1、说明

Sentinel主要用来流控，熔断降级保护目标资源用的，常用集成SCG，SpringBoot，SprinMVC这些，但底层本质没变，但是体现形式上会有差别。例如SCG底层是Netty 和 SpringWebFlux 采用Reactor Stream处理，SpringBoot内部通过AOP处理流控这些。

以网关形式展现Sentinel调用流程吧，网关Sentinel都会了，其它应用集成就更好理解了

2、准备

依赖

org.springframework.cloudspring-cloud-starter-gatewaycom.alibaba.cspsentinel-spring-cloud-gateway-adapter1.8.1

SCG集成Sentinel所需配置

@Configuration
public class GatewayConfiguration {private final List viewResolvers;private final ServerCodecConfigurer serverCodecConfigurer;public GatewayConfiguration(ObjectProvider> viewResolversProvider,ServerCodecConfigurer serverCodecConfigurer) {this.viewResolvers = viewResolversProvider.getIfAvailable(Collections::emptyList);this.serverCodecConfigurer = serverCodecConfigurer;}@Bean@Order(Ordered.HIGHEST_PRECEDENCE)public SentinelGatewayBlockExceptionHandler sentinelGatewayBlockExceptionHandler() {// Register the block exception handler for Spring Cloud Gateway.return new SentinelGatewayBlockExceptionHandler(viewResolvers, serverCodecConfigurer);}@Bean@Order(-1)public GlobalFilter sentinelGatewayFilter() {return new SentinelGatewayFilter();}
}

往IOC容器中注入两个javaBean，SentinelGatewayBlockExceptionHandler Sentinel异常处理器，SentinelGatewayFilter Sentinel流控过滤器

3、大致流程

4、Sentinel执行流程

4.1、SentinelGlobalFilter

public class SentinelGatewayFilter implements GatewayFilter, GlobalFilter, Ordered {private final int order;// filterChain 中  filter执行优先级public SentinelGatewayFilter() {this(Ordered.HIGHEST_PRECEDENCE);}public SentinelGatewayFilter(int order) {this.order = order;}private final GatewayParamParser paramParser = new GatewayParamParser<>(new ServerWebExchangeItemParser());@Overridepublic Mono filter(ServerWebExchange exchange, GatewayFilterChain chain) {// 获取当前路由Route route = exchange.getAttribute(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR);Mono asyncResult = chain.filter(exchange);if (route != null) {String routeId = route.getId();// 解析参数 以备参数流控Object[] params = paramParser.parseParameterFor(routeId, exchange,r -> r.getResourceMode() == SentinelGatewayConstants.RESOURCE_MODE_ROUTE_ID);String origin = Optional.ofNullable(GatewayCallbackManager.getRequestOriginParser()).map(f -> f.apply(exchange)).orElse("");// 定义流控发布者，准备流控/熔断asyncResult = asyncResult.transform(new SentinelReactorTransformer<>(new EntryConfig(routeId, ResourceTypeConstants.COMMON_API_GATEWAY,EntryType.IN, 1, params, new ContextConfig(contextName(routeId), origin))));}Set matchingApis = pickMatchingApiDefinitions(exchange);for (String apiName : matchingApis) {// 解析参数 以备参数流控Object[] params = paramParser.parseParameterFor(apiName, exchange,r -> r.getResourceMode() == SentinelGatewayConstants.RESOURCE_MODE_CUSTOM_API_NAME);// 定义流控发布者，准备流控/熔断asyncResult = asyncResult.transform(new SentinelReactorTransformer<>(new EntryConfig(apiName, ResourceTypeConstants.COMMON_API_GATEWAY,EntryType.IN, 1, params)));}return asyncResult;}private String contextName(String route) {return SentinelGatewayConstants.GATEWAY_CONTEXT_ROUTE_PREFIX + route;}Set pickMatchingApiDefinitions(ServerWebExchange exchange) {return GatewayApiMatcherManager.getApiMatcherMap().values().stream().filter(m -> m.test(exchange)).map(WebExchangeApiMatcher::getApiName).collect(Collectors.toSet());}@Overridepublic int getOrder() {return order;}
}

SCG中只要匹配到route之后，通过FilteringWebHandler构建FiterChain，SentinelGlobalFilter会被执行。

4.2 EntryConfig

定义限流资源配置（Sentinel访问令牌配置）
new EntryConfig(routeId, ResourceTypeConstants.COMMON_API_GATEWAY, EntryType.IN, 1, params, new ContextConfig(contextName(routeId), origin))

• resourceName：默认将路由id作为访问的资源名
• entryType：EntryType.IN
• resourceType：网关类型，集成在网关
• acquireCount：入口流量默认1
• args: 热点参数流控项
• contextConfig 上下文配置

4.3、SentinelReactorTransformer

SentinelReactorTransformer 主要用来将publisher发布者进行转换的

接收到EntryConfig之后，进行存储

 @Overridepublic Publisher apply(Publisher publisher) {if (publisher instanceof Mono) {return new MonoSentinelOperator<>((Mono) publisher, entryConfig);}if (publisher instanceof Flux) {return new FluxSentinelOperator<>((Flux) publisher, entryConfig);}throw new IllegalStateException("Publisher type is not supported: " + publisher.getClass().getCanonicalName());}

