推荐一篇写的不错的文章：限流降级神器-哨兵

源码地址

使用

有关于sentinel的使用方法和工作原理，在官方文档中都有详细的介绍，并且源码中也已经给出了一系列的demo，以下是示例：

<dependency>
    <groupId>com.alibaba.csp</groupId>
    <artifactId>sentinel-core</artifactId>
    <version>1.4.0-SNAPSHOT</version>
</dependency>

public class AuthorityDemo {

    private static final String RESOURCE_NAME = "testABC";

    public static void main(String[] args) {
        System.out.println("========Testing for black list========");
        initBlackRules();
        testFor(RESOURCE_NAME, "appA");
        testFor(RESOURCE_NAME, "appB");
        testFor(RESOURCE_NAME, "appC");
        testFor(RESOURCE_NAME, "appE");

        System.out.println("========Testing for white list========");
        initWhiteRules();
        testFor(RESOURCE_NAME, "appA");
        testFor(RESOURCE_NAME, "appB");
        testFor(RESOURCE_NAME, "appC");
        testFor(RESOURCE_NAME, "appE");
    }

    private static void testFor(/*@NonNull*/ String resource, /*@NonNull*/ String origin) {
        ContextUtil.enter(resource, origin);
        Entry entry = null;
        try {
            entry = SphU.entry(resource);
            System.out.println(String.format("Passed for resource %s, origin is %s", resource, origin));
        } catch (BlockException ex) {
            System.err.println(String.format("Blocked for resource %s, origin is %s", resource, origin));
        } finally {
            if (entry != null) {
                entry.exit();
            }
            ContextUtil.exit();
        }
    }

    private static void initWhiteRules() {
        AuthorityRule rule = new AuthorityRule();
        rule.setResource(RESOURCE_NAME);
        rule.setStrategy(RuleConstant.AUTHORITY_WHITE);
        rule.setLimitApp("appA,appE");
        AuthorityRuleManager.loadRules(Collections.singletonList(rule));
    }

    private static void initBlackRules() {
        AuthorityRule rule = new AuthorityRule();
        rule.setResource(RESOURCE_NAME);
        rule.setStrategy(RuleConstant.AUTHORITY_BLACK);
        rule.setLimitApp("appA,appB");
        AuthorityRuleManager.loadRules(Collections.singletonList(rule));
    }
}

原理

SphU.entry(resource);

// SphU.java
public static Entry entry(String name) throws BlockException {
    return Env.sph.entry(name, EntryType.OUT, 1, OBJECTS0);
}

// Env.java
public static final Sph sph = new CtSph();

// CtSph.java
public Entry entry(String name, EntryType type, int count, Object... args) throws BlockException {
    StringResourceWrapper resource = new StringResourceWrapper(name, type);
    return entry(resource, count, args);
}

入口其实就是在CtSph类的entry方法中。这里引出了一个“资源”的概念，“资源”在sentinel中可以是任何东西：服务，服务里的方法，甚至是一段代码，比如上面demo中的RESOURCE_NAME就是一个资源。当然这只是我们字面上理解的“资源”，sentinel对资源做了抽象，即：ResourceWrapper。比如这里的RESOURCE_NAME是一个字符串，所以对应StringResourceWrapper，StringResourceWrapper 是ResourceWrapper的子类。

entry的具体实现如下，前面是一些校验项目，重点关注lookProcessChain方法，其实就是ProcessorSlotChain的生成过程

public Entry entry(ResourceWrapper resourceWrapper, int count, Object... args) throws BlockException {
    Context context = ContextUtil.getContext();
    if (context instanceof NullContext) {
        // The {@link NullContext} indicates that the amount of context has exceeded the threshold,
        // so here init the entry only. No rule checking will be done.
        return new CtEntry(resourceWrapper, null, context);
    }

    if (context == null) {
        // Using default context.
        context = MyContextUtil.myEnter(Constants.CONTEXT_DEFAULT_NAME, "", resourceWrapper.getType());
    }

    // Global switch is close, no rule checking will do.
    if (!Constants.ON) {
        return new CtEntry(resourceWrapper, null, context);
    }

    ProcessorSlot<Object> chain = lookProcessChain(resourceWrapper);

    /*
        * Means amount of resources (slot chain) exceeds {@link Constants.MAX_SLOT_CHAIN_SIZE},
        * so no rule checking will be done.
        */
    if (chain == null) {
        return new CtEntry(resourceWrapper, null, context);
    }

    Entry e = new CtEntry(resourceWrapper, chain, context);
    try {
        chain.entry(context, resourceWrapper, null, count, args);
    } catch (BlockException e1) {
        e.exit(count, args);
        throw e1;
    } catch (Throwable e1) {
        // This should not happen, unless there are errors existing in Sentinel internal.
        RecordLog.info("Sentinel unexpected exception", e1);
    }
    return e;
}

