在讨论熔断-限流-降级，我们再回顾下服务雪崩

一、服务雪崩

• 服务雪崩原因
  
• 服务雪崩应对策略

---

二、熔断-限流-降级概述

限流和熔断最终都会导致用户的体验降级

• 限流：流量2k，但是我的服务能力只有1k，所以这个时候多出来的流量怎么办？
  • a.拒绝；
  • b.排队等待；
• 用户体验
  • 用户体验不好：当前访问用户过多，请稍后重试
  • 用户体验降级：原本是访问流畅，下单流畅 -> 当前访问用户过多，请稍后重试
• 熔断：
  • 比如A服务访问B服务，这时候B服务很慢（B服务压力过大，导致了出现不少请求错误），调用方很容易出现一个问题：每次调用都超时
  • 结果这个时候数据库出现了问题，超时重试，导致网络2k的流量突然变成了3k，这让原本满负荷的B服务雪上加霜，B服务宕机
  • 如果这时候有一种熔断机制（比较恰当的比喻如保险丝）
    • a.发现了大部分请求很慢，50%请求都很慢
    • b.发现请求有50%都错误了
    • c.错误数量很多，比如1s出现了20个错误

---

三、熔断限流技术选型

• Hystrix官网：https://github.com/Netflix/Hystrix；Netflix开源的，已经不维护了
• Sentinel：https://github.com/alibaba/Sentinel；阿里开源的，一直在维护
• sentinel-golang：https://github.com/alibaba/sentinel-golang

---

四、sentinel限流

1 - 基于QPS限流

• 基于QPS限流：

  • Entry表示入口，base.Inbound表示入流量
  • StatIntervalInMs: 规则对应的流量控制器的独立统计结构的统计周期。如果StatIntervalInMs是1000，也就是统计QPS
  • 测试结果是之前10次都是检查通过，第11次和第12次限流

• ControlBehavior：表示表示流量控制器的控制行为，目前 Sentinel 支持两种控制行为

  • Reject：表示如果当前统计周期内，统计结构统计的请求数超过了阈值，就直接拒绝
  • Throttling：表示匀速排队的统计策略。它的中心思想是，以固定的间隔时间让请求通过

• Throttling匀速排队
  

• Reject策略

package main

import (
	"fmt"
	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/base"
	"github.com/alibaba/sentinel-golang/core/flow"
	"log"
)

func main() {
	//先初始化sentinel
	err := sentinel.InitDefault()
	if err != nil {
		log.Fatalf("初始化sentinel 异常: %v", err)
	}

	//配置限流规则
	_, err = flow.LoadRules([]*flow.Rule{
		{
			Resource:               "some-test",
			TokenCalculateStrategy: flow.Direct,
			ControlBehavior:        flow.Reject, //匀速通过
			Threshold:              10,
			StatIntervalInMs:       1000,
		},

		{
			Resource:               "some-test2",
			TokenCalculateStrategy: flow.Direct,
			ControlBehavior:        flow.Reject, //直接拒绝
			Threshold:              10,
			StatIntervalInMs:       1000,
		},
	})

	if err != nil {
		log.Fatalf("加载规则失败: %v", err)
	}

	for i := 0; i < 12; i++ {
		e, b := sentinel.Entry("some-test", sentinel.WithTrafficType(base.Inbound))
		if b != nil {
			fmt.Println("限流了")
		} else {
			fmt.Println("检查通过")
			e.Exit()
		}
	}
}

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2 - Throttling策略

• Throttling策略：修改 ControlBehavior: flow.Throttling, //匀速通过

package main

import (
	"fmt"
	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/base"
	"github.com/alibaba/sentinel-golang/core/flow"
	"log"
)

func main() {
	//先初始化sentinel
	err := sentinel.InitDefault()
	if err != nil {
		log.Fatalf("初始化sentinel 异常: %v", err)
	}

