Sentinel是阿里開源的一個限流方案框架具有以下特徵:

• 豐富的應用場景：Sentinel 承接了阿里巴巴近 10 年的雙十一大促流量的核心場景，例如秒殺（即突發流量控制在系統容量可以承受的範圍）、訊息削峰填谷、叢集流量控制、實時熔斷下游不可用應用等。
• 完備的實時監控：Sentinel 同時提供實時的監控功能。您可以在控制檯中看到接入應用的單臺機器秒級資料，甚至 500 臺以下規模的叢集的彙總執行情況。
• 廣泛的開源生態：Sentinel 提供開箱即用的與其它開源框架/庫的整合模組，例如與 Spring Cloud、Dubbo、gRPC 的整合。您只需要引入相應的依賴並進行簡單的配置即可快速地接入 Sentinel。
• 完善的 SPI 擴充套件點：Sentinel 提供簡單易用、完善的 SPI 擴充套件介面。您可以通過實現擴充套件介面來快速地定製邏輯。例如定製規則管理、適配動態資料來源等。

 一、搭建監控平臺

       1.下載相應的jar包，地址https://github.com/alibaba/Sentinel/releases

       2.啟動  java -Dserver.port=8080 -Dcsp.sentinel.dashboard.server=localhost:8080 -Dproject.name=sentinel-dashboard -jar                                        -Dsentinel.dashboard.auth.username=sentinel -Dsentinel.dashboard.auth.password=123456         sentinel-dashboard-1.6.3.jar

       -Dsentinel.dashboard.auth.username 指定登入名

       -Dsentinel.dashboard.auth.password  指定登入密碼

      3.訪問localhost:8080 ，輸入使用者密碼

     4.看到如下效果搭建成功

二、客戶端使用

以註解使用為例

1.maven依賴

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

	<modelVersion>4.0.0</modelVersion>

	<artifactId>sentinel-demo-annotation-spring-aop</artifactId>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.6.RELEASE</version>
	</parent>



	<dependencies>
		<dependency>
			<groupId>com.alibaba.csp</groupId>
			<artifactId>sentinel-core</artifactId>
			<version>1.6.3</version>
		</dependency>
		<dependency>
			<groupId>com.alibaba.csp</groupId>
			<artifactId>sentinel-transport-simple-http</artifactId>
			<version>1.6.3</version>
		</dependency>
		<dependency>
			<groupId>com.alibaba.csp</groupId>
			<artifactId>sentinel-annotation-aspectj</artifactId>
			<version>1.6.3</version>
		</dependency>


		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-aop</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>
	</dependencies>
</project>

2.配置限流規則方案

/*
 * Copyright 1999-2018 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.csp.sentinel.demo.annotation.aop.config;

import com.alibaba.csp.sentinel.annotation.aspectj.SentinelResourceAspect;
import com.alibaba.csp.sentinel.slots.block.RuleConstant;
import com.alibaba.csp.sentinel.slots.block.flow.FlowRule;
import com.alibaba.csp.sentinel.slots.block.flow.FlowRuleManager;

import java.util.ArrayList;
import java.util.List;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

/**
 * @author Eric Zhao
 */
@Configuration
public class AopConfiguration {

    @Bean
    public SentinelResourceAspect sentinelResourceAspect() {
        //配置固定限流規則，每秒為20qps,真對test
        List<FlowRule> rules = new ArrayList<>();
  	    FlowRule rule = new FlowRule();
  	    rule.setResource("test");
  	    rule.setGrade(RuleConstant.FLOW_GRADE_QPS);
  	    // Set limit QPS to 20.
  	    rule.setCount(20);
  	    rules.add(rule);
  	    FlowRuleManager.loadRules(rules);
        return new SentinelResourceAspect();
    }
}

3.實際應用

/*
 * Copyright 1999-2018 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.csp.sentinel.demo.annotation.aop.service;

import org.springframework.stereotype.Service;

import com.alibaba.csp.sentinel.annotation.SentinelResource;

/**
 * @author Eric Zhao
 */
@Service
public class TestServiceImpl implements TestService {

    @Override
    @SentinelResource(value = "test", blockHandler = "handleException", blockHandlerClass = {ExceptionUtil.class})
    public void test() {
        System.out.println("Test");
    }