通过transfer调用apply封装

4.4 MonoSentinelOperator

流控发布者

当请求进来的时候，消费者（subscriber）会订阅MonoSentinelOperator，FluxSentinelOperator，调用内部的subscribe方法，正式触发Sentinel流控

4.5 SentinelReactorSubscriber

订阅者负责处理请求，进行流控，SCG流控真正的入口

private void entryWhenSubscribed() {// 获取上下文配置ContextConfig sentinelContextConfig = entryConfig.getContextConfig();if (sentinelContextConfig != null) {// 一般情况下走不到这里，因为会自己创建ContextConfig// If current we're already in a context, the context config won't work.// 如果没有配置上下文，尝试获取上下文，并缓存ContextUtil.enter(sentinelContextConfig.getContextName(), sentinelContextConfig.getOrigin());}try {// 获取资源访问令牌AsyncEntry entry = SphU.asyncEntry(entryConfig.getResourceName(), entryConfig.getResourceType(),entryConfig.getEntryType(), entryConfig.getAcquireCount(), entryConfig.getArgs());this.currentEntry = entry;actual.onSubscribe(this);} catch (BlockException ex) {// Mark as completed (exited) explicitly.entryExited.set(true);// Signal cancel and propagate the {@code BlockException}.cancel();actual.onSubscribe(this);// 派发异常信号actual.onError(ex);} finally {// 清除线程上下文，SCG底层采用netty实现，io多路复用，// 一个线程处理多个请求不清除上下文，会出现多个请求共用一个上下的问题if (sentinelContextConfig != null) {ContextUtil.exit();}}}

1、一般情况下访问目标资源会创建上下文，什么意思？

在SCG网关中，一个访问资源route对应一个上下文，内部一般情况下只会存在一个调用链路，因为routeId既是资源访问者，又是受保护的资源。 是不是有点绕？ 哈哈哈哈哈哈哈哈哈。因为网关核心作用是转发请求到目标服务，以目标路由id，作为资源访问者，目标路由也是受保护的资源 。 资源访问者—》受保护资源 不就一条路径嘛？

只能是一个路由对应一个上下文嘛？不是的，可以修改源码自定义，例如将调用方AppKey作为上下文入口，访问目标路由a1,a2 不就是在一个上下文中存在两条调用链路嘛？

在SpringMVC中，请求者对应一个上下文，请求者访问不同的受限资源，会创建不同的调用链路

2、ContextUtil.enter(sentinelContextConfig.getContextName(), sentinelContextConfig.getOrigin()); 干什么？

从线程中获取context，没有就创建

public static Context enter(String name, String origin) {if (Constants.CONTEXT_DEFAULT_NAME.equals(name)) {throw new ContextNameDefineException("The " + Constants.CONTEXT_DEFAULT_NAME + " can't be permit to defined!");}return trueEnter(name, origin);}protected static Context trueEnter(String name, String origin) {// 尝试着从ThreadLocal中获取ContextContext context = contextHolder.get();// 若ThreadLocal中没有context，则尝试着从缓存map中获取if (context == null) {// 缓存map的key为context名称，value为EntranceNodeMap localCacheNameMap = contextNameNodeMap;// 获取EntranceNode——双重检测锁DCL——为了防止并发创建DefaultNode node = localCacheNameMap.get(name);if (node == null) {// 若缓存map的size 大于 context数量的最大阈值，则直接返回NULL_CONTEXTif (localCacheNameMap.size() > Constants.MAX_CONTEXT_NAME_SIZE) {setNullContext();return NULL_CONTEXT;} else {LOCK.lock();try {node = contextNameNodeMap.get(name);if (node == null) {if (contextNameNodeMap.size() > Constants.MAX_CONTEXT_NAME_SIZE) {setNullContext();return NULL_CONTEXT;} else {// 创建一个EntranceNodenode = new EntranceNode(new StringResourceWrapper(name, EntryType.IN), null);// Add entrance node.将新建的node添加到ROOTConstants.ROOT.addChild(node);// 将新建node写入到缓存map// 为了防止“迭代稳定性问题”——iterate stable——对于共享集合的写操作Map newMap = new HashMap<>(contextNameNodeMap.size() + 1);newMap.putAll(contextNameNodeMap);newMap.put(name, node);contextNameNodeMap = newMap;}}} finally {LOCK.unlock();}}}// 将context的name与entranceNode封装为contextcontext = new Context(node, name);// 初始化context的来源context.setOrigin(origin);// 将context写入到ThreadLocalcontextHolder.set(context);}return context;}

3、AsyncEntry entry = SphU.asyncEntry(entryConfig.getResourceName(), entryConfig.getResourceType(),
entryConfig.getEntryType(), entryConfig.getAcquireCount(), entryConfig.getArgs()); 获取资源访问令牌

Sentinel初始化的源码分析，之前讲过，忘了可以看之前的文章

4.6、ProcessSlotChain

// 找到资源对应的 slotChain ProcessorSlot