Monthly Archives: October 2021

Spring Cloud Tutorial for Beginners

What is Spring Cloud? In this post, I will cover Spring Cloud Tutorial for beginners. If you are new to Spring Framework, I will suggest you start with Spring Boot and Microservices and Simplifying Spring Security.

As the official documentation on the Spring website says:

“Spring Cloud provides tools for developers to quickly build common patterns in distributed systems – configuration management, service discovery,  circuit breakers, intelligent routing, microproxy, control bus, one-time tokens”

Spring Cloud Tutorial - The Complete Guide

  • What is Spring Cloud?
  • Spring Cloud Features
  • Spring Cloud Example in action
  • Conclusion

What is Spring Cloud?

Spring Cloud provides readymade patterns to develop distributed system applications. Most of these patterns are common when building such applications.

One example is when there are multiple microservices and they interact with each other. You have to secure each service. Each service communicates with other services securely. Henceforth, how to secure these services? How do they communicate securely? And how do they get deployed seamlessly? What are the other automation tasks used for different requirements?

Using Spring Cloud, a developer can quickly build an application that implements these design patterns and deploy the application on cloud platforms( like Heroku or Cloud Foundry).

Spring Cloud Features

Spring framework is fundamental to building a Spring Cloud application. So what are the different features that Spring Cloud added?

Service Registration and Discovery

Spring Boot became popular with microservice architecture. When you have multiple services interacting with each other, you need a service to register each service, this is mostly Configuration Service. Then you need a discovery service to find other services.

Distributing Messaging

Basically, Spring cloud provides different tools to make our microservice-based architecture successful. Spring Boot helps rapid development of these applications. Spring Cloud assists in coordinating and deploying these applications. One such feature with Spring Cloud is distributed messaging.

Microservices communicate synchronously or asynchronously. Overall, Spring Cloud Bus offers a message broker that links nodes of a distributed system. Equally, Spring Cloud Stream offers a framework to build event-driven microservices. Nevertheless, this feature works well with messaging services like Kafka or ActiveMQ.

Service to Service Communication

Spring Cloud provides a feature for service-to-service communication. Usually, the flow goes like this

  • Register the service
  • Fetch the registry
  • Find the target downstream service
  • Call the REST endpoint of that service

Distributed Configuration

Particularly, the spring cloud config server allows externalized configuration on the client-side for the distributed systems.

Other than these features, Spring Cloud provides tools to build resilient and robust services. One such tool is circuit breakers.

As an illustration, we will create two microservices and one microservice will call another. We will use the feature of registry service (from Spring Cloud) to register these microservices.

Spring Cloud Example in Action

Build Eureka Server for Registry Service

First, we will create a service that will use the Eureka service and act as a registry service. As a result, add the following dependency in a new Spring Boot application:

plugins {
	id 'org.springframework.boot' version '2.5.5'
	id 'io.spring.dependency-management' version '1.0.11.RELEASE'
	id 'java'
}

group = 'com.betterjavacode'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
	mavenCentral()
}

ext {
	set('springCloudVersion', "2020.0.4")
}

dependencies {
	implementation 'org.springframework.cloud:spring-cloud-starter-netflix-eureka-server'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

dependencyManagement {
	imports {
		mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
	}
}

test {
	useJUnitPlatform()
}

Once we have that dependency, we can enable the eureka server in our main class.

package com.betterjavacode.eurekaserver;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@SpringBootApplication
@EnableEurekaServer
public class EurekaserverApplication {

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

}

Add the following properties to application.yml

server:
  port: 7000

# Discovery Server Access
eureka:
  instance:
    hostname: localhost
  client:
    registerWithEureka: false
    fetchRegistry: false
  serviceUrl:
    defaultZone: http://${eureka.instance.hostname}:${server.port}/eureka/

The properties eureka.instance.client.register-with-eureka=false and eureka.instance.client.fetch-registry=false indicates that this is a registry server and won’t use itself to register.

