In the previous  Microservice Tutorial ,we have learned about How to use Zuul API gateway. In this Tutorial , we will learn about Hystrix which act as a Circuit breaker of the services. Circuit breaker — the term is new to you in terms of Software Architecture? Don’t  worry I will discuss in detail regarding the same.
But before that Let’s discuss with a well-known incident who are working in Support Project(Monolith).
Birth of the Night’s King:

Folks who are in On call Support how many times it happens, You got a call in the night saying, System is not responding, it is a priority 1 issue. You wake up in odd time opens your laptop,
Check health check pages found some servers are down, Some servers have a huge memory spike.So immediately you take a thread dump and all the necessary details then restarted all the server in the pool. After restarting you will found things are quite normal and go to sleep if you are lucky enough then you got a good sleep but if you unlucky again in the morning you may face the same scenario.

So, Next day when you and your team researching why this happened , what is the root cause of the birth of White walkers, which ate up all precious resources and the server eventually got unresponsive.

You may find there is a resource leak in somewhere may be in code level–Someone forgot to close a precious resource, like connection. Or there were an unnecessary open threads. Or there is a blocking session in the database etc.

But hold on why we can’t find this resource leak/birth of Night King at the first time? Why the Night’s king grows up silently and when he is in action then we got notified?

So, It opens our eyes that there is a problem in our Architecture(King’s Landing), there are no techniques for early detection of a Resource leak( No Jon Snow to Watch the Wall!!!).
A practical Scenario.
Let examine a simple scenario which may cause this type of scenario, Say we have an architecture where service A and Service B dependent on Service C.  Both Service A and B query  Service C API to get some result. Now Service C is used the underlying database to fetch result but unfortunately, programmer does not close the connection in finally block he does it in the try block.

Now in production, if any error occurs in Service C regarding Database connection/query, It does not release the connection so Connections are not back in Connection pools(Connection pool has finite resources).  But Service A and B does not aware of this scenario it queries Service C as the request comes and Service C ate up one by one free Connection from Connections pool. So after a certain time, all Connections are eaten up by Service C and there is no connection available as free in Connection pool and Night Walkers(Service C) eaten up your System. After restarting all the server its gives you relief for sometimes but if the Service C error continues (Programming fault) then again you might have to wake up in the morning (Night King’s is back).

It all happens due to Service A and B, They are not aware the Service C is not responding the way it should be. If they aware they just simply stop the querying then We should not have faced this situation. Here the concept of the Circuit breaker(In GOT NightWatch) comes up.

Resourse Leak--Birth of Night King


Circuit Breaker Pattern :

The Circuit breaker concept is same as an electrical circuit When the Circuit is closed electrons flow through the circuit but if any unusual thing happens it trips the circuit, Circuit is opened up so there is no flow of electrons through the circuit. It provides the circuit to recover itself after a certain amount of time, Circuit closes and flows of the electrons continues.

Netflix hystrix is such a framework which works on the same principle.

It always monitoring the calls so if any dependent service response is greater than the threshold limit it trips the circuit,  so no further calls will not flow to the dependent service. It gives dependent service to recover itself. In that time there is a fallback policy, all the request goes to that fallback path. After a certain amount of time again the circuit is closed and request flows as it is.

Please note that we can enable Hystrix(Jon Snow– The King of North) in Spring cloud. previously, it supports only Service and Component level @Service or @Component. With the latest, it supports in @Controller also.
Hystrinx As JonSnow
Coding Time:

Lets recap the EmployeeDashBoardService , It calls EmployeeSearchService to find employee based on the id. Currently ig EmployeeSearch service is unavailable then EmployeeDashBoard Service does not got the result and show the error. But we want to show a Default Employee Value if EmployeeSeracgservice is not available so to incorporate the change in EmployeeDashboardService we have to do the following changes.
Step 1 : Add Hystrix plugin into pom.xml

<dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-hystrix</artifactId>
</dependency>
Step 2 : Add @EnableCircuitBreaker on top of  EmployeeDashBoardService, to enable Hystrix for this service.

