技术干货分享 | SpringCloud Gateway 全链路实现分析

1. 背景

随着微服务架构的流行，服务按照不同的维度进行拆分，一次请求往往需要涉及到多个服务。而诸多的服务可能分布在了几千台服务器，横跨多个不同的数据中心。为了快速定位和解决故障，应用性能进行分析，全链路监控组件就在这样的问题背景下产生了。最出名的是谷歌公开的论文提到的 Google Dapper。想要在这个上下文中理解分布式系统的行为，就需要监控那些横跨了不同的应用、不同的服务器之间的关联动作。

1.1 全链路原理

通过业务调用过程中添加并传递调用链ID，实现应用间生成链路数据，最终串联成一条完整的调用链。 其中整个调用过程中每个请求都要透传TxId、SpanId和pSpanId。

1.2 Spring Cloud Gateway

作为Spring Cloud官方推出的第二代网关框架，Spring cloud gateway是基于Spring 5.0、Spring Boot2.0和Reactor等技术开发的网关，采用了NIO模型进行通信。

1.2.1 Spring WebFlux

Spring Boot 2.0 包括一个新的 spring-webflux 模块，名称中的 Flux 来源于 Reactor 中的类 Flux。该模块包含对响应式 HTTP 和 WebSocket 客户端的支持，以及对 REST、HTML 和 WebSocket 交互等程序的支持。 一般来说，Spring MVC 用于同步处理；Spring Webflux 用于异步处理。

1.2.2 Mono与Flux

Mono表示的是包含 0 或者 1 个元素的异步序列，即要么成功发布元素，要么错误

Flux 表示的是包含 0 到 N 个元素的异步序列 ，即要么成功发布 0 到 N 个元素，要么错误

Flux和Mono之间可以相互转换，比如对一个 Flux 序列进行计数操作，得到的结果是一个 Mono对象，或者把两个 Mono 序列合并在一起，得到的是一个 Flux 对象。

2. Spring Cloud Gateway不做监控

Spring Cloud Gateway作为入口网关，主要负责服务的路由转发。如果网关没有进行监控，则全链路会缺失网关节点，直接展示为用户访问后续应用；不能有效定位用户请求慢是网关问题还是后续节点。

3. Spring Cloud Gateway进行监控

由于Spring Cloud Gateway采用了Reactor框架非阻塞式调用，任务之间会跨线程执行，导致HTTP头信息所需的调用链ID不好传递。

• Gateway接收线程
• Gateway返回线程
• 路由转发线程

3.1 Spring Cloud Gateway流程

现对Spring Cloud Gateway的流程进行梳理，本篇由于只涉及到跨线程服务调度，不讨论路由过程。

3.1.1 Spring Cloud Gateway的请求入口

org.springframework.http.server.reactive.ReactorHttpHandlerAdapter#apply 先将接收到的HttpServerRequest或者最终需要返回的HttpServerResponse包装转换为ReactorServerHttpRequest和ReactorServerHttpResponse，再处理请求。

public Mono<Void> apply(HttpServerRequest request, HttpServerResponse response) {

NettyDataBufferFactory bufferFactory = new NettyDataBufferFactory(response.alloc());

ServerHttpRequest adaptedRequest;

ServerHttpResponse adaptedResponse;

try {

adaptedRequest = new ReactorServerHttpRequest(request, bufferFactory);

adaptedResponse = new ReactorServerHttpResponse(response, bufferFactory);

} catch (URISyntaxException ex) {

logger.error("Invalid URL " + ex.getMessage(), ex);

response.status(HttpResponseStatus.BAD_REQUEST);

return Mono.empty();

}

if (adaptedRequest.getMethod() == HttpMethod.HEAD) {

adaptedResponse = new HttpHeadResponseDecorator(adaptedResponse);

}

return this.httpHandler.handle(adaptedRequest, adaptedResponse)

.doOnError(ex -> logger.error("Handling completed with error", ex))

.doOnSuccess(aVoid -> logger.debug("Handling completed with success"));

}

3.1.2 构造网关上下文

org.springframework.web.server.adapter.HttpWebHandlerAdapter#handle

• createExchange()构造网关上下文ServerWebExchange
• getDelegate()通过委托的方式获取一系列需要处理的WebHandler

public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {

ServerWebExchange exchange = createExchange(request, response);

return getDelegate().handle(exchange)

.onErrorResume(ex -> handleFailure(request, response, ex))

.then(Mono.defer(response::setComplete));

}

 

protected ServerWebExchange createExchange(ServerHttpRequest request, ServerHttpResponse response) {

return new DefaultServerWebExchange(request, response, this.sessionManager,

getCodecConfigurer(), getLocaleContextResolver(), this.applicationContext);

}

3.1.3 进入Filter链

org.springframework.cloud.gateway.handler.FilteringWebHandler#handle 获得 GatewayFilter 数组，并根据获得的 GatewayFilter 数组创建DefaultGatewayFilterChain，过滤处理请求。

public Mono<Void> handle(ServerWebExchange exchange) {

Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);

List<GatewayFilter> gatewayFilters = route.getFilters();

List<GatewayFilter> combined = new ArrayList<>(this.globalFilters);

combined.addAll(gatewayFilters);

 

AnnotationAwareOrderComparator.sort(combined);

logger.debug("Sorted gatewayFilterFactories: "+ combined);

return new DefaultGatewayFilterChain(combined).filter(exchange);

}

3.1.4 执行Filter链

org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain#filter 过滤器的链式调用

public Mono<Void> filter(ServerWebExchange exchange) {

return Mono.defer(() -> {

if (this.index < filters.size()) {

GatewayFilter filter = filters.get(this.index);

