Table of Contents

RabbitMQ RPC and the Micronaut Framework

Use RabbitMQ RPC to use request-reply pattern in your Micronaut applications.

Authors: Iván López

Micronaut Version: 4.6.3

1. Getting Started

In this guide, we will create three microservices that will communicate with each other with RabbitMQ using the request-response pattern with RPC (Remote Procedure Call).

