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1. Introduction

In this tutorial, we'll understand the basics of creating reactive systems in Java using Spring and other tools and frameworks.

In the process, we'll discuss how reactive programming is just a driver towards creating a reactive system. This will help us understand the rationale for creating reactive systems and different specifications, libraries, and standards it has inspired along the way.

2. What are Reactive Systems?

Over the last few decades, the technology landscape has seen several disruptions that have led to a complete transformation in the way we see value in technology. The computing world before the Internet could never have imagined the ways and means in which it will change our current day.

With the reach of the Internet to the masses and the ever-evolving experience it promises, application architects need to be on their toes to meet their demand.

Fundamentally, this means that we can never design an application in the way we used to earlier. A highly responsive application is no longer a luxury but a necessity.

That, too, is in the face of random failures and unpredictable load. The need of the hour is not just to get the correct result but get to get it fast! It's quite important to drive the amazing user experiences we promise to deliver.

This is what creates the need for an architectural style that can give us Reactive Systems.

2.1. Reactive Manifesto

Back in the year 2013, a team of developers, lead by Jonas Boner came together to define a set of core principles in a document known as the Reactive Manifesto. This is what laid the foundation for an architecture style to create Reactive Systems. Since then, this manifesto has gathered a lot of interest from the developer community.

Basically, this document prescribes the recipe for a reactive system to be flexible, loosely-coupled, and scalable. This makes such systems easy to develop, tolerant of failures, and most importantly highly responsive, the underpinning for incredible user experiences.

So what is this secret recipe? Well, it's hardly any secret! The manifesto defines the fundamental characteristics or principles of a reactive system:

  • Responsive: A reactive system should provide a rapid and consistent response time and hence a consistent quality of service
  • Resilient: A reactive system should remain responsive in case of random failures through replication and isolation
  • Elastic: Such a system should remain responsive under unpredictable workloads through cost-effective scalability
  • Message-Driven: It should rely on asynchronous message passing between system components

These principles sound simple and sensible but aren't always easier to implement in complex enterprise architecture. In this tutorial, we'll develop a sample system in Java with these principles in mind!

3. What is Reactive Programming?

Before we proceed, it's important to understand the difference between reactive programming and reactive systems. We use both these terms quite often and easily misunderstand one for the other. As we've seen earlier, reactive systems are a result of a specific architectural style.

In contrast, reactive programming is a programming paradigm where the focus is on developing asynchronous and non-blocking components. The core of reactive programming is a data stream that we can observe and react to, even apply back pressure as well. This leads to non-blocking execution and hence to better scalability with fewer threads of execution.

Now, this doesn't mean that reactive systems and reactive programming are mutually exclusive. In fact, reactive programming is an important step towards realizing a reactive system, but it's not everything!

3.1. Reactive Streams

Reactive Streams is a community initiative that started back in the year 2013 to provide a standard for asynchronous stream processing with non-blocking backpressure. The objective here was to define a set of interfaces, methods, and protocols that can describe the necessary operations and entities.

Since then, several implementations in multiple programming languages have emerged which conforms to the reactive streams specification. These include Akka Streams, Ratpack, and Vert.x to name a few.

3.2. Reactive Libraries for Java

One of the initial objectives behind the reactive streams was to be eventually included as an official Java standard library. As a result, the reactive streams specification is semantically equivalent to the Java Flow library, introduced in Java 9.

Apart from that, there are a few popular choices to implement reactive programming in Java:

  • Reactive Extensions: Popularly known as ReactiveX, they provide API for asynchronous programming with observable streams. These are available for multiple programming languages and platforms, including Java where it's known as RxJava
  • Project Reactor: This is another reactive library, grounds-up based on the reactive streams specification, targetted towards building non-applications on the JVM. It also happens to be the foundation of the reactive stack in the Spring ecosystem

4. A Simple Application

For the purpose of this tutorial, we'll develop a simple application based on microservices architecture with a minimal frontend. The application architecture should have enough elements to create a reactive system.

For our application, we'll adopt end-to-end reactive programming and other patterns and tools to accomplish the fundamental characteristics of a reactive system.

4.1. Architecture

We'll begin by defining a simple application architecture which doesn't necessarily exhibit the characteristics of reactive systems. From there on, we'll make the necessary changes to achieve these characteristics one by one.