DefaultGatewayFilterChain chain = new DefaultGatewayFilterChain(this, this.index + 1);

return filter.filter(exchange, chain);

} else {

return Mono.empty(); // complete

}

});

}

3.1.5 Gateway Filter适配器

org.springframework.cloud.gateway.handler.FilteringWebHandler$GatewayFilterAdapter#filter GatewayFilterAdapter是GlobalFilter过滤器的包装类，最终委托Global Filter进行执行。

private static class GatewayFilterAdapter implements GatewayFilter {
	private final GlobalFilter delegate;
	public GatewayFilterAdapter(GlobalFilter delegate) {
		this.delegate = delegate;
	}

	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		return this.delegate.filter(exchange, chain);
	}
}

3.1.6 Netty路由网关过滤器

org.springframework.cloud.gateway.filter.NettyRoutingFilter#filter

GlobalFilter实现有很多，此处只分析NettyRoutingFIlter和NettyWriteResponseFilter。而NettyRoutingFilter负责使用 Netty HttpClient 代理对下游的请求。

public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {

// 获得 requestUrl

URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);

 

// 判断是否能够处理，http或https前缀

String scheme = requestUrl.getScheme();

if (isAlreadyRouted(exchange) || (!"http".equals(scheme) && !"https".equals(scheme))) {

return chain.filter(exchange);

}

// 设置已经路由

setAlreadyRouted(exchange);

 

ServerHttpRequest request = exchange.getRequest();

// 创建Netty Request Method对象

final HttpMethod method = HttpMethod.valueOf(request.getMethod().toString());

final String url = requestUrl.toString();

HttpHeaders filtered = filterRequest(this.headersFilters.getIfAvailable(), exchange);

 

final DefaultHttpHeaders httpHeaders = new DefaultHttpHeaders();

filtered.forEach(httpHeaders::set);

 

String transferEncoding = request.getHeaders().getFirst(HttpHeaders.TRANSFER_ENCODING);

boolean chunkedTransfer = "chunked".equalsIgnoreCase(transferEncoding);

boolean preserveHost = exchange.getAttributeOrDefault(PRESERVE_HOST_HEADER_ATTRIBUTE, false);

// 请求后端服务

return this.httpClient.request(method, url, req -> {

final HttpClientRequest proxyRequest = req.options(NettyPipeline.SendOptions::flushOnEach)

.headers(httpHeaders)

.chunkedTransfer(chunkedTransfer)

.failOnServerError(false)

.failOnClientError(false);

if (preserveHost) {

String host = request.getHeaders().getFirst(HttpHeaders.HOST);

proxyRequest.header(HttpHeaders.HOST, host);

}

return proxyRequest.sendHeaders() //发送请求头

.send(request.getBody().map(dataBuffer -> // 发送请求Body

((NettyDataBuffer)dataBuffer).getNativeBuffer()));

}).doOnNext(res -> {

ServerHttpResponse response = exchange.getResponse();

// put headers and status so filters can modify the response

HttpHeaders headers = new HttpHeaders();

res.responseHeaders().forEach(entry -> headers.add(entry.getKey(), entry.getValue()));

exchange.getAttributes().put("original_response_content_type", headers.getContentType());

HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(

this.headersFilters.getIfAvailable(), headers, exchange, Type.RESPONSE);

response.getHeaders().putAll(filteredResponseHeaders);

 

HttpStatus status = HttpStatus.resolve(res.status().code());

if (status != null) {

response.setStatusCode(status);

} else if (response instanceof AbstractServerHttpResponse) {

// https://jira.spring.io/browse/SPR-16748

((AbstractServerHttpResponse) response).setStatusCodeValue(res.status().code());

} else {

throw new IllegalStateException("Unable to set status code on response: " +res.status().code()+", "+response.getClass());

}

// Defer committing the response until all route filters have run

// Put client response as ServerWebExchange attribute and write response later NettyWriteResponseFilter

exchange.getAttributes().put(CLIENT_RESPONSE_ATTR, res);

}).then(chain.filter(exchange));

}

3.1.7 Netty 回写响应网关过滤器

org.springframework.cloud.gateway.filter.NettyWriteResponseFilter#filter NettyWriteResponseFilter 与NettyRoutingFilter成对出现，负责将代理响应写回网关客户端响应。

public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {

// then方法实现After Filter逻辑

return chain.filter(exchange).then(Mono.defer(() -> {

// 获得Netty Response

HttpClientResponse clientResponse = exchange.getAttribute(CLIENT_RESPONSE_ATTR);

if (clientResponse == null) {

return Mono.empty();

}

log.trace("NettyWriteResponseFilter start");

ServerHttpResponse response = exchange.getResponse();

// 将Netty Response回写给客户端

NettyDataBufferFactory factory = (NettyDataBufferFactory) response.bufferFactory();

//TODO: what if it's not netty

final Flux<NettyDataBuffer> body = clientResponse.receive()

.retain() // ByteBufFlux => ByteBufFlux

.map(factory::wrap); // ByteBufFlux => Flux<NettyDataBuffer> 

MediaType contentType = response.getHeaders().getContentType();

return (isStreamingMediaType(contentType) ?

response.writeAndFlushWith(body.map(Flux::just)) : 

response.writeWith(body));

}));

}

4. Spring Cloud Gateway进行监控

当最终将网关也进行监控，可以全局的看到应用请求流转，网关的请求业务分流，各应用的调用负载情况。

 

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来源：oschina

链接：https://my.oschina.net/harmonycloud0/blog/3185222