package com.example.EmployeeDashBoardService;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.feign.EnableFeignClients;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@EnableDiscoveryClient
@EnableCircuitBreaker
@EnableFeignClients
@SpringBootApplication
public class EmployeeDashBoardService {

   public static void main(String[] args) {
      SpringApplication.run(EmployeeDashBoardService.class, args);
   }

   @Bean
   public RestTemplate restTemplate(RestTemplateBuilder builder) {
      return builder.build();
   }
}
Step 3 :  Now we will change the EmployeeInfoController.java so it can be Hystrix enable.

package com.example.EmployeeDashBoardService.controller;

import java.util.Collection;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.cloud.context.config.annotation.RefreshScope;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

import com.example.EmployeeDashBoardService.domain.model.EmployeeInfo;
import com.netflix.appinfo.InstanceInfo;
import com.netflix.discovery.EurekaClient;
import com.netflix.discovery.shared.Application;
import com.netflix.hystrix.contrib.javanica.annotation.HystrixCommand;

@RefreshScope
@RestController
public class EmployeeInfoController {
   
    @Autowired
    private RestTemplate restTemplate;
   
    @Autowired
    private EurekaClient eurekaClient;
   
    @Value(“${service.employyesearch.serviceId}”)
    private String employeeSearchServiceId;
   @RequestMapping(“/dashboard/{myself}”)
   @HystrixCommand(fallbackMethod=”defaultMe”)
   public EmployeeInfo findme(@PathVariable Long myself){
      Application application = eurekaClient.getApplication(employeeSearchServiceId);
       InstanceInfo instanceInfo = application.getInstances().get(0);
       String url = “http://”+instanceInfo.getIPAddr()+ “:”+instanceInfo.getPort()+”/”+”employee/find/”+myself;
       System.out.println(“URL” + url);
       EmployeeInfo emp = restTemplate.getForObject(url, EmployeeInfo.class);
       System.out.println(“RESPONSE ” + emp);
       return emp;
   }
   
   private EmployeeInfo defaultMe(Long id){
      EmployeeInfo info = new EmployeeInfo();
      info.setEmployeeId(id);
      info.setName(“Hystrix fallback”);
      info.setCompanyInfo(“Netfilx”);
      info.setDesignation(“Fallback”);
      return info;
   }
   
   
   @RequestMapping(“/dashboard/peers”)
   public  Collection<EmployeeInfo> findPeers(){
      Application application = eurekaClient.getApplication(employeeSearchServiceId);
       InstanceInfo instanceInfo = application.getInstances().get(0);
       String url = “http://”+instanceInfo.getIPAddr()+ “:”+instanceInfo.getPort()+”/”+”employee/findall”;
       System.out.println(“URL” + url);
       Collection<EmployeeInfo> list= restTemplate.getForObject(url, Collection.class);
        System.out.println(“RESPONSE ” + list);
       return list;
   }
}
Carefully note the method named findme, It actually calls the EmployeeService, So I use a
@HystrixCommand(fallbackMethod=”defaultMe”) annotation on top of this method, by doing we instruct Spring to proxy this method, so that if any error will occur or Employee Service is not available it goes through the fallback method and called it, and shows the default value rather than showing an error

For that, we add the attribute fallbackmethod=defaultMe where default me is the default method, Please note that method signature and return type must be the same of the findme method. Unless you facing an error no Such method found. It internally uses Spring AOP which intercept the method call.

If the EmployeeService is not available then it calls defaultMe Method and returns the default employee.

Let’s check the same,

Start Config server, Eureka server, and EmployeeDashBoard service, intentionally I not started the EmployeeSearchService so it is unavailable when we call findme method

If you hit the following URL

http://localhost:8081/dashboard/2

You will see the following response as the Actual EmployeeSearchService is down.

{
  “employeeId”:2,
  “name”:”Hystrix fallback”,
  “practiceArea”:null,
  “designation”:”Fallback”,
  “companyInfo”:”Netfilx”
}

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Microservices Communication: Hystrix As The Jon Snow

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