	//配置限流规则
	_, err = flow.LoadRules([]*flow.Rule{
		{
			Resource:               "some-test",
			TokenCalculateStrategy: flow.Direct,
			ControlBehavior:        flow.Throttling, //匀速通过
			Threshold:              10,
			StatIntervalInMs:       1000,
		},
	})

	if err != nil {
		log.Fatalf("加载规则失败: %v", err)
	}

	for i := 0; i < 12; i++ {
		e, b := sentinel.Entry("some-test", sentinel.WithTrafficType(base.Inbound))
		if b != nil {
			fmt.Println("限流了")
		} else {
			fmt.Println("检查通过")
			e.Exit()
		}
	}
}

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• 验证Throttling策略：我们在每个Entry后添加sleep 101ms，结果就是全部通过

package main

import (
	"fmt"
	"log"
	"time"

	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/base"
	"github.com/alibaba/sentinel-golang/core/flow"
)

func main() {
	//先初始化sentinel
	err := sentinel.InitDefault()
	if err != nil {
		log.Fatalf("初始化sentinel 异常: %v", err)
	}

	//配置限流规则
	_, err = flow.LoadRules([]*flow.Rule{
		{
			Resource:               "some-test",
			TokenCalculateStrategy: flow.Direct,
			ControlBehavior:        flow.Throttling, //匀速通过
			Threshold:              10,
			StatIntervalInMs:       1000,
		},
	})

	if err != nil {
		log.Fatalf("加载规则失败: %v", err)
	}

	for i := 0; i < 12; i++ {
		e, b := sentinel.Entry("some-test", sentinel.WithTrafficType(base.Inbound))
		if b != nil {
			fmt.Println("限流了")
		} else {
			fmt.Println("检查通过")
			e.Exit()
		}
		time.Sleep(101 * time.Millisecond)
	}
}

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3 - sentinel预热/冷启动

• WarmUp：即预热/冷启动方式。当系统长期处于低水位的情况下，当流量突然增加时，直接把系统拉升到高水位可能瞬间把系统压垮。通过"冷启动"，让通过的流量缓慢增加，在一定时间内逐渐增加到阈值上限，给冷系统一个预热的时间，避免冷系统被压垮

• 测试预热

package main

import (
	"fmt"
	"log"
	"math/rand"
	"time"

	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/base"
	"github.com/alibaba/sentinel-golang/core/flow"
)

func main() {
	//先初始化sentinel
	err := sentinel.InitDefault()
	if err != nil {
		log.Fatalf("初始化sentinel 异常: %v", err)
	}

	var globalTotal int
	var passTotal int
	var blockTotal int
	ch := make(chan struct{})

	//配置限流规则
	_, err = flow.LoadRules([]*flow.Rule{
		{
			Resource:               "some-test",
			TokenCalculateStrategy: flow.WarmUp, //冷启动策略
			ControlBehavior:        flow.Reject, //直接拒绝
			Threshold:              1000,
			WarmUpPeriodSec:        30,
		},
	})

	if err != nil {
		log.Fatalf("加载规则失败: %v", err)
	}

	//我会在每一秒统计一次，这一秒只能 你通过了多少，总共有多少， block了多少, 每一秒会产生很多的block
	for i := 0; i < 100; i++ {
		go func() {
			for {
				globalTotal++
				e, b := sentinel.Entry("some-test", sentinel.WithTrafficType(base.Inbound))
				if b != nil {
					//fmt.Println("限流了")
					blockTotal++
					time.Sleep(time.Duration(rand.Uint64()%10) * time.Millisecond)
				} else {
					passTotal++
					time.Sleep(time.Duration(rand.Uint64()%10) * time.Millisecond)
					e.Exit()
				}
			}
		}()
	}

	go func() {
		var oldTotal int //过去1s总共有多少个
		var oldPass int  //过去1s总共pass多少个
		var oldBlock int //过去1s总共block多少个
		for {
			oneSecondTotal := globalTotal - oldTotal
			oldTotal = globalTotal

			oneSecondPass := passTotal - oldPass
			oldPass = passTotal

			oneSecondBlock := blockTotal - oldBlock
			oldBlock = blockTotal

			time.Sleep(time.Second)
			fmt.Printf("total:%d, pass:%d, block:%d\n", oneSecondTotal, oneSecondPass, oneSecondBlock)
		}
	}()

