spring-cloud-gateway filter

The LoadBalancerClient Filter

LoadBlancerClientFilter

The LoadBalancerClientFilter looks for a URI in the exchange attribute named ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a scheme of lb (such as lb://myservice), it uses the Spring Cloud LoadBalancerClient to resolve the name (myservice in this case) to an actual host and port and replaces the URI in the same attribute. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute. The filter also looks in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR attribute to see if it equals lb. If so, the same rules apply. The following listing configures a LoadBalancerClientFilter:

application.yml

spring:
  cloud:
    gateway:
      routes:
      - id: myRoute
        uri: lb://service
        predicates:
        - Path=/service/**

/**
 * @deprecated in favour of {@link ReactiveLoadBalancerClientFilter}
 * @author Spencer Gibb
 * @author Tim Ysewyn
 */
@Deprecated
public class LoadBalancerClientFilter implements GlobalFilter, Ordered {

	/**
	 * Filter order for {@link LoadBalancerClientFilter}.
	 */
	public static final int LOAD_BALANCER_CLIENT_FILTER_ORDER = 10100;

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

	protected final LoadBalancerClient loadBalancer;

	private LoadBalancerProperties properties;

	public LoadBalancerClientFilter(LoadBalancerClient loadBalancer,
			LoadBalancerProperties properties) {
		this.loadBalancer = loadBalancer;
		this.properties = properties;
	}

	@Override
	public int getOrder() {
		return LOAD_BALANCER_CLIENT_FILTER_ORDER;
	}

	@Override
	@SuppressWarnings("Duplicates")
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
	    // lb://order-service/myprefix/102/order?id=180
		URI url = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
		String schemePrefix = exchange.getAttribute(GATEWAY_SCHEME_PREFIX_ATTR);
		// 未配置 或者非lb开头的
		if (url == null
				|| (!"lb".equals(url.getScheme()) && !"lb".equals(schemePrefix))) {
			return chain.filter(exchange);
		}
		// preserve the original url
		addOriginalRequestUrl(exchange, url);

		if (log.isTraceEnabled()) {
			log.trace("LoadBalancerClientFilter url before: " + url);
		}
        // 负载均衡  得到具体的1台机器 
		final ServiceInstance instance = choose(exchange);

		if (instance == null) {
			throw NotFoundException.create(properties.isUse404(),
					"Unable to find instance for " + url.getHost());
		}
		
		URI uri = exchange.getRequest().getURI();

		// if the `lb:<scheme>` mechanism was used, use `<scheme>` as the default,
		// if the loadbalancer doesn't provide one.
		String overrideScheme = instance.isSecure() ? "https" : "http";
		if (schemePrefix != null) {
			overrideScheme = url.getScheme();
		}
		// 替换后具体的服务
		URI requestUrl = loadBalancer.reconstructURI(
				new DelegatingServiceInstance(instance, overrideScheme), uri);

		if (log.isTraceEnabled()) {
			log.trace("LoadBalancerClientFilter url chosen: " + requestUrl);
		}

		exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
		return chain.filter(exchange);
	}

	protected ServiceInstance choose(ServerWebExchange exchange) {
		return loadBalancer.choose(
				((URI) exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR)).getHost());
	}

}