    @Override
    @SentinelResource(value = "hello", fallback = "helloFallback")
    public String hello(long s) {
        if (s < 0) {
            throw new IllegalArgumentException("invalid arg");
        }
        return String.format("Hello at %d", s);
    }

    @Override
    @SentinelResource(value = "helloAnother", defaultFallback = "defaultFallback",
        exceptionsToIgnore = {IllegalStateException.class})
    public String helloAnother(String name) {
        if (name == null || "bad".equals(name)) {
            throw new IllegalArgumentException("oops");
        }
        if ("foo".equals(name)) {
            throw new IllegalStateException("oops");
        }
        return "Hello, " + name;
    }

    public String helloFallback(long s, Throwable ex) {
        // Do some log here.
        ex.printStackTrace();
        return "Oops, error occurred at " + s;
    }

    public String defaultFallback() {
        System.out.println("Go to default fallback");
        return "default_fallback";
    }
}

/*
 * Copyright 1999-2018 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.csp.sentinel.demo.annotation.aop.controller;

import com.alibaba.csp.sentinel.demo.annotation.aop.service.TestService;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

/**
 * @author Eric Zhao
 */
@RestController
public class DemoController {

    @Autowired
    private TestService service;

    @GetMapping("/foo")
    public String apiFoo(@RequestParam(required = false) Long t) throws Exception {
        if (t == null) {
            t = System.currentTimeMillis();
        }
        service.test();
        return service.hello(t);
    }

    @GetMapping("/baz/{name}")
    public String apiBaz(@PathVariable("name") String name) {
        return service.helloAnother(name);
    }
}

4.啟動應用，-Dproject.name=test -Dcsp.sentinel.dashboard.server=localhost:8080 訪問介面就可以看到監控資訊了

-Dproject.name=test  指定監控名稱

-Dcsp.sentinel.dashboard.server 指定監控平臺地址

三、簡單基於註解原理分析

1.sentinel 主要的核心功能由sentinel-core 提供，基於註解的應用主要是依賴於切面的使用，具體的實現如下

/*
 * Copyright 1999-2018 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.csp.sentinel.annotation.aspectj;

import com.alibaba.csp.sentinel.Entry;
import com.alibaba.csp.sentinel.EntryType;
import com.alibaba.csp.sentinel.SphU;
import com.alibaba.csp.sentinel.annotation.SentinelResource;
import com.alibaba.csp.sentinel.slots.block.BlockException;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;

import java.lang.reflect.Method;

/**
 * Aspect for methods with {@link SentinelResource} annotation.
 *
 * @author Eric Zhao
 */
@Aspect
public class SentinelResourceAspect extends AbstractSentinelAspectSupport {

    @Pointcut("@annotation(com.alibaba.csp.sentinel.annotation.SentinelResource)")
    public void sentinelResourceAnnotationPointcut() {
    }

    @Around("sentinelResourceAnnotationPointcut()")
    public Object invokeResourceWithSentinel(ProceedingJoinPoint pjp) throws Throwable {
        Method originMethod = resolveMethod(pjp);

        SentinelResource annotation = originMethod.getAnnotation(SentinelResource.class);
        if (annotation == null) {
            // Should not go through here.
            throw new IllegalStateException("Wrong state for SentinelResource annotation");
        }
        String resourceName = getResourceName(annotation.value(), originMethod);
        EntryType entryType = annotation.entryType();
        int resourceType = annotation.resourceType();
        Entry entry = null;
        try {
            entry = SphU.entry(resourceName, resourceType, entryType, pjp.getArgs());
            Object result = pjp.proceed();
            return result;
        } catch (BlockException ex) {
            return handleBlockException(pjp, annotation, ex);
        } catch (Throwable ex) {
            Class<? extends Throwable>[] exceptionsToIgnore = annotation.exceptionsToIgnore();
            // The ignore list will be checked first.
            if (exceptionsToIgnore.length > 0 && exceptionBelongsTo(ex, exceptionsToIgnore)) {
                throw ex;
            }
            if (exceptionBelongsTo(ex, annotation.exceptionsToTrace())) {
                traceException(ex, annotation);
                return handleFallback(pjp, annotation, ex);
            }

            // No fallback function can handle the exception, so throw it out.
            throw ex;
        } finally {
            if (entry != null) {
                entry.exit(1, pjp.getArgs());
            }
        }
    }
}

與單機的應用其實是一樣的，初始化規則，呼叫SphU的entry方法