ProcessorSlotChain生成

ProcessorSlot<Object> chain = lookProcessChain(resourceWrapper);

ProcessorSlot<Object> lookProcessChain(ResourceWrapper resourceWrapper) {
    ProcessorSlotChain chain = chainMap.get(resourceWrapper);
    // 双重校验
    if (chain == null) {
        synchronized (LOCK) {
            chain = chainMap.get(resourceWrapper);
            if (chain == null) {
                // Entry size limit.
                if (chainMap.size() >= Constants.MAX_SLOT_CHAIN_SIZE) {
                    return null;
                }

                chain = SlotChainProvider.newSlotChain();
                Map<ResourceWrapper, ProcessorSlotChain> newMap = new HashMap<ResourceWrapper, ProcessorSlotChain>(
                    chainMap.size() + 1);
                newMap.putAll(chainMap);
                newMap.put(resourceWrapper, chain);
                chainMap = newMap;
            }
        }
    }
    return chain;
}

该方法主要就是根据资源获取到对应的ProcessorSlotChain，这里通过一个HashMap将资源和ProcessorSlotChain的关系缓存起来了，如果根据资源没有在缓存中找到ProcessorSlotChain，则创建一个新的ProcessorSlotChain。而ProcessorSlotChain则是具体限流、降级等操作的入口。在sentinel中定义了一系列的功能插槽(Solt)，目前有7个：NodeSelectorSlot、ClusterBuilderSlot、LogSlot、StatisticSlot、SystemSlot、AuthoritySlot、FlowSlot、DegradeSlot。每个插槽对应不同的功能，比如FlowSlot负责流量控制、DegradeSlot用来做熔断降级，具体的可以查看官方文档，每个资源可以对应一个或多个Solt。ProcessorSlotChain主要就是针对资源调用具体插槽的逻辑，将一个或多个插槽泡拼装成一条链，在执行完当期插槽逻辑的之后，出发下一个插槽的逻辑，直到整条链调用完成。

SlotChainProvider.newSlotChain()的具体逻辑如下：

private static volatile SlotChainBuilder builder = null;

private static final ServiceLoader<SlotChainBuilder> LOADER = ServiceLoader.load(SlotChainBuilder.class);

public static ProcessorSlotChain newSlotChain() {
    if (builder != null) {
        return builder.build();
    }

    resolveSlotChainBuilder();

    if (builder == null) {
        RecordLog.warn("[SlotChainProvider] Wrong state when resolving slot chain builder, using default");
        builder = new DefaultSlotChainBuilder();
    }
    return builder.build();
}

这里引入了SPI的概念，在sentinel-core模块的resource/META-INF/services目录下，有一个名为com.alibaba.csp.sentinel.slotchain.SlotChainBuilder的文件，文件内容如下：

# Default slot chain builder
com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder

即，这里引用的是DefaultSlotChainBuilder，同时这也说明我们可以自定义SlotChainBuilder实现。

public class DefaultSlotChainBuilder implements SlotChainBuilder {

    @Override
    public ProcessorSlotChain build() {
        ProcessorSlotChain chain = new DefaultProcessorSlotChain();
        chain.addLast(new NodeSelectorSlot());
        chain.addLast(new ClusterBuilderSlot());
        chain.addLast(new LogSlot());
        chain.addLast(new StatisticSlot());
        chain.addLast(new SystemSlot());
        chain.addLast(new AuthoritySlot());
        chain.addLast(new FlowSlot());
        chain.addLast(new DegradeSlot());

        return chain;
    }

}

DefaultProcessorSlotChain 中的部分代码

public class DefaultProcessorSlotChain extends ProcessorSlotChain {

    AbstractLinkedProcessorSlot<?> first = new AbstractLinkedProcessorSlot<Object>() {

        @Override
        public void entry(Context context, ResourceWrapper resourceWrapper, Object t, int count, Object... args)
            throws Throwable {
            super.fireEntry(context, resourceWrapper, t, count, args);
        }

        @Override
        public void exit(Context context, ResourceWrapper resourceWrapper, int count, Object... args) {
            super.fireExit(context, resourceWrapper, count, args);
        }

    };
    AbstractLinkedProcessorSlot<?> end = first;

    @Override
    public void addFirst(AbstractLinkedProcessorSlot<?> protocolProcessor) {
        protocolProcessor.setNext(first.getNext());
        first.setNext(protocolProcessor);
        if (end == first) {
            end = protocolProcessor;
        }
    }

    @Override
    public void addLast(AbstractLinkedProcessorSlot<?> protocolProcessor) {
        end.setNext(protocolProcessor);
        end = protocolProcessor;
    }
}

