Aggregator Design Pattern In Java Microservices with Examples

The Aggregator design pattern in Microservice architecture is a design pattern used to compose a complex service by aggregating the responses of multiple independent microservices. It's also one of the essential Microservices Design patterns along with SAGA, CQRS, and Event Sourcing. This pattern is proper when a client request requires data or functionality distributed across multiple microservices. It can improve the performance and scalability of the system by allowing each microservice to focus on a specific task and reducing the workload of a single microservice. In this article, we will discuss how the Aggregator Microservice Pattern can be implemented in Java using various approaches, such as asynchronous communication, synchronous communication, or a combination of both. We will also provide examples of code to illustrate each approach.

Aggregator Microservice Pattern In Java With Examples

In Java, the Aggregator Microservice Pattern can be implemented using various approaches, such as asynchronous communication, synchronous communication, or a combination of both.

1. Asynchronous Communication

One way to implement the Aggregator Microservice Pattern in Java is by using asynchronous communication between the microservices. In this approach, the client sends a request to the aggregator microservice, which then sends requests to the individual microservices in parallel. 

Each microservice processes the request and sends the response back to the aggregator microservice, which then aggregates the responses and returns the result to the client.

This approach has the advantage of improving the performance of the system by allowing the microservices to process the requests concurrently. However, it requires the use of an asynchronous communication mechanism, such as message queues or event-driven architectures, which can introduce additional complexity to the system.

Here is an example of an Aggregator Microservice that uses asynchronous communication in Java:

public class AsyncAggregatorMicroservice {
    private final ExecutorService executorService;
    private final Microservice1Client microservice1Client;
    private final Microservice2Client microservice2Client;
    private final Microservice3Client microservice3Client;

    public AsyncAggregatorMicroservice(ExecutorService executorService, Microservice1Client microservice1Client, Microservice2Client microservice2Client, Microservice3Client microservice3Client) {
        this.executorService = executorService;
        this.microservice1Client = microservice1Client;
        this.microservice2Client = microservice2Client;
        this.microservice3Client = microservice3Client;
    }

    public CompletableFuture<AggregatedResponse> processRequest(Request request) {
        CompletableFuture<Response1> response1Future 
       = CompletableFuture.supplyAsync(() 
            -> microservice1Client.processRequest(request), executorService);
        CompletableFuture<Response2> response2Future 
      = CompletableFuture.supplyAsync(() 
           -> microservice2Client.processRequest(request), executorService);
        CompletableFuture<Response3> response3Future 
      = CompletableFuture.supplyAsync(() 
           -> microservice3Client.processRequest(request), executorService);

        return CompletableFuture.allOf(response1Future, response2Future, response3Future)
                .thenApply(v -> 
         new AggregatedResponse(response1Future.join(), response2Future.join(), 
              response3Future.join()));
    }
}

In this example, the AsyncAggregatorMicroservice class uses the CompletableFuture class from the Java Concurrency API to send requests to the individual microservices asynchronously. The CompletableFuture.allOf() method is used to wait for all the responses to be received, and the thenApply() method is used to aggregate the responses and return the result to the client.

2. Synchronous Communication

Another way to implement the Aggregator Microservice Pattern in Java is by using synchronous communication between the microservices. In this approach, the client sends a request to the aggregator microservice, which then sends requests to the individual microservices sequentially. 

 Each microservice processes the request and sends the response back to the aggregator microservice, which then aggregates the responses and returns the result to the client.

This approach has the advantage of simplicity, as it does not require the use of asynchronous communication mechanisms. However, it can have a negative impact on the performance of the system, as the aggregator microservice has to wait for each microservice to complete its task before moving on to the next one.

Here is an example of an Aggregator Microservice that uses synchronous communication in Java:

public class SyncAggregatorMicroservice {
    private final Microservice1Client microservice1Client;
    private final Microservice2Client microservice2Client;
    private final Microservice3Client microservice3Client;

    public SyncAggregatorMicroservice(Microservice1Client microservice1Client, 
              Microservice2Client microservice2Client, 
              Microservice3Client microservice3Client) {
        this.microservice1Client = microservice1Client;
        this.microservice2Client = microservice2Client;
        this.microservice3Client = microservice3Client;
    }

    public AggregatedResponse processRequest(Request request) {
        Response1 response1 = microservice1Client.processRequest(request);
        Response2 response2 = microservice2Client.processRequest(request);
        Response3 response3 = microservice3Client.processRequest(request);
        return new AggregatedResponse(response1, response2, response3);
    }
}

In this example, the SyncAggregatorMicroservice class sends requests to the individual microservices synchronously, one after the other. The responses are then aggregated and returned to the client.

Combination of Asynchronous and Synchronous Communication

It is also possible to implement the Aggregator Microservice Pattern in Java by combining asynchronous and synchronous communication. In this approach, the client sends a request to the aggregator microservice, which then sends requests to some of the microservices asynchronously and to others synchronously, depending on the requirements of the system.

This approach allows for a balance between performance and simplicity, as it allows the microservices to process the requests concurrently where possible, while still keeping the implementation straightforward.

Here is an example of an Aggregator Microservice that uses a combination of asynchronous and synchronous communication in Java:

public class HybridAggregatorMicroservice {
    private final ExecutorService executorService;
    private final Microservice1Client microservice1Client;
    private final Microservice2Client microservice2Client;
    private final Microservice3Client microservice3Client;

    public HybridAggregatorMicroservice(ExecutorService executorService,
      Microservice1Client microservice1Client, Microservice2Client microservice2Client,
      Microservice3Client microservice3Client) {
        this.executorService = executorService;
        this.microservice1Client = microservice1Client;
        this.microservice2Client = microservice2Client;
        this.microservice3Client = microservice3Client;
    }


public AggregatedResponse processRequest(Request request) {
CompletableFuture<Response1> response1Future 
= CompletableFuture.supplyAsync(() -> microservice1Client.processRequest(request), 
executorService);

Response2 response2 = microservice2Client.processRequest(request);
CompletableFuture<Response3> response3Future 
= CompletableFuture.supplyAsync(() -> microservice3Client.processRequest(request), 
   executorService);


    return CompletableFuture.allOf(response1Future, response3Future)
            .thenApply(v -> new AggregatedResponse(response1Future.join(), 
                 response2, response3Future.join()));
}

In this example, the `HybridAggregatorMicroservice` class sends requests to the `microservice1Client` and `microservice3Client` asynchronously, and to the `microservice2Client` synchronously. The responses are then aggregated and returned to the client.

Conclusion

The Aggregator Microservice Pattern in Java is a useful design pattern for composing complex services by aggregating the responses of multiple independent microservices. In Java, this pattern can be implemented using asynchronous communication, synchronous communication, or a combination of both, depending on the requirements of the system. 

 Asynchronous communication can improve the performance of the system, but it requires the use of additional communication mechanisms. Synchronous communication is simpler to implement, but it can have a negative impact on performance.

A combination of asynchronous and synchronous communication allows for a balance between performance and simplicity.

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