So, first, let's begin by defining a simple architecture:

This is quite a simple architecture that has a bunch of microservices to facilitate a commerce use-case where we can place an order. It also has a frontend for user experience, and all communication happens as REST over HTTP. Moreover, every microservice manages their data in individual databases, a practice known as database-per-service.

We'll go ahead and create this simple application in the following sub-sections. This will be our base to understand the fallacies of this architecture and ways and means to adopt principles and practices so that we can transform this into a reactive system.

4.3. Inventory Microservice

Inventory microservice will be responsible for managing a list of products and their current stock. It will also allow altering the stock as orders are processed. We'll use Spring Boot with MongoDB to develop this service.

Let's begin by defining a controller to expose some endpoints:

@GetMapping
public List<Product> getAllProducts() {
    return productService.getProducts();
}
 
@PostMapping
public Order processOrder(@RequestBody Order order) {
    return productService.handleOrder(order);
}
 
@DeleteMapping
public Order revertOrder(@RequestBody Order order) {
    return productService.revertOrder(order);
}

and a service to encapsulate our business logic:

@Transactional
public Order handleOrder(Order order) {       
    order.getLineItems()
      .forEach(l -> {
          Product> p = productRepository.findById(l.getProductId())
            .orElseThrow(() -> new RuntimeException("Could not find the product: " + l.getProductId()));
          if (p.getStock() >= l.getQuantity()) {
              p.setStock(p.getStock() - l.getQuantity());
              productRepository.save(p);
          } else {
              throw new RuntimeException("Product is out of stock: " + l.getProductId());
          }
      });
    return order.setOrderStatus(OrderStatus.SUCCESS);
}

@Transactional
public Order revertOrder(Order order) {
    order.getLineItems()
      .forEach(l -> {
          Product p = productRepository.findById(l.getProductId())
            .orElseThrow(() -> new RuntimeException("Could not find the product: " + l.getProductId()));
          p.setStock(p.getStock() + l.getQuantity());
          productRepository.save(p);
      });
    return order.setOrderStatus(OrderStatus.SUCCESS);
}

Note that we're persisting the entities within a transaction, which ensures that no inconsistent state results in case of exceptions.

Apart from these, we'll also have to define the domain entities, the repository interface, and a bunch of configuration classes necessary for everything to work properly.

But since these are mostly boilerplate, we'll avoid going through them, and they can be referred to in the GitHub repository provided in the last section of this article.

4.4. Shipping Microservice

The shipping microservice will not be very different either. This will be responsible for checking if a shipment can be generated for the order and create one if possible.

As before we'll define a controller to expose our endpoints, in fact just a single endpoint:

@PostMapping
public Order process(@RequestBody Order order) {
    return shippingService.handleOrder(order);
}

and a service to encapsulate the business logic related to order shipment:

public Order handleOrder(Order order) {
    LocalDate shippingDate = null;
    if (LocalTime.now().isAfter(LocalTime.parse("10:00"))
      && LocalTime.now().isBefore(LocalTime.parse("18:00"))) {
        shippingDate = LocalDate.now().plusDays(1);
    } else {
        throw new RuntimeException("The current time is off the limits to place order.");
    }
    shipmentRepository.save(new Shipment()
      .setAddress(order.getShippingAddress())
      .setShippingDate(shippingDate));
    return order.setShippingDate(shippingDate)
      .setOrderStatus(OrderStatus.SUCCESS);
}

Our simple shipping service is just checking the valid time window to place orders. We'll avoid discussing the rest of the boilerplate code as before.

4.5. Order Microservice

Finally, we'll define an order microservice which will be responsible for creating a new order apart from other things. Interestingly, it'll also play as an orchestrator service where it will communicate with the inventory service and the shipping service for the order.

Let's define our controller with the required endpoints:

@PostMapping
public Order create(@RequestBody Order order) {
    Order processedOrder = orderService.createOrder(order);
    if (OrderStatus.FAILURE.equals(processedOrder.getOrderStatus())) {
        throw new RuntimeException("Order processing failed, please try again later.");
    }
    return processedOrder;
}
@GetMapping
public List<Order> getAll() {
    return orderService.getOrders();
}

