6. Client Side Load Balancer: Ribbon

To include Ribbon in your project use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-netflix-ribbon. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.

You can configure some bits of a Ribbon client using external properties in <client>.ribbon.*, which is no different than using the Netflix APIs natively, except that you can use Spring Boot configuration files. The native options can be inspected as static fields in CommonClientConfigKey (part of ribbon-core).

Spring Cloud also lets you take full control of the client by declaring additional configuration (on top of the RibbonClientConfiguration) using @RibbonClient. Example:

@Configuration
@RibbonClient(name = "foo", configuration = FooConfiguration.class)
public class TestConfiguration {
}

In this case the client is composed from the components already in RibbonClientConfiguration together with any in FooConfiguration (where the latter generally will override the former).

Warning

The FooConfiguration has to be @Configuration but take care that it is not in a @ComponentScan for the main application context, otherwise it will be shared by all the @RibbonClients. If you use @ComponentScan (or @SpringBootApplication) you need to take steps to avoid it being included (for instance put it in a separate, non-overlapping package, or specify the packages to scan explicitly in the @ComponentScan).

Spring Cloud Netflix provides the following beans by default for ribbon (BeanType beanName: ClassName):

Creating a bean of one of those type and placing it in a @RibbonClient configuration (such as FooConfiguration above) allows you to override each one of the beans described. Example:

@Configuration
protected static class FooConfiguration {
	@Bean
	public ZonePreferenceServerListFilter serverListFilter() {
		ZonePreferenceServerListFilter filter = new ZonePreferenceServerListFilter();
		filter.setZone("myTestZone");
		return filter;
	}

	@Bean
	public IPing ribbonPing() {
		return new PingUrl();
	}
}

This replaces the NoOpPing with PingUrl and provides a custom serverListFilter

A default configuration can be provided for all Ribbon Clients using the @RibbonClients annotation and registering a default configuration as shown in the following example:

@RibbonClients(defaultConfiguration = DefaultRibbonConfig.class)
public class RibbonClientDefaultConfigurationTestsConfig {

	public static class BazServiceList extends ConfigurationBasedServerList {
		public BazServiceList(IClientConfig config) {
			super.initWithNiwsConfig(config);
		}
	}
}

@Configuration
class DefaultRibbonConfig {

	@Bean
	public IRule ribbonRule() {
		return new BestAvailableRule();
	}

	@Bean
	public IPing ribbonPing() {
		return new PingUrl();
	}

	@Bean
	public ServerList<Server> ribbonServerList(IClientConfig config) {
		return new RibbonClientDefaultConfigurationTestsConfig.BazServiceList(config);
	}

	@Bean
	public ServerListSubsetFilter serverListFilter() {
		ServerListSubsetFilter filter = new ServerListSubsetFilter();
		return filter;
	}

}

Starting with version 1.2.0, Spring Cloud Netflix now supports customizing Ribbon clients using properties to be compatible with the Ribbon documentation.

This allows you to change behavior at start up time in different environments.

The supported properties are listed below and should be prefixed by <clientName>.ribbon.:

	Note
	Classes defined in these properties have precedence over beans defined using @RibbonClient(configuration=MyRibbonConfig.class) and the defaults provided by Spring Cloud Netflix.

To set the IRule for a service name users you could set the following:

application.yml. 

users:
  ribbon:
    NIWSServerListClassName: com.netflix.loadbalancer.ConfigurationBasedServerList
    NFLoadBalancerRuleClassName: com.netflix.loadbalancer.WeightedResponseTimeRule

See the Ribbon documentation for implementations provided by Ribbon.

When Eureka is used in conjunction with Ribbon (i.e., both are on the classpath) the ribbonServerList is overridden with an extension of DiscoveryEnabledNIWSServerList which populates the list of servers from Eureka. It also replaces the IPing interface with NIWSDiscoveryPing which delegates to Eureka to determine if a server is up. The ServerList that is installed by default is a DomainExtractingServerList and the purpose of this is to make physical metadata available to the load balancer without using AWS AMI metadata (which is what Netflix relies on). By default the server list will be constructed with "zone" information as provided in the instance metadata (so on the remote clients set eureka.instance.metadataMap.zone), and if that is missing it can use the domain name from the server hostname as a proxy for zone (if the flag approximateZoneFromHostname is set). Once the zone information is available it can be used in a ServerListFilter. By default it will be used to locate a server in the same zone as the client because the default is a ZonePreferenceServerListFilter. The zone of the client is determined the same way as the remote instances by default, i.e. via eureka.instance.metadataMap.zone.