	<-ch
}

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---

五、sentinel熔断

1 - 熔断器模型

• sentinel熔断器模型：Sentinel 熔断降级基于熔断器模式 (circuit breaker pattern) 实现。熔断器内部维护了一个熔断器的状态机

• 熔断器有三种状态
  • Closed 状态：也是初始状态，该状态下，熔断器会保持闭合，对资源的访问直接通过熔断器的检查
  • Open 状态：断开状态，熔断器处于开启状态，对资源的访问会被切断
  • Half-Open 状态：半开状态，该状态下除了探测流量，其余对资源的访问也会被切断。探测流量指熔断器处于半开状态时，会周期性的允许一定数目的探测请求通过，如果探测请求能够正常的返回，代表探测成功，此时熔断器会重置状态到 Closed 状态，结束熔断；如果探测失败，则回滚到 Open 状态
• 静默期：
  • Sentinel 熔断器的三种熔断策略都支持静默期 (规则中通过MinRequestAmount字段表示)
  • 静默期是指一个最小的静默请求数，在一个统计周期内，如果对资源的请求数小于设置的静默数，那么熔断器将不会基于其统计值去更改熔断器的状态
  • 静默期的设计理由也很简单，举个例子，假设在一个统计周期刚刚开始时候，第 1 个请求碰巧是个慢请求，这个时候这个时候的慢调用比例就会是 100%，很明显是不合理，所以存在一定的巧合性
  • 所以静默期提高了熔断器的精准性以及降低误判可能性

2 - 基于错误数熔断

package main

import (
	"errors"
	"fmt"
	"log"
	"math/rand"
	"time"

	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/circuitbreaker"
	"github.com/alibaba/sentinel-golang/core/config"
	"github.com/alibaba/sentinel-golang/logging"
	"github.com/alibaba/sentinel-golang/util"
)

type stateChangeTestListener struct {
}

func (s *stateChangeTestListener) OnTransformToClosed(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Closed, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToOpen(prev circuitbreaker.State, rule circuitbreaker.Rule, snapshot interface{}) {
	fmt.Printf("rule.steategy: %+v, From %s to Open, snapshot: %d, time: %d\n", rule.Strategy, prev.String(), snapshot, util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToHalfOpen(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Half-Open, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func main() {
	total := 0
	totalPass := 0
	totalBlock := 0
	totalErr := 0
	conf := config.NewDefaultConfig()
	// for testing, logging output to console
	conf.Sentinel.Log.Logger = logging.NewConsoleLogger()
	err := sentinel.InitWithConfig(conf)
	if err != nil {
		log.Fatal(err)
	}
	ch := make(chan struct{})
	// Register a state change listener so that we could observer the state change of the internal circuit breaker.
	circuitbreaker.RegisterStateChangeListeners(&stateChangeTestListener{})

	_, err = circuitbreaker.LoadRules([]*circuitbreaker.Rule{
		// Statistic time span=10s, recoveryTimeout=3s, maxErrorCount=50
		{
			Resource:         "abc",
			Strategy:         circuitbreaker.ErrorCount,
			RetryTimeoutMs:   3000, //3s之后尝试恢复
			MinRequestAmount: 10,   //静默数
			StatIntervalMs:   5000,
			Threshold:        50,
		},
	})
	if err != nil {
		log.Fatal(err)
	}

	logging.Info("[CircuitBreaker ErrorCount] Sentinel Go circuit breaking demo is running. You may see the pass/block metric in the metric log.")
	go func() {
		for {
			total++
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g1 blocked
				totalBlock++
				fmt.Println("协程熔断了")
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				totalPass++
				if rand.Uint64()%20 > 9 {
					totalErr++
					// Record current invocation as error.
					sentinel.TraceError(e, errors.New("biz error"))
				}
				// g1 passed
				time.Sleep(time.Duration(rand.Uint64()%20+10) * time.Millisecond)
				e.Exit()
			}
		}
	}()
	go func() {
		for {
			total++
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g2 blocked
				totalBlock++
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				// g2 passed
				totalPass++
				time.Sleep(time.Duration(rand.Uint64()%80) * time.Millisecond)
				e.Exit()
			}
		}
	}()

	go func() {
		for {
			time.Sleep(time.Second)
			fmt.Println(totalErr)
		}
	}()
	<-ch
}