And, a service to encapsulate the business logic related to orders:

public Order createOrder(Order order) {
    boolean success = true;
    Order savedOrder = orderRepository.save(order);
    Order inventoryResponse = null;
    try {
        inventoryResponse = restTemplate.postForObject(
          inventoryServiceUrl, order, Order.class);
    } catch (Exception ex) {
        success = false;
    }
    Order shippingResponse = null;
    try {
        shippingResponse = restTemplate.postForObject(
          shippingServiceUrl, order, Order.class);
    } catch (Exception ex) {
        success = false;
        HttpEntity<Order> deleteRequest = new HttpEntity<>(order);
        ResponseEntity<Order> deleteResponse = restTemplate.exchange(
          inventoryServiceUrl, HttpMethod.DELETE, deleteRequest, Order.class);
    }
    if (success) {
        savedOrder.setOrderStatus(OrderStatus.SUCCESS);
        savedOrder.setShippingDate(shippingResponse.getShippingDate());
    } else {
        savedOrder.setOrderStatus(OrderStatus.FAILURE);
    }
    return orderRepository.save(savedOrder);
}

public List<Order> getOrders() {
    return orderRepository.findAll();
}

The handling of orders where we're orchestrating calls to inventory and shipping services is far from ideal. Distributed transactions with multiple microservices is a complex topic in itself and beyond the scope of this tutorial.

However, we'll see later in this tutorial how a reactive system can avoid the need for distributed transactions to a certain extent.

As before, we'll not go through the rest of the boilerplate code. However, this can be referenced in the GitHub repo.

4.6. Front-end

Let's also add a user interface to make the discussion complete. The user interface will be based on Angular and will be a simple single-page application.

We'll need to create a simple component in Angular to handle create and fetch orders. Of specific importance is the part where we call our API to create the order:

createOrder() {
    let headers = new HttpHeaders({'Content-Type': 'application/json'});
    let options = {headers: headers}
    this.http.post('http://localhost:8080/api/orders', this.form.value, options)
      .subscribe(
        (response) => {
          this.response = response
        },
        (error) => {
          this.error = error
        }
      )
}

The above code snippet expects order data to be captured in a form and available within the scope of the component. Angular offers fantastic support for creating simple to complex forms using reactive and template-driven forms.

Also important is the part where we get previously created orders:

getOrders() {
  this.previousOrders = this.http.get(''http://localhost:8080/api/orders'')
}

Please note that the Angular HTTP module is asynchronous in nature and hence returns RxJS Observables. We can handle the response in our view by passing them through an async pipe:

<div class="container" *ngIf="previousOrders !== null">
  <h2>Your orders placed so far:</h2>
  <ul>
    <li *ngFor="let order of previousOrders | async">
      <p>Order ID: {{ order.id }}, Order Status: {{order.orderStatus}}, Order Message: {{order.responseMessage}}</p>
    </li>
  </ul>
</div>

Of course, Angular will require templates, styles, and configurations to work, but these can be referred to in the GitHub repository. Please note that we have bundled everything in a single component here, which is ideally not something we should do.

But, for this tutorial, those concerns are not in scope.

4.7. Deploying the Application

Now that we've created all individual parts of the application, how should we go about deploying them? Well, we can always do this manually. But we should be careful that it can soon become tedious.

For this tutorial, we'll use Docker Compose to build and deploy our application on a Docker Machine. This will require us to add a standard Dockerfile in each service and create a Docker Compose file for the entire application.

Let's see how this docker-compose.yml file looks:

version: '3'
services:
  frontend:
    build: ./frontend
    ports:
      - "80:80"
  order-service:
    build: ./order-service
    ports:
      - "8080:8080"
  inventory-service:
    build: ./inventory-service
    ports:
      - "8081:8081"
  shipping-service:
    build: ./shipping-service
    ports:
      - "8082:8082"

This is a fairly standard definition of services in Docker Compose and does not require any special attention.

4.8. Problems with this Architecture

Now that we have a simple application in place with multiple services interacting with each other, we can discuss the problems in this architecture. There are what we'll try to address in the following sections and eventually get to the state where we would have transformed our application into a reactive system!

While this application is far from a production-grade software and there are several issues, we'll focus on the issues that pertain to the motivations for reactive systems:

  • Failure in either inventory service or shipping service can have a cascading effect
  • The calls to external systems and database are all blocking in nature
  • The deployment cannot handle failures and fluctuating loads automatically

5. Reactive Programming

Blocking calls in any program often result in critical resources just waiting for things to happen. These include database calls, calls to web services, and file system calls. If we can free up threads of execution from this waiting and provide a mechanism to circle back once results are available, it will yield much better resource utilization.

