many requests.
            var waitAndRetryPolicy = Policy
                .Handle<HttpRequestException>()
                .OrResult<HttpResponseMessage>(e => e.StatusCode == HttpStatusCode.ServiceUnavailable ||
                    e.StatusCode == (System.Net.HttpStatusCode)429 || e.StatusCode == (System.Net.HttpStatusCode)403)
                .WaitAndRetryAsync(10, // Retry 10 times with a delay between retries before ultimately giving up
                    attempt => TimeSpan.FromSeconds(0.25 * Math.Pow(2, attempt)), // Back off!  2, 4, 8, 16 etc times 1/4-second
                                                                                  //attempt => TimeSpan.FromSeconds(6), // Wait 6 seconds between retries
                    (exception, calculatedWaitDuration) =>
                    {
                        _log.LogWarning($"Computer Vision API server is throttling our requests. Automatically delaying for {calculatedWaitDuration.TotalMilliseconds}ms");
                    }
                );

            // Define our first CircuitBreaker policy: Break if the action fails 4 times in a row.
            // This is designed to handle Exceptions from the Computer Vision API, as well as
            // a number of recoverable status messages, such as 500, 502, and 504.
            var circuitBreakerPolicyForRecoverable = Policy
                .Handle<HttpResponseException>()
                .OrResult<HttpResponseMessage>(r => httpStatusCodesWorthRetrying.Contains(r.StatusCode))
                .CircuitBreakerAsync(
                    handledEventsAllowedBeforeBreaking: 3,
                    durationOfBreak: TimeSpan.FromSeconds(3),
                    onBreak: (outcome, breakDelay) =>
                    {
                        _log.LogWarning($"Polly Circuit Breaker logging: Breaking the circuit for {breakDelay.TotalMilliseconds}ms due to: {outcome.Exception?.Message ?? outcome.Result.StatusCode.ToString()}");
                    },
                    onReset: () => _log.LogInformation("Polly Circuit Breaker logging: Call ok... closed the circuit again"),
                    onHalfOpen: () => _log.LogInformation("Polly Circuit Breaker logging: Half-open: Next call is a trial")
                );

            // Combine the waitAndRetryPolicy and circuit breaker policy into a PolicyWrap. This defines our resiliency strategy.
            return Policy.WrapAsync(waitAndRetryPolicy, circuitBreakerPolicyForRecoverable);
        }