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3 - 基于错误率熔断

package main

import (
	"errors"
	"fmt"
	"log"
	"math/rand"
	"time"

	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/circuitbreaker"
	"github.com/alibaba/sentinel-golang/core/config"
	"github.com/alibaba/sentinel-golang/logging"
	"github.com/alibaba/sentinel-golang/util"
)

type stateChangeTestListener struct {
}

func (s *stateChangeTestListener) OnTransformToClosed(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Closed, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToOpen(prev circuitbreaker.State, rule circuitbreaker.Rule, snapshot interface{}) {
	fmt.Printf("rule.steategy: %+v, From %s to Open, snapshot: %.2f, time: %d\n", rule.Strategy, prev.String(), snapshot, util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToHalfOpen(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Half-Open, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func main() {
	total := 0
	totalPass := 0
	totalBlock := 0
	totalErr := 0
	conf := config.NewDefaultConfig()
	// for testing, logging output to console
	conf.Sentinel.Log.Logger = logging.NewConsoleLogger()
	err := sentinel.InitWithConfig(conf)
	if err != nil {
		log.Fatal(err)
	}
	ch := make(chan struct{})
	// Register a state change listener so that we could observer the state change of the internal circuit breaker.
	circuitbreaker.RegisterStateChangeListeners(&stateChangeTestListener{})

	_, err = circuitbreaker.LoadRules([]*circuitbreaker.Rule{
		// Statistic time span=10s, recoveryTimeout=3s, maxErrorCount=50
		{
			Resource:         "abc",
			Strategy:         circuitbreaker.ErrorRatio,
			RetryTimeoutMs:   3000, //3s之后尝试恢复
			MinRequestAmount: 10,   //静默数
			StatIntervalMs:   5000,
			Threshold:        0.4,
		},
	})
	if err != nil {
		log.Fatal(err)
	}

	logging.Info("[CircuitBreaker ErrorCount] Sentinel Go circuit breaking demo is running. You may see the pass/block metric in the metric log.")
	go func() {
		for {
			total++
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g1 blocked
				totalBlock++
				fmt.Println("协程熔断了")
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				totalPass++
				if rand.Uint64()%20 > 9 {
					totalErr++
					// Record current invocation as error.
					sentinel.TraceError(e, errors.New("biz error"))
				}
				// g1 passed
				time.Sleep(time.Duration(rand.Uint64()%40+10) * time.Millisecond)
				e.Exit()
			}
		}
	}()
	go func() {
		for {
			total++
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g2 blocked
				totalBlock++
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				// g2 passed
				totalPass++
				time.Sleep(time.Duration(rand.Uint64()%80) * time.Millisecond)
				e.Exit()
			}
		}
	}()

	go func() {
		for {
			time.Sleep(time.Second)
			fmt.Println(float64(totalErr) / float64(total))
		}
	}()
	<-ch
}

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4 - 基于慢请求熔断

package main

import (
	"errors"
	"fmt"
	"log"
	"math/rand"
	"time"

	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/circuitbreaker"
	"github.com/alibaba/sentinel-golang/core/config"
	"github.com/alibaba/sentinel-golang/logging"
	"github.com/alibaba/sentinel-golang/util"
)

type stateChangeTestListener struct {
}

func (s *stateChangeTestListener) OnTransformToClosed(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Closed, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToOpen(prev circuitbreaker.State, rule circuitbreaker.Rule, snapshot interface{}) {
	fmt.Printf("rule.steategy: %+v, From %s to Open, snapshot: %.2f, time: %d\n", rule.Strategy, prev.String(), snapshot, util.CurrentTimeMillis())
}

func (s *stateChangeTestListener) OnTransformToHalfOpen(prev circuitbreaker.State, rule circuitbreaker.Rule) {
	fmt.Printf("rule.steategy: %+v, From %s to Half-Open, time: %d\n", rule.Strategy, prev.String(), util.CurrentTimeMillis())
}

func main() {
	conf := config.NewDefaultConfig()
	// for testing, logging output to console
	conf.Sentinel.Log.Logger = logging.NewConsoleLogger()
	err := sentinel.InitWithConfig(conf)
	if err != nil {
		log.Fatal(err)
	}
	ch := make(chan struct{})
	// Register a state change listener so that we could observer the state change of the internal circuit breaker.
	circuitbreaker.RegisterStateChangeListeners(&stateChangeTestListener{})