This is what adopting the reactive programming paradigm does for us. While it's possible to switch over to a reactive library for many of these calls, it may not be possible for everything. For us, fortunately, Spring makes it much easier to use reactive programming with MongoDB and REST APIs:

Spring Data Mongo has support for reactive access through the MongoDB Reactive Streams Java Driver. It provides ReactiveMongoTemplate and ReactiveMongoRepository, both of which have extensive mapping functionality.

Spring WebFlux provides the reactive-stack web framework for Spring, enabling non-blocking code and Reactive Streams backpressure. It leverages the Reactor as its reactive library. Further, it provides WebClient for performing HTTP requests with Reactive Streams backpressure. It uses Reactor Netty as the HTTP client library.

5.1. Inventory Service

We'll begin by changing our endpoints to emit reactive publishers:

@GetMapping
public Flux<Product> getAllProducts() {
    return productService.getProducts();
}
@PostMapping
public Mono<Order> processOrder(@RequestBody Order order) {
    return productService.handleOrder(order);
}

@DeleteMapping
public Mono<Order> revertOrder(@RequestBody Order order) {
    return productService.revertOrder(order);
}

Obviously, we'll have to make necessary changes to the service as well:

@Transactional
public Mono<Order> handleOrder(Order order) {
    return Flux.fromIterable(order.getLineItems())
      .flatMap(l -> productRepository.findById(l.getProductId()))
      .flatMap(p -> {
          int q = order.getLineItems().stream()
            .filter(l -> l.getProductId().equals(p.getId()))
            .findAny().get()
            .getQuantity();
          if (p.getStock() >= q) {
              p.setStock(p.getStock() - q);
              return productRepository.save(p);
          } else {
              return Mono.error(new RuntimeException("Product is out of stock: " + p.getId()));
          }
      })
      .then(Mono.just(order.setOrderStatus("SUCCESS")));
}

@Transactional
public Mono<Order> revertOrder(Order order) {
    return Flux.fromIterable(order.getLineItems())
      .flatMap(l -> productRepository.findById(l.getProductId()))
      .flatMap(p -> {
          int q = order.getLineItems().stream()
            .filter(l -> l.getProductId().equals(p.getId()))
            .findAny().get()
            .getQuantity();
          p.setStock(p.getStock() + q);
          return productRepository.save(p);
      })
      .then(Mono.just(order.setOrderStatus("SUCCESS")));
}

5.2. Shipping Service

Similarly, we'll change the endpoint of our shipping service:

@PostMapping
public Mono<Order> process(@RequestBody Order order) {
    return shippingService.handleOrder(order);
}

And, corresponding changes in the service to leverage reactive programming:

public Mono<Order> handleOrder(Order order) {
    return Mono.just(order)
      .flatMap(o -> {
          LocalDate shippingDate = null;
          if (LocalTime.now().isAfter(LocalTime.parse("10:00"))
            && LocalTime.now().isBefore(LocalTime.parse("18:00"))) {
              shippingDate = LocalDate.now().plusDays(1);
          } else {
              return Mono.error(new RuntimeException("The current time is off the limits to place order."));
          }
          return shipmentRepository.save(new Shipment()
            .setAddress(order.getShippingAddress())
            .setShippingDate(shippingDate));
      })
      .map(s -> order.setShippingDate(s.getShippingDate())
        .setOrderStatus(OrderStatus.SUCCESS));
    }

5.3. Order Service

We'll have to make similar changes in the endpoints of the order service:

@PostMapping
public Mono<Order> create(@RequestBody Order order) {
    return orderService.createOrder(order)
      .flatMap(o -> {
          if (OrderStatus.FAILURE.equals(o.getOrderStatus())) {
              return Mono.error(new RuntimeException("Order processing failed, please try again later. " + o.getResponseMessage()));
          } else {
              return Mono.just(o);
          }
      });
}