A microservice to return products

In order to show how we will use the registry service as part of the entire Spring Cloud integration, we will create a new microservice. This REST-based microservice will return a list of products.

plugins {
	id 'org.springframework.boot' version '2.5.5'
	id 'io.spring.dependency-management' version '1.0.11.RELEASE'
	id 'java'
}

group = 'com.betterjavacode'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
	mavenCentral()
}

ext {
	set('springCloudVersion', "2020.0.4")
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter-web'
	implementation 'org.springframework.cloud:spring-cloud-starter-netflix-eureka-client'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

dependencyManagement {
	imports {
		mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
	}
}

test {
	useJUnitPlatform()
}

With this in mind, RESTController for this service will look like below:

package com.betterjavacode.productservice.controllers;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.ArrayList;
import java.util.List;

@RestController
public class ProductController
{
    @GetMapping("/products")
    public List getAllProducts ()
    {
        List products = new ArrayList<>();
        products.add("Shampoo");
        products.add("Soap");
        products.add("Cleaning Supplies");
        products.add("Dishes");

        return products;
    }
}

And the application.yml file for this application will be like this

spring:
  application:
    name: product-service

server:
  port: 8083

eureka:
  client:
    registerWithEureka: true
    fetchRegistry: true
    serviceUrl:
      defaultZone: http://localhost:7000/eureka/
  instance:
    hostname: localhost

Here we have eureka.client.registerWithEureka=true and eureka.client.fetchRegistry=true as we want our service to be registered with our Eureka server running registry service. Subsequently, our main class for this service will have an annotation @EnableDiscoveryClient that will allow this service to be discovered by Eureka Server.

Client Service to call Product Service

Now, let’s create another service which will be a client service to product service. It will be very similar to Product Service except it will be based on MVC, so we will use a thymeleaf template to call this service.

package com.betterjavacode.productserviceclient.controllers;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.discovery.DiscoveryClient;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.client.RestTemplate;

import java.util.List;

@Controller
public class ProductController
{
    @Autowired
    private DiscoveryClient discoveryClient;

    @GetMapping("/")
    public String home(Model model)
    {
        List serviceInstances = discoveryClient.getInstances("product" +
                "-service");

        if(serviceInstances != null && !serviceInstances.isEmpty())
        {
            ServiceInstance serviceInstance = serviceInstances.get(0);
            String url = serviceInstance.getUri().toString();
            url = url + "/products";
            RestTemplate restTemplate = new RestTemplate();
            List products = restTemplate.getForObject(url, List.class);
            model.addAttribute("products", products);
        }

        return "home";
    }
}

application.yml for this service will look like below:


spring:
  application:
    name: product-service-client

server:
  port: 8084


eureka:
  client:
    registerWithEureka: true
    fetchRegistry: true
    serviceUrl:
      defaultZone: http://localhost:7000/eureka/
  instance:
    hostname: localhost

Thymeleaf template for home will basically list the products in a table form.

Run the services

Shortly, run all the services – starting with Eureka server, product service and product-service-client. Now if we access eureka server, we will see the list of services registered with it as below:

Spring Cloud Tutorial for Beginners

You can see both services registered. And if we access our product-service-client application at http://localhost:8084/, we will see the list of products

Spring Cloud Tutorial for Beginners - ProductServiceClient

At last, we saw a simple demo of using Eureka server as a registry service with Spring Cloud. If you want to learn more about Spring Cloud Config, I definitely recommend this course Distributed configuration with Spring Cloud Config from udemy.

Conclusion

In this post, we learned about Spring Cloud. There are a number of features to evaluate in Spring Cloud. I have covered only a feature that most developers have to use while using Spring Cloud. A developer can also combine Spring Cloud Function with AWS Lambda to learn more about Spring Cloud.

If you are still looking to learn about Spring Security, you can my book here.

Note – Links for Udemy or Educative courses are affiliate links. If you end up buying those courses, I get a percentage of the total price. I also recommend only those courses that I have taken or have learned about that topic myself.

Details of Liskov Substitution Principle Example

In this post, I will cover the details of the Liskov Substitution Principle(LSP) with an example. This is a key principle to validate the object-oriented design of your system. Hopefully, you will be able to use this in your design and find out if there are any violations. You can learn more about other object oriented design principles. Let’s start with the basics of Liskov Substitution Principle first.

Liskov Substitution Principle (LSP)

Basically, the principle states that if in an object-oriented program you substitute superclass object reference with any of its subclass objects, it should not break the program.

Wikipedia definition says – If S is a subtype of T, then the objects of type T may be replaced with objects of S without altering any of the desirable properties of the program.

LSP comes into play when you have super-sub class OR interface implementation type of inheritance. Usually, when you define a superclass or an interface, it is a contract. Any inherited object from this superclass or interface implementation class must follow the contract. Any of the objects that fail to follow the contract, will violate the Liskov Substitution Principle. If you want to learn more about Object-Oriented Design, get this course from educative.