	_, err = circuitbreaker.LoadRules([]*circuitbreaker.Rule{
		// Statistic time span=10s, recoveryTimeout=3s, slowRtUpperBound=50ms, maxSlowRequestRatio=50%
		{
			Resource:         "abc",
			Strategy:         circuitbreaker.SlowRequestRatio,
			RetryTimeoutMs:   3000,
			MinRequestAmount: 10,
			StatIntervalMs:   5000,
			MaxAllowedRtMs:   50,
			Threshold:        0.5,
		},
	})
	if err != nil {
		log.Fatal(err)
	}

	logging.Info("[CircuitBreaker SlowRtRatio] Sentinel Go circuit breaking demo is running. You may see the pass/block metric in the metric log.")
	go func() {
		for {
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g1 blocked
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				if rand.Uint64()%20 > 9 {
					// Record current invocation as error.
					sentinel.TraceError(e, errors.New("biz error"))
				}
				// g1 passed
				time.Sleep(time.Duration(rand.Uint64()%80+10) * time.Millisecond)
				e.Exit()
			}
		}
	}()
	go func() {
		for {
			e, b := sentinel.Entry("abc")
			if b != nil {
				// g2 blocked
				time.Sleep(time.Duration(rand.Uint64()%20) * time.Millisecond)
			} else {
				// g2 passed
				time.Sleep(time.Duration(rand.Uint64()%80) * time.Millisecond)
				e.Exit()
			}
		}
	}()
	<-ch
}

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---

六、gin集成sentinel

• 我们在goods_web中对grpc的GoodsList做限流：熔断跟限流差不多，只是配置不同而已，根据需求进行修改即可
• goods_web/initialize/init_sentinel.go：初始化sentinel

package initialize

import (
	sentinel "github.com/alibaba/sentinel-golang/api"
	"github.com/alibaba/sentinel-golang/core/flow"
	"go.uber.org/zap"
)

func InitSentinel() {
	err := sentinel.InitDefault()
	if err != nil {
		zap.S().Fatalf("初始化sentinel 异常: %v", err)
	}

	//配置限流规则
	//这种配置应该从nacos中读取
	_, err = flow.LoadRules([]*flow.Rule{
		{
			Resource:               "goods-list",
			TokenCalculateStrategy: flow.Direct,
			ControlBehavior:        flow.Reject,
			//Threshold:              20,
			Threshold:        3, //为了测试，6秒钟只允许3个请求
			StatIntervalInMs: 6000,
		},
	})

	if err != nil {
		zap.S().Fatalf("加载规则失败: %v", err)
	}
}

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• goods_web/main.go：main中添加初始化逻辑

func main() {
	//1. 初始化logger
	initialize.InitLogger()
	//2. 初始化配置文件
	initialize.InitConfig()
	//3. 初始化routers
	Router := initialize.Routers()
	//4. 初始化翻译
	if err := initialize.InitTrans("zh"); err != nil {
		panic(err)
	}

	//5. 初始化srv的连接
	initialize.InitSrvConn()

	//6.初始化sentinel
	initialize.InitSentinel()

	//省略。。。
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• goods_web/api/goods/api_goods.go

func List(ctx *gin.Context) {
	//省略。。。
	e, b := sentinel.Entry("goods-list", sentinel.WithTrafficType(base.Inbound))
	if b != nil {
		ctx.JSON(http.StatusTooManyRequests, gin.H{
			"msg": "请求过于频繁，请稍后重试",
		})
		return
	}

	请求商品的service服务
	r, err := global.GoodsSrvClient.GoodsList(context.WithValue(context.Background(), "ginContext", ctx), request)
	if err != nil {
		zap.S().Errorw("[List] 查询 【商品列表】失败")
		api.HandleGrpcErrorToHttp(err, ctx)
		return
	}

	e.Exit()

	reMap := map[string]interface{}{
		"total": r.Total,
	}
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