@GetMapping
public Flux<Order> getAll() {
    return orderService.getOrders();
}

The changes to service will be more involved as we'll have to make use of Spring WebClient to invoke the inventory and shipping reactive endpoints:

public Mono<Order> createOrder(Order order) {
    return Mono.just(order)
      .flatMap(orderRepository::save)
      .flatMap(o -> {
          return webClient.method(HttpMethod.POST)
            .uri(inventoryServiceUrl)
            .body(BodyInserters.fromValue(o))
            .exchange();
      })
      .onErrorResume(err -> {
          return Mono.just(order.setOrderStatus(OrderStatus.FAILURE)
            .setResponseMessage(err.getMessage()));
      })
      .flatMap(o -> {
          if (!OrderStatus.FAILURE.equals(o.getOrderStatus())) {
              return webClient.method(HttpMethod.POST)
                .uri(shippingServiceUrl)
                .body(BodyInserters.fromValue(o))
                .exchange();
          } else {
              return Mono.just(o);
          }
      })
      .onErrorResume(err -> {
          return webClient.method(HttpMethod.POST)
            .uri(inventoryServiceUrl)
            .body(BodyInserters.fromValue(order))
            .retrieve()
            .bodyToMono(Order.class)
            .map(o -> o.setOrderStatus(OrderStatus.FAILURE)
              .setResponseMessage(err.getMessage()));
      })
      .map(o -> {
          if (!OrderStatus.FAILURE.equals(o.getOrderStatus())) {
              return order.setShippingDate(o.getShippingDate())
                .setOrderStatus(OrderStatus.SUCCESS);
          } else {
              return order.setOrderStatus(OrderStatus.FAILURE)
                .setResponseMessage(o.getResponseMessage());
          }
      })
      .flatMap(orderRepository::save);
}

public Flux<Order> getOrders() {
    return orderRepository.findAll();
}

This kind of orchestration with reactive APIs is no easy exercise and often error-prone as well as hard to debug. We'll see how this can be simplified in the next section.

5.4. Front-end

Now, that our APIs are capable of streaming events as they occur, it's quite natural that we should be able to leverage that in our front-end as well. Fortunately, Angular supports EventSource, the interface for Server-Sent Events.

Let's see how can we pull and process all our previous orders as a stream of events:

getOrderStream() {
    return Observable.create((observer) => {
        let eventSource = new EventSource('http://localhost:8080/api/orders')
        eventSource.onmessage = (event) => {
            let json = JSON.parse(event.data)
            this.orders.push(json)
            this._zone.run(() => {
                observer.next(this.orders)
            })
        }
        eventSource.onerror = (error) => {
            if(eventSource.readyState === 0) {
                eventSource.close()
                this._zone.run(() => {
                    observer.complete()
                })
            } else {
                this._zone.run(() => {
                    observer.error('EventSource error: ' + error)
                })
            }
        }
    })
}

6. Message-Driven Architecture

The first problem we're going to address is related to service-to-service communication. Right now, these communications are synchronous, which presents several problems. These include cascading failures, complex orchestration, and distributed transactions to name a few.

An obvious way to solve this problem is to make these communications asynchronous. A message broker for facilitating all service-to-service communication can do the trick for us. We'll use Kafka as our message broker and Spring for Kafka to produce and consume messages:

We'll use a single topic to produce and consume order messages with different order statuses for services to react.

Let's see how each service needs to change.

6.1. Inventory Service

Let's begin by defining the message producer for our inventory service:

@Autowired
private KafkaTemplate<String, Order> kafkaTemplate;

public void sendMessage(Order order) {
    this.kafkaTemplate.send("orders", order);
}

Next, we'll have to define a message consumer for inventory service to react to different messages on the topic:

@KafkaListener(topics = "orders", groupId = "inventory")
public void consume(Order order) throws IOException {
    if (OrderStatus.RESERVE_INVENTORY.equals(order.getOrderStatus())) {
        productService.handleOrder(order)
          .doOnSuccess(o -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.INVENTORY_SUCCESS));
          })
          .doOnError(e -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.INVENTORY_FAILURE)
                .setResponseMessage(e.getMessage()));
          }).subscribe();
    } else if (OrderStatus.REVERT_INVENTORY.equals(order.getOrderStatus())) {
        productService.revertOrder(order)
          .doOnSuccess(o -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.INVENTORY_REVERT_SUCCESS));
          })
          .doOnError(e -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.INVENTORY_REVERT_FAILURE)
                .setResponseMessage(e.getMessage()));
          }).subscribe();
    }
}

This also means that we can safely drop some of the redundant endpoints from our controller now. These changes are sufficient to achieve asynchronous communication in our application.