Let’s take a simple look before we look at this in detail.


public class Bird
{
    void fly()
    {
       // Fly function for bird
    }
}

public class Parrot extends Bird
{
    @Override
    void fly()
    {

    }
}

public class Ostrich extends Bird
{
   // can't implement fly since Ostrich doesn't fly
}

If you look at the above classes, Ostrich is not a bird. Technically, we can still implement the fly method in Ostrich class, but it will be without implementation or throwing some exception. In this case, Ostrich is violating LSP.

Object-Oriented Design can violate the LSP in the following circumstances:

  1. If a subclass returns an object that is completely different from what the superclass returns.
  2. If a subclass throws an exception that is not defined in the superclass.
  3. There are any side effects in subclass methods that were not part of the superclass definition.

How do programmers break this principle?

Sometimes, if a programmer ends up extending a superclass without completely following the contract of the superclass, a programmer will have to use instanceofcheck for the new subclass. If there are more similar subclasses are added to the code, it can increase the complexity of the code and violate the LSP.

Supertype abstract intends to help programmers, but instead, it can end up hindering and add more bugs in the code. That’s why it is important for programmers to be careful when creating a new subclass of a superclass.

Liskov Substitution Principle Example

Now, let’s look at an example in detail.

Many banks offer a basic account as well as a premium account. They also charge fees for these premium accounts while basic accounts are free. So, we will have an abstract class to represent BankAccount.

public abstract class BankAccount
{
   public boolean withDraw(double amount);

   public void deposit(double amount);

}

The class BankAccount has two methods withDraw and deposit.

Consequently, let’s create a class that represents a basic account.


public class BasicAccount extends BankAccount
{
    private double balance;

    @Override
    public boolean withDraw(double amount)
    {
       if(balance > 0)
       {
           balance -= amount;
           if(balance < 0)
           {
              return false;
           }
           else 
           {
              return true;
           }
       }
       else
       {
          return false;
       } 
    }

    @Override
    public void deposit(double amount)
    {
       balance += amount;
    }
}

Now, a premium account is a little different. Of course, an account holder will still be able to deposit or withdraw from that account. But with every transaction, the account holder also earns rewards points.


public class PremiumAccount extends BankAccount
{
   private double balance;
   private double rewardPoints;

   @Override
   public boolean withDraw(double amount)
   {
      if(balance > 0)
       {
           balance -= amount;
           if(balance < 0)
           {
              return false;
           }
           else 
           {
              return true;
              updateRewardsPoints();
           }
       }
       else
       {
          return false;
       } 
   }
   
   @Override
   public void deposit(double amount)
   {
      this.balance += amount;
      updateRewardsPoints();
   }

   public void updateRewardsPoints()
   {
      this.rewardsPoints++;
   }
}

So far so good. Everything looks ok. If you want to use the same class of BankAccount to create a new investment account that an account holder can’t withdraw from, it will look like below:


public class InvestmentAccount extends BankAccount
{
   private double balance;

   @Override
   public boolean withDraw(double amount)
   {
      throw new Expcetion("Not supported");
   }
   
   @Override
   public void deposit(double amount)
   {
      this.balance += amount;
   }

}

Even though, this InvestmentAccount follows most of the contract of BankAccount, it does not implement withDraw method and throws an exception that is not in the superclass. In short, this subclass violates the LSP.

How to avoid violating LSP in the design?

So how can we avoid violating LSP in our above example? There are a few ways you can avoid violating Liskov Substitution Principle. First, the superclass should contain the most generic information. We will use some other object-oriented design principles to do design changes in our example to avoid violating LSP.

  • Use Interface instead of Program.
  • Composition over inheritance

So now, we can fix BankAccount class by creating an interface that classes can implement.


public interface IBankAccount
{
  boolean withDraw(double amount) throws InvalidTransactionException;
  void deposit(double amount) throws InvalidTransactionException;
}

Now if we create classes that implement this interface, we can also include a InvestingAccount that won’t implement withDraw method.

If you have used Object-oriented programming, you must have heard both the terms composition and inheritance. Also in object-oriented design, composition over inheritance is a common pattern. One can always make objects more generic. Using composition over inheritance can help to avoid violating LSP.

Combine Object-Oriented Design principles with fundamentals of distributed system design and you will be good at system design.

Conclusion

In this post, we talked about Liskov Substitution Principle and its example, how designers usually violate this principle, and how we can avoid violating it.