主要就是将不同的插槽拼装成一条链路，addFirst表示加在链表的头部，主要通过改变first的next指向来实现；addLast表示加在链表的尾部，主要通过改变end的next指向来实现，如果不是很理解，在纸上比划比划就很清楚了。

ProcessorSlotChain执行

以上是有关于ProcessorSlotChain的生成逻辑，接下来看看ProcessorSlotChain的执行逻辑，继续回到Ctsph中的entry方法，在上面已经粘贴过一次，这里省略部分非关键代码：

public Entry entry(ResourceWrapper resourceWrapper, int count, Object... args) throws BlockException {
    ProcessorSlot<Object> chain = lookProcessChain(resourceWrapper);
    // 如果chain为空，说明资源数已经超过sentinel设置的最带值了，默认是6000
    if (chain == null) {
        return new CtEntry(resourceWrapper, null, context);
    }

    Entry e = new CtEntry(resourceWrapper, chain, context);
    try {
        // ProcessorSlotChain执行入口
        chain.entry(context, resourceWrapper, null, count, args);
    } catch (BlockException e1) {
        e.exit(count, args);
        throw e1;
    } catch (Throwable e1) {
        // This should not happen, unless there are errors existing in Sentinel internal.
        RecordLog.info("Sentinel unexpected exception", e1);
    }
    return e;
}

不难发现，入口在chain.entry(context, resourceWrapper, null, count, args)，从上面的ProcessorSlotChain生成逻辑可以发现，生成的是DefaultProcessorSlotChain，所以主要关注DefaultProcessorSlotChain的entry方法

// DefaultProcessorSlotChain.java
public void entry(Context context, ResourceWrapper resourceWrapper, Object t, int count, Object... args)
    throws Throwable {
    first.transformEntry(context, resourceWrapper, t, count, args);
}

// AbstractLinkedProcessorSlot.java
void transformEntry(Context context, ResourceWrapper resourceWrapper, Object o, int count, Object... args)
    throws Throwable {
    T t = (T)o;
    entry(context, resourceWrapper, t, count, args);
}

//DefaultProcessorSlotChain.java
public void entry(Context context, ResourceWrapper resourceWrapper, Object t, int count, Object... args)
    throws Throwable {
    super.fireEntry(context, resourceWrapper, t, count, args);
}

// AbstractLinkedProcessorSlot.java
public void fireEntry(Context context, ResourceWrapper resourceWrapper, Object obj, int count, Object... args)
    throws Throwable {
    // 这里的next其实就是指具体的插槽实现了
    if (next != null) {
        next.transformEntry(context, resourceWrapper, obj, count, args);
    }
}

最关键的部分其实就在fireEntry方法中，这里的next其实就是指具体的插槽实现，比如这里以FlowSlot为例：

public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, Object... args)
    throws Throwable {
    // FlowSlot具体的插槽逻辑
    checkFlow(resourceWrapper, context, node, count);
    
    // 通过调用AbstractLinkedProcessorSlot的fireEntry方法，用来触发下一个插槽逻辑的调用
    fireEntry(context, resourceWrapper, node, count, args);
}

其实这就是插槽链的调用，比如SpringAOP中的Intercepterl链、Mybatis中的plugin链路，虽然具体的实现方式不同，但是目的都是一样的：执行完整条链上的逻辑。

上面的调用都是理想的情况，即：所有的请求都通过，没有被限制的情况。如果请求被拒绝，该怎么处理？这里还是以FlowSlot为例：

public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, Object... args)
    throws Throwable {
    checkFlow(resourceWrapper, context, node, count);

    fireEntry(context, resourceWrapper, node, count, args);
}

void checkFlow(ResourceWrapper resource, Context context, DefaultNode node, int count) throws BlockException {
    // Flow rule map cannot be null.
    Map<String, List<FlowRule>> flowRules = FlowRuleManager.getFlowRules();

    List<FlowRule> rules = flowRules.get(resource.getName());
    if (rules != null) {
        for (FlowRule rule : rules) {
            if (!canPassCheck(rule, context, node, count)) {
                throw new FlowException(rule.getLimitApp());
            }
        }
    }
}

可以看到，如果请求被拒绝，即：被限流了，则抛出BlockException异常，在外层如果捕获到BlockException异常，则在里面处理对应的逻辑。

sentinel-dashboard

sentinel-dashboard是sentinel的轻量级控制台，该控制台主要提供两个功能：监控、配置。即：针对资源的监控和针对资源的配置，比如：可以配置一些规则。

sentinel-dashboard是基于spring-boot2，所以直接启动DashboardApplication就可以了，当然也可以以jar包的方式启动，启动之后的界面效果如下：