6.2. Shipping Service

The changes in shipping service are relatively similar to what we did earlier with the inventory service. The message producer is the same, and the message consumer is specific to shipping logic:

@KafkaListener(topics = "orders", groupId = "shipping")
public void consume(Order order) throws IOException {
    if (OrderStatus.PREPARE_SHIPPING.equals(order.getOrderStatus())) {
        shippingService.handleOrder(order)
          .doOnSuccess(o -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.SHIPPING_SUCCESS)
                .setShippingDate(o.getShippingDate()));
          })
          .doOnError(e -> {
              orderProducer.sendMessage(order.setOrderStatus(OrderStatus.SHIPPING_FAILURE)
                .setResponseMessage(e.getMessage()));
          }).subscribe();
    }
}

We can safely drop all the endpoints in our controller now as we no longer need them.

6.3. Order Service

The changes in order service will be a little more involved as this is where we were doing all the orchestration earlier.

Nevertheless, the message producer remains unchanged, and message consumer takes on order service-specific logic:

@KafkaListener(topics = "orders", groupId = "orders")
public void consume(Order order) throws IOException {
    if (OrderStatus.INITIATION_SUCCESS.equals(order.getOrderStatus())) {
        orderRepository.findById(order.getId())
          .map(o -> {
              orderProducer.sendMessage(o.setOrderStatus(OrderStatus.RESERVE_INVENTORY));
              return o.setOrderStatus(order.getOrderStatus())
                .setResponseMessage(order.getResponseMessage());
          })
          .flatMap(orderRepository::save)
          .subscribe();
    } else if ("INVENTORY-SUCCESS".equals(order.getOrderStatus())) {
        orderRepository.findById(order.getId())
          .map(o -> {
              orderProducer.sendMessage(o.setOrderStatus(OrderStatus.PREPARE_SHIPPING));
              return o.setOrderStatus(order.getOrderStatus())
                .setResponseMessage(order.getResponseMessage());
          })
          .flatMap(orderRepository::save)
          .subscribe();
    } else if ("SHIPPING-FAILURE".equals(order.getOrderStatus())) {
        orderRepository.findById(order.getId())
          .map(o -> {
              orderProducer.sendMessage(o.setOrderStatus(OrderStatus.REVERT_INVENTORY));
              return o.setOrderStatus(order.getOrderStatus())
                .setResponseMessage(order.getResponseMessage());
          })
          .flatMap(orderRepository::save)
          .subscribe();
    } else {
        orderRepository.findById(order.getId())
          .map(o -> {
              return o.setOrderStatus(order.getOrderStatus())
                .setResponseMessage(order.getResponseMessage());
          })
          .flatMap(orderRepository::save)
          .subscribe();
    }
}

The consumer here is merely reacting to order messages with different order statuses. This is what gives us the choreography between different services.

Lastly, our order service will also have to change to support this choreography:

public Mono<Order> createOrder(Order order) {
    return Mono.just(order)
      .flatMap(orderRepository::save)
      .map(o -> {
          orderProducer.sendMessage(o.setOrderStatus(OrderStatus.INITIATION_SUCCESS));
          return o;
      })
      .onErrorResume(err -> {
          return Mono.just(order.setOrderStatus(OrderStatus.FAILURE)
            .setResponseMessage(err.getMessage()));
      })
      .flatMap(orderRepository::save);
}

Note that this is far simpler than the service we had to write with reactive endpoints in the last section. Asynchronous choreography often results in far simpler code, although it does come at the cost of eventual consistency and complex debugging and monitoring. As we may guess, our front-end will no longer get the final status of the order immediately.

7. Container Orchestration Service

The last piece of the puzzle that we want to solve is related to deployment.

What we want in the application is ample redundancy and a tendency to scale up or down depending upon the need automatically.

We've already achieved containerization of services through Docker and are managing dependencies between them through Docker Compose. While these are fantastic tools in their own right, they do not help us to achieve what we want.

Hence, we need a container orchestration service that can take care of redundancy and scalability in our application. While there are several options, one of the popular ones includes Kubernetes. Kubernetes provides us with a cloud vendor-agnostic way to achieve highly scalable deployments of containerized workloads.

Kubernetes wraps containers like Docker into Pods, which are the smallest unit of deployment. Further, we can use Deployment to describe the desired state declaratively.

Deployment creates ReplicaSets, which internally is responsible for bringing up the pods. We can describe a minimum number of identical pods that should be running at any point in time. This provides redundancy and hence high availability.

