1.权重路由

1.1 权重路由使用场景

在开发或者测试的时候，或者线上发布，线上服务多版本控制的时候，需要对服务提供权重路由，最常见的使用就是，一个服务有两个版本，旧版本V1，新版本v2。在线上灰度的时候，需要通过网关动态实时推送，路由权重信息。比如95%的流量走服务v1版本，5%的流量走服务v2版本。

issue: The Spring Cloud Gateway issue of Allow Rolling Deployments https://github.com/spring-cloud/spring-cloud-gateway/issues/67

1.2 Spring Cloud Gateway权重路由原理

Spring Cloud Gateway中提供了org.springframework.cloud.gateway.handler.predicate.WeightRoutePredicateFactory去实现根据分组设置权重进行路由，因此使用起来相对比较简单，有兴趣的可以debug阅读源码。

public class WeightRoutePredicateFactory extends AbstractRoutePredicateFactory<WeightConfig> implements ApplicationEventPublisherAware {

    private static final Log log = LogFactory.getLog(WeightRoutePredicateFactory.class);

    public static final String GROUP_KEY = WeightConfig.CONFIG_PREFIX + ".group";
    public static final String WEIGHT_KEY = WeightConfig.CONFIG_PREFIX + ".weight";

    private ApplicationEventPublisher publisher;

    public WeightRoutePredicateFactory() {
        super(WeightConfig.class);
    }

    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher publisher) {
        this.publisher = publisher;
    }

    @Override
    public List<String> shortcutFieldOrder() {
        return Arrays.asList(GROUP_KEY, WEIGHT_KEY);
    }

    @Override
    public String shortcutFieldPrefix() {
            return WeightConfig.CONFIG_PREFIX;
    }

    @Override
    public void beforeApply(WeightConfig config) {
        if (publisher != null) {
            publisher.publishEvent(new WeightDefinedEvent(this, config));
        }
    }

    @Override
    public Predicate<ServerWebExchange> apply(WeightConfig config) {
        return exchange -> {
            Map<String, String> weights = exchange.getAttributeOrDefault(WEIGHT_ATTR,
                    Collections.emptyMap());

            String routeId = exchange.getAttribute(GATEWAY_PREDICATE_ROUTE_ATTR);

            // all calculations and comparison against random num happened in
            // WeightCalculatorWebFilter
            String group = config.getGroup();
            if (weights.containsKey(group)) {

                String chosenRoute = weights.get(group);
                if (log.isTraceEnabled()) {
                    log.trace("in group weight: "+ group + ", current route: " + routeId +", chosen route: " + chosenRoute);
                }

                return routeId.equals(chosenRoute);
            }

            return false;
        };
    }
}

2.Spring Cloud Gateway中的权重路由案例

2.1 案例代码地址

https://github.com/SoftwareKing/sc-gateway/tree/master/ch4

2.2 Spring Cloud Gateway Server说明

Spring Cloud Gateway will dispatch 95% of the requests to version 1 and 5% of the traffic to version 2 of a specified service, as shown by the following figure.

我们通过在Spring Cloud Gateway中会配置不同的权重信息到不同URL上，Spring Cloud Gateway会根据我们配置的路由权重信息，将请求分发到不同的源服务组，权重信息如ch4/ch4-gateway中的application.yml所示，主要配置信息如下。

spring:
  application:
    name: ch4-gateway
  cloud:
    gateway:
      routes:

      - id: service1_v1
        uri: http://localhost:8081/v1
        predicates:
        - Path=/test
        - Weight=service1, 95

      - id: service1_v2
        uri: http://localhost:8081/v2
        predicates:
        - Path=/test
        - Weight=service1, 5

Weight=service1, 95，Weight=service1, 5就是路由的权重信息。

2.3 源服务

源服务在本案例中源服务如ch4-service-provider所示，主要提提供Gateway Server权重路由对应的后端源服务。因为比较简单因此不做详细说明，主要代码如下所示。

package org.xujin.sc.service;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import reactor.core.publisher.Mono;

@RestController
public class ServiceController {

    @RequestMapping(value = "/v1", produces = "text/plain;charset=UTF-8")
    public Mono<String> v1() {
        return Mono.just("v1");
    }

    @RequestMapping(value = "/v2", produces = "text/plain;charset=UTF-8")
    public Mono<String> v2() {
        return Mono.just("v2");
    }
}

2.4 测试

分别启动ch4-gateway，ch4-service-provider进行访问:http://localhost:8080/test 测试,发现会根据所设权重进行路由。