In the previous Microservice tutorial , we have learned How to communicate with other Microservice using Feign as a REST client and Eureka server as a Service discovery.
In all cases, We consider only one instance of a Microservice-- which calls another instance of dependent Microservice(EmployeeDasBoard service call to EmployeeSearch service).
This is good for demo purpose or when you are practicing How to develop Microservice.
In production, Certainly it is not the case-- we break Monolith application to Microservice applications because we can scale each service based on the payload. So Single instance of a service is unimaginable in production-- so what we generally do is, using a load balancer which balancing the payload among multiple instances of a service.
Before digging into Ribbon the Client side Load Balancer for Microservice architecture, Let discuss How our old fashioned Java EE services AKA Monolith maintains Load balancing.
Server Side Load Balancing : In java EE architecture we deploy our war/ear files into multiple application servers, then we create a pool of server and put a load balancer(Netscaler)in front of it. Which has a public IP. The client makes a request using that public IP and Netscaler decides in which internal application server it forwards the request by Round robin or Sticky session algorithm. We call it Server side load balancing.
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| Server Side Load Balancing |
Problem : The problem of server side load balancing is if one or more servers stop responding we have to manually remove those servers from Load balancer by updating IP table of the Load balancer.
Another problem is we have to implement failover policy to provide the client a seamless experience.
But Microservice not using the server side load balancing. It uses client side Load balancing.
Client side Load Balancing : To understand Client Side Load balancing let's recap the Microservice architecture. We generally create a Service discovery like Eureka or Consul where each service instance register when bootstrapped. Eureka server maintains a Service registry, it maintains all the instances of the service as Key/value map.Where {service id} of your Microservice serves as Key and instance serve as Value. Now if one Microservice wants to communicate other Microservice it generally looks up the service registry using DiscoveryClient and Eureka server returns all the instances of the calling Microservices to the caller service. Now it is Caller service headache which instance it calls. Here Client side Load balancing stepped in. Client side Load Balancer maintains Algorithm like Round robin or Zone specific by which it can invoke instances of calling services. The advantage is as Service registry always updated itself if one instance goes down it removes it from its registry so When Client side Load balancer talks to Eureka server it always updates itself so there is no manual intervention unlike server side load balancing to remove an Instance.
Another Advantage is as Load balancer is in client side you can control its Load balancing algorithm programmatically.
Ribbon provides this facility so we will use Ribbon for Client side Load balancing.
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| Client Side Load Balancing |
Coding Time
We will configure Ribbon in Our EmployeeDashBoradService which will communicate with Eureka to fetch EmployeeSearchservice instances.
Step 1: To enable Ribbon in EmployeeDashBoard we have to add the following dependency in pom.xml
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-ribbon</artifactId>
</dependency>
Step 2: Now we have to Enable Ribbon so it can Load balance the EmployeeSerach Application so for that we need to put @RibbonClient(name="EmployeeSearch") on top of the EmployeeServiceProxy interface. By doing this we instruct Spring boot to communicate Eureka server and get the list of instances for service id EmployeeSerach. Please note that this is the {service-id} for the Employeeserach application.
package com.example.EmployeeDashBoardService.controller;
import java.util.Collection;
import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.cloud.netflix.ribbon.RibbonClient;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import com.example.EmployeeDashBoardService.domain.model.EmployeeInfo;
@FeignClient(name="EmployeeSearch" )
@RibbonClient(name="EmployeeSearch")
public interface EmployeeServiceProxy {
@RequestMapping("/employee/find/{id}")
public EmployeeInfo findById(@PathVariable(value="id") Long id);
@RequestMapping("/employee/findall")
public Collection<EmployeeInfo> findAll();
}
Our Ribbon Client is ready now.
Testing time:
Start Configserver and Eureka server first.
Then Start EmployeeService it will up on port 8080 as we mentioned in bootstrap.preoperties.
Now Run another instance but this time starts with -Dserver.port=8082 so another instance up on 8082 port.
After that run the EmployeeDashBoard service.
Now check the Eureka server GUI it will look like following
Now if you hit the following URL
You can see the following response
{
"employeeId": 1,
"name": "Shamik Mitra",
"practiceArea": "Java",
"designation": "Architect",
"companyInfo": "Cognizant"
}
Now open the EmployeedashBorad Console you can see following lines are printed in console
DynamicServerListLoadBalancer for client EmployeeSearch initialized: DynamicServerListLoadBalancer:{NFLoadBalancer:name=EmployeeSearch,current list of Servers=[192.168.0.103:8080, localhost:8082],Load balancer stats=Zone stats: {defaultzone=[Zone:defaultzone; Instance count:2; Active connections count: 0; Circuit breaker tripped count: 0; Active connections per server: 0.0;]
},Server stats: [[Server:localhost:8082; Zone:defaultZone; Total Requests:0; Successive connection failure:0; Total blackout seconds:0; Last connection made:Thu Jan 01 05:30:00 IST 1970; First connection made: Thu Jan 01 05:30:00 IST 1970; Active Connections:0; total failure count in last (1000) msecs:0; average resp time:0.0; 90 percentile resp time:0.0; 95 percentile resp time:0.0; min resp time:0.0; max resp time:0.0; stddev resp time:0.0]
, [Server:192.168.0.103:8080; Zone:defaultZone; Total Requests:0; Successive connection failure:0; Total blackout seconds:0; Last connection made:Thu Jan 01 05:30:00 IST 1970; First connection made: Thu Jan 01 05:30:00 IST 1970; Active Connections:0; total failure count in last (1000) msecs:0; average resp time:0.0; 90 percentile resp time:0.0; 95 percentile resp time:0.0; min resp time:0.0; max resp time:0.0; stddev resp time:0.0]
]}ServerList:org.springframework.cloud.netflix.ribbon.eureka.DomainExtractingServerList@a1df28c
2017-08-04 22:56:47.180 INFO 3293 --- [erListUpdater-0] c.netflix.config.ChainedDynamicProperty : Flipping property: EmployeeSearch.ribbon.ActiveConnectionsLimit to use NEXT property: niws.loadbalancer.availabilityFilteringRule.activeConnectionsLimit = 2147483647
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