Let's see how can we define a Kubernetes deployment for our applications:

apiVersion: apps/v1
kind: Deployment
metadata: 
  name: inventory-deployment
spec: 
  replicas: 3
  selector:
    matchLabels:
      name: inventory-deployment
  template: 
    metadata: 
      labels: 
        name: inventory-deployment
    spec: 
      containers:
      - name: inventory
        image: inventory-service-async:latest
        ports: 
        - containerPort: 8081
---
apiVersion: apps/v1
kind: Deployment
metadata: 
  name: shipping-deployment
spec: 
  replicas: 3
  selector:
    matchLabels:
      name: shipping-deployment
  template: 
    metadata: 
      labels: 
        name: shipping-deployment
    spec: 
      containers:
      - name: shipping
        image: shipping-service-async:latest
        ports: 
        - containerPort: 8082
---
apiVersion: apps/v1
kind: Deployment
metadata: 
  name: order-deployment
spec: 
  replicas: 3
  selector:
    matchLabels:
      name: order-deployment
  template: 
    metadata: 
      labels: 
        name: order-deployment
    spec: 
      containers:
      - name: order
        image: order-service-async:latest
        ports: 
        - containerPort: 8080

Here we're declaring our deployment to maintain three identical replicas of pods at any time. While this is a good way to add redundancy, it may not be sufficient for varying loads. Kubernetes provides another resource known as the Horizontal Pod Autoscaler which can scale the number of pods in a deployment based on observed metrics like CPU utilization.

Please note that we have just covered the scalability aspects of the application hosted on a Kubernetes cluster. This does not necessarily imply that the underlying cluster itself is scalable. Creating a high availability Kubernetes cluster is a non-trivial task and beyond the scope of this tutorial.

8. Resulting Reactive System

Now that we've made several improvements in our architecture, it's perhaps time to evaluate this against the definition of a Reactive System. We'll keep the evaluation against the four characteristics of a Reactive Systems we discussed earlier in the tutorial:

  • Responsive: The adoption of the reactive programming paradigm should help us achieve end-to-end non-blocking and hence a responsive application
  • Resilient: Kubernetes deployment with ReplicaSet of the desired number of pods should provide resilience against random failures
  • Elastic: Kubernetes cluster and resources should provide us the necessary support to be elastic in the face of unpredictable loads
  • Message-Driven: Having all service-to-service communication handled asynchronously through a Kafka broker should help us here

While this looks quite promising, it's far from over. To be honest, the quest for a truly reactive system should be a continuous exercise of improvements. We can never preempt all that can fail in a highly complex infrastructure, where our application is just a small part.

A reactive system thus will demand reliability from every part that makes the whole. Right from the physical network to infrastructure services like DNS, they all should fall in line to help us achieve the end goal.

Often, it may not be possible for us to manage and provide the necessary guarantees for all these parts. And this is where a managed cloud infrastructure helps alleviate our pain. We can choose from a host of services like IaaS (Infeastrure-as-a-Service), BaaS (Backend-as-a-Service), and PaaS (Platform-as-a-Service) to delegate the responsibilities to external parties. This leaves us with the responsibility of our application as far as possible.

9. Conclusion

In this tutorial, we went through the basics of reactive systems and how does it compare with reactive programming. We created a simple application with multiple microservices and highlighted the problems we intend to solve with a reactive system.

Further, we went ahead, introducing reactive programming, message-based architecture, and container orchestration service in the architecture to realize a reactive system.

Lastly, we discussed the resulting architecture and how it remains a journey towards the reactive system! This tutorial does not introduce us to all the tools, frameworks, or patterns which can help us create a reactive system, but it introduces us to the journey.

As usual, the source code for this article can be found over on GitHub.

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Vitor Hugo Oliveira
Vitor Hugo Oliveira
29 days ago

I know that the focus of the article is not the front end, but I think it is important to show in the article, the image of the final result of the project, as well as the structure of the project in eclipse for example.

Loredana Crusoveanu
23 days ago

Hi Vitor,
We prefer not to add screenshots to our articles as we think it might clutter the write-up and hence include only essential diagrams or architectural drawings. We invite you to try the code out from our Github link and look at the project structure and final UI.

Cheers.

Mohammed Salman Shaikh
Mohammed Salman Shaikh
28 days ago

Amazing article. I just realised the use of Kafka after reading this article. Would love to see such complete articles more!