Use Case
My use case is very simple - I want to specify the URL(s) that a rest call is invoked against. This may appear straightforward to start with however there are a few catches to consider. By default if Spring Cloud sees Eureka related libraries in the classpath the behavior is to use Eureka to discover the instances of a service and loadbalance across the instances.Approach 1 - Use a non-loadbalanced Rest Template
An approach that will work is to use an instance of RestTemplate that does not use Ribbon at all:@Bean
public RestOperations nonLoadbalancedRestTemplate() {
return new RestTemplate();
}
Now, where you need the Rest Template, you can inject this instance in, knowing that Spring Cloud also would have instantiated another instance that supports Eureka, so this injection will have to be done by name this way:
@Service("restTemplateDirectPongClient")
public class RestTemplateDirectPongClient implements PongClient {
private final RestOperations restTemplate;
@Autowired
public RestTemplateDirectPongClient(@Qualifier("nonLoadbalancedRestTemplate") RestOperations restTemplate) {
this.restTemplate = restTemplate;
}
...
}
The big catch with the approach however is now that we have bypassed Ribbon all the features that Ribbon provides are lost - we would not have features like automatic retry, read and connect timeouts, loadbalancing in case we had multiple urls. So a better approach may be the following.
Approach 2 - Customize Ribbon based Rest Template
In the earlier blog post I had shown some basic customization of Ribbon which can be made using a configuration file:samplepong:
ribbon:
DeploymentContextBasedVipAddresses: sample-pong
ReadTimeout: 5000
MaxAutoRetries: 2
All the customizations that you would normally do through a configuration file for Ribbon however do not carry over, in this specific instance I want to use a list of server instances that I specify instead of letting Ribbon figure out via a Eureka call. Using raw ribbon it is specified the following way:
samplepong.ribbon.NIWSServerListClassName=com.netflix.loadbalancer.ConfigurationBasedServerList samplepong.ribbon.listOfServers=127.0.0.1:8082
This specific configuration will not work with Spring Cloud however, the way to specify a list of servers is by specifying a configuration file along these lines:
package org.bk.noscan.consumer.ribbon;
import com.netflix.client.config.IClientConfig;
import com.netflix.loadbalancer.ConfigurationBasedServerList;
import com.netflix.loadbalancer.Server;
import com.netflix.loadbalancer.ServerList;
import org.springframework.cloud.netflix.ribbon.RibbonClientConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class PongDirectCallRibbonConfiguration extends RibbonClientConfiguration {
@Bean
@Override
public ServerList<Server> ribbonServerList(IClientConfig clientConfig) {
ConfigurationBasedServerList serverList = new ConfigurationBasedServerList();
serverList.initWithNiwsConfig(clientConfig);
return serverList;
}
}
and telling Ribbon to use this configuration for the specific "named client" that we are concerned about:
@RibbonClients({
@RibbonClient(name = "samplepongdirect", configuration = PongDirectCallRibbonConfiguration.class),
})
With this configuration in place, the list of servers can now be specified using configuration this way:
samplepongdirect:
ribbon:
DeploymentContextBasedVipAddresses: sample-pong
listOfServers: localhost:8082
ReadTimeout: 5000
MaxAutoRetries: 2
One thing to note is that since the Ribbon Configuration is a normal Spring configuration it will likely get picked up as part of the @ComponentScan annotation, since this is very specific for Ribbon we would not want this configuration to be picked up this way. I have avoided that by specifying a package not in the normal classpath scan "org.bk.noscan.*" package!, I am not sure if there is another clean way to do this but this approach has worked well for me.
This approach is little more extensive than the first approach, however the advantage is that once this is in place all the features of Ribbon carry over.