没错，什么都没有，因为这时候没有可监控的应用。

接入到sentinel-dashboard的流程也很简单，新建一个应用， 添加以下依赖

<!-- sentinel-dashboard -->
<dependency>
    <groupId>com.alibaba.csp</groupId>
    <artifactId>sentinel-transport-simple-http</artifactId>
    <version>1.4.0-SNAPSHOT</version>
</dependency>

以下是测试代码，其实就是源码中的demo，这里直接搬过来

package com.hand.sxy.sentinel;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

import com.alibaba.csp.sentinel.util.TimeUtil;
import com.alibaba.csp.sentinel.Entry;
import com.alibaba.csp.sentinel.SphU;
import com.alibaba.csp.sentinel.slots.block.BlockException;
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;

public class FlowQps {

    private static final String KEY = "abc";

    private static AtomicInteger pass = new AtomicInteger();
    private static AtomicInteger block = new AtomicInteger();
    private static AtomicInteger total = new AtomicInteger();

    private static volatile boolean stop = false;

    private static final int threadCount = 32;

    private static int seconds = 600000 + 40;

    public static void main(String[] args) throws Exception {
        initFlowQpsRule();

        tick();
        // first make the system run on a very low condition
        simulateTraffic();

        System.out.println("===== begin to do flow control");
        System.out.println("only 20 requests per second can pass");

    }

    private static void initFlowQpsRule() {
        List<FlowRule> rules = new ArrayList<FlowRule>();
        FlowRule rule1 = new FlowRule();
        rule1.setResource(KEY);
        // set limit qps to 20
        rule1.setCount(20);
        rule1.setGrade(RuleConstant.FLOW_GRADE_QPS);
        rule1.setLimitApp("default");
        rules.add(rule1);
        FlowRuleManager.loadRules(rules);
    }

    private static void simulateTraffic() {
        for (int i = 0; i < threadCount; i++) {
            Thread t = new Thread(new RunTask());
            t.setName("simulate-traffic-Task");
            t.start();
        }
    }

    private static void tick() {
        Thread timer = new Thread(new TimerTask());
        timer.setName("sentinel-timer-task");
        timer.start();
    }

    static class TimerTask implements Runnable {

        @Override
        public void run() {
            long start = System.currentTimeMillis();
            System.out.println("begin to statistic!!!");

            long oldTotal = 0;
            long oldPass = 0;
            long oldBlock = 0;
            while (!stop) {
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {
                }
                long globalTotal = total.get();
                long oneSecondTotal = globalTotal - oldTotal;
                oldTotal = globalTotal;

                long globalPass = pass.get();
                long oneSecondPass = globalPass - oldPass;
                oldPass = globalPass;

                long globalBlock = block.get();
                long oneSecondBlock = globalBlock - oldBlock;
                oldBlock = globalBlock;

                System.out.println(seconds + " send qps is: " + oneSecondTotal);
                System.out.println(TimeUtil.currentTimeMillis() + ", total:" + oneSecondTotal
                        + ", pass:" + oneSecondPass
                        + ", block:" + oneSecondBlock);

                if (seconds-- <= 0) {
                    stop = true;
                }
            }

            long cost = System.currentTimeMillis() - start;
            System.out.println("time cost: " + cost + " ms");
            System.out.println("total:" + total.get() + ", pass:" + pass.get()
                    + ", block:" + block.get());
            System.exit(0);
        }
    }

    static class RunTask implements Runnable {
        @Override
        public void run() {
            while (!stop) {
                Entry entry = null;

                try {
                    entry = SphU.entry(KEY);
                    // token acquired, means pass
                    pass.addAndGet(1);
                } catch (BlockException e1) {
                    block.incrementAndGet();
                } catch (Exception e2) {
                    // biz exception
                } finally {
                    total.incrementAndGet();
                    if (entry != null) {
                        entry.exit();
                    }
                }

                Random random2 = new Random();
                try {
                    TimeUnit.MILLISECONDS.sleep(random2.nextInt(50));
                } catch (InterruptedException e) {
                    // ignore
                }
            }
        }
    }
}

启动的时候，添加以下参数：

-Djava.net.preferIPv4Stack=true
-Dcsp.sentinel.dashboard.server=localhost:8080
-Dcsp.sentinel.api.port=8720
-Dproject.name=我的APP

启动之后，刷新界面，查看效果

有关于控制台的一些功能就不过多介绍了，有兴趣可以自己看看。

Dubbo适配

看官方文档，除了dubbo适配，文档上还有与其它主流框架适配的介绍。dubbo适配主要是涉及到两个Filtter：SentinelDubboConsumerFilter、SentinelDubboProviderFilter。从名字上也可以看出来，SentinelDubboConsumerFilter主要是限制调用方请求；SentinelDubboProviderFilter主要就是限制提供方提供。有关于这两个应用场景，推荐看看dubbo的官方文档，上面有详细的说明，并且还列举了比较好的例子，例如：