.NET Polly 重试、熔断、降级
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<h2>一、安装依赖</h2>
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<pre><code># 基础包
Install-Package Polly</code></pre>
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<pre><code># HTTP 集成（推荐） Install-Package Microsoft.Extensions.Http.Polly </code></pre>
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<h2>二、策略与代码</h2>
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<h3>1. 重试（Retry）：自愈瞬时故障</h3>
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<div>作用：网络抖动、偶发超时、5xx 等临时错误自动重试，避免单次失败影响业务。
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常用模式：固定次数、指数退避（重试间隔越来越长）、带抖动防风暴（前两者基础上加随机时间）。</div>
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<h4>示例：HTTP 请求指数退避重试（3 次）</h4>
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<pre><code>using Polly;
using Polly.Retry;
using System.Net.Http;
// 1. 定义重试策略（异步）
var retryPolicy = Policy<HttpResponseMessage>
.Handle<HttpRequestException>() // 捕获网络异常
.OrResult(r => !r.IsSuccessStatusCode)// 捕获非成功状态码
.WaitAndRetryAsync(
retryCount: 3, // 最多重试3次
sleepDurationProvider: attempt => // 指数退避：1s→2s→4s + 随机抖动
TimeSpan.FromSeconds(Math.Pow(2, attempt))
+ TimeSpan.FromMilliseconds(new Random().Next(100, 300)),
onRetryAsync: (ex, span, attempt, _) =>
{
Console.WriteLine($"第{attempt}次重试，等待{span.TotalSeconds:F1}s，异常：{ex?.Message}");
return Task.CompletedTask;
}
);
// 2. 执行受保护的调用
var httpClient = new HttpClient();
var response = await retryPolicy.ExecuteAsync(async () =>
{
var res = await httpClient.GetAsync("https://api.example.com/data");
res.EnsureSuccessStatusCode();
return res;
});
</code></pre>
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<h3>2. 熔断（Circuit Breaker）：防雪崩</h3>
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<div>作用：连续失败达阈值后，主动切断请求（快速失败），熔断期内不发起真实调用；到期进入半开试探，成功则恢复。
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状态：Closed（正常）→ Open（熔断）→ Half-Open（试探）→ Closed/Open。</div>
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<h4>示例：连续 5 次失败熔断 10 秒</h4>
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<pre><code>using Polly.CircuitBreaker;
// 定义熔断策略
var circuitPolicy = Policy<HttpResponseMessage>
.Handle<HttpRequestException>()
.OrResult(r => r.StatusCode == HttpStatusCode.InternalServerError)
.CircuitBreakerAsync(
exceptionsAllowedBeforeBreaking: 5,// 连续5次失败触发熔断
durationOfBreak: TimeSpan.FromSeconds(10), // 熔断10秒
onBreak: (ex, span) => Console.WriteLine($"【熔断】断开{span.TotalSeconds}s，异常：{ex.Message}"),
onReset: () => Console.WriteLine("【熔断】恢复正常"),
onHalfOpen: () => Console.WriteLine("【熔断】半开，试探请求1次，成功则恢复正常，失败则继续熔断10秒")
);
// 执行
try
{
var response = await circuitPolicy.ExecuteAsync(async () =>
{
var res = await httpClient.GetAsync("https://api.example.com/data");
res.EnsureSuccessStatusCode();
return res;
});
}
catch (BrokenCircuitException)
{
Console.WriteLine("熔断器已打开，拒绝请求");
}
</code></pre>
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<h3>3. 降级（Fallback）：兜底响应</h3>
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<div>作用：所有重试 / 熔断都失败后，返回预设降级结果（默认值、缓存、简化数据），保证流程不中断。</div>
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<h4>示例：HTTP 失败返回空数据降级</h4>
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<pre><code>using Polly.Fallback;
// 定义降级策略
var fallbackPolicy = Policy<HttpResponseMessage>
.Handle<Exception>()
.OrResult(r => !r.IsSuccessStatusCode)
.FallbackAsync(
fallbackAction: _ =>
{
// 降级响应：与原接口同结构、同 Content-Type
var fallback = new HttpResponseMessage(HttpStatusCode.OK)
{
Content = new StringContent(
"{\"code\":0,\"data\":null,\"msg\":\"服务降级，稍后重试\"}",
Encoding.UTF8,
"application/json"
)
};
return Task.FromResult(fallback);
},
onFallbackAsync: (ex, _) =>
{
Console.WriteLine($"【降级】触发，异常：{ex?.Message}");
return Task.CompletedTask;
}
);
</code></pre>
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<h2>三、策略组合</h2>
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<h3>1. 组合顺序</h3>
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<div>外层：熔断 → 中层：重试 → 内层：降级
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熔断控制整体开关，重试处理瞬时故障，降级兜底所有失败。</div>
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<h4>示例：熔断 + 重试 + 降级组合（PolicyWrap）</h4>
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<pre><code>// 组合策略（Wrap）
var resilientPolicy = Policy.WrapAsync(
circuitPolicy, // 外层：熔断（先判断是否熔断）
retryPolicy, // 中层：重试（熔断关闭时才重试）
fallbackPolicy // 内层：降级（都失败则兜底）
);
// 最终调用
var finalResponse = await resilientPolicy.ExecuteAsync(async () =>
{
var res = await httpClient.GetAsync("https://api.example.com/data");
res.EnsureSuccessStatusCode();
return res;
});
</code></pre>
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<h3>2. ASP.NET Core + HttpClientFactory 集成</h3>
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<pre><code>// Program.cs
var builder = WebApplication.CreateBuilder();
// 1. 定义策略
static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
{
return HttpPolicyExtensions
.HandleTransientHttpError()
.WaitAndRetryAsync(3, attempt => TimeSpan.FromSeconds(Math.Pow(2, attempt)));
}
static IAsyncPolicy<HttpResponseMessage> GetCircuitPolicy()
{
return HttpPolicyExtensions
.HandleTransientHttpError()
.CircuitBreakerAsync(5, TimeSpan.FromSeconds(10));
}
static IAsyncPolicy<HttpResponseMessage> GetFallbackPolicy()
{
return Policy<HttpResponseMessage>
.Handle<Exception>()
.OrResult(r => !r.IsSuccessStatusCode)
.FallbackAsync(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
{
Content = new StringContent("{\"data\":null}", Encoding.UTF8, "application/json")
}));
}
// 2. 注册带策略的 HttpClient
builder.Services.AddHttpClient("ResilientClient")
.AddPolicyHandler(GetCircuitPolicy())
.AddPolicyHandler(GetRetryPolicy())
.AddPolicyHandler(GetFallbackPolicy());
var app = builder.Build();
app.Run();
</code></pre>
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<h2>四、实践</h2>
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<ol>
<li>重试：只用于瞬时、可自愈故障（网络、5xx）；不用于业务错误（400/404）；加抖动防重试风暴。</li>
<li>熔断：全局单例 Policy，避免多实例导致计数失效；阈值按业务调整（敏感易抖动，宽松易雪崩）。</li>
<li>降级：降级响应与原接口结构 / Content-Type 一致，避免上游解析异常；记录降级日志用于监控。</li>
<li>组合：熔断在外、重试在内，防止熔断打开后仍做无效重试。</li>
<li>监控：监听 <code>onBreak/onReset/onFallback</code>，接入 Prometheus/Grafana 告警。</li>
</ol><br><br>
来源：https://www.cnblogs.com/chuansheng/p/19916139
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