	Note
	The orthodox "archaius" way to set the client zone is via a configuration property called "@zone", and Spring Cloud will use that in preference to all other settings if it is available (note that the key will have to be quoted in YAML configuration).

	Note
	If there is no other source of zone data then a guess is made based on the client configuration (as opposed to the instance configuration). We take eureka.client.availabilityZones, which is a map from region name to a list of zones, and pull out the first zone for the instance’s own region (i.e. the eureka.client.region, which defaults to "us-east-1" for comatibility with native Netflix).

Eureka is a convenient way to abstract the discovery of remote servers so you don’t have to hard code their URLs in clients, but if you prefer not to use it, Ribbon and Feign are still quite amenable. Suppose you have declared a @RibbonClient for "stores", and Eureka is not in use (and not even on the classpath). The Ribbon client defaults to a configured server list, and you can supply the configuration like this

application.yml. 

stores:
  ribbon:
    listOfServers: example.com,google.com

Setting the property ribbon.eureka.enabled = false will explicitly disable the use of Eureka in Ribbon.

application.yml. 

ribbon:
  eureka:
   enabled: false

You can also use the LoadBalancerClient directly. Example:

public class MyClass {
    @Autowired
    private LoadBalancerClient loadBalancer;

    public void doStuff() {
        ServiceInstance instance = loadBalancer.choose("stores");
        URI storesUri = URI.create(String.format("http://%s:%s", instance.getHost(), instance.getPort()));
        // ... do something with the URI
    }
}

Each Ribbon named client has a corresponding child Application Context that Spring Cloud maintains, this application context is lazily loaded up on the first request to the named client. This lazy loading behavior can be changed to instead eagerly load up these child Application contexts at startup by specifying the names of the Ribbon clients.

application.yml. 

ribbon:
  eager-load:
    enabled: true
    clients: client1, client2, client3

If you change zuul.ribbonIsolationStrategy to THREAD, the thread isolation strategy for Hystrix will be used for all routes. In this case, the HystrixThreadPoolKey is set to "RibbonCommand" as default. It means that HystrixCommands for all routes will be executed in the same Hystrix thread pool. This behavior can be changed using the following configuration and it will result in HystrixCommands being executed in the Hystrix thread pool for each route.

application.yml. 

zuul:
  threadPool:
    useSeparateThreadPools: true

The default HystrixThreadPoolKey in this case is same with service ID for each route. To add a prefix to HystrixThreadPoolKey, set zuul.threadPool.threadPoolKeyPrefix to a value that you want to add. For example:

application.yml. 

zuul:
  threadPool:
    useSeparateThreadPools: true
    threadPoolKeyPrefix: zuulgw

If you need to provide your own IRule implementation to handle a special routing requirement like a canary test, you probably want to pass some information to the choose method of IRule.

com.netflix.loadbalancer.IRule.java. 

public interface IRule{
    public Server choose(Object key);
         :

You can provide some information that will be used to choose a target server by your IRule implementation like the following:

RequestContext.getCurrentContext()
              .set(FilterConstants.LOAD_BALANCER_KEY, "canary-test");

If you put any object into the RequestContext with a key FilterConstants.LOAD_BALANCER_KEY, it will be passed to the choose method of IRule implementation. Above code must be executed before RibbonRoutingFilter is executed and Zuul’s pre filter is the best place to do that. You can easily access HTTP headers and query parameters via RequestContext in pre filter, so it can be used to determine LOAD_BALANCER_KEY that will be passed to Ribbon. If you don’t put any value with LOAD_BALANCER_KEY in RequestContext, null will be passed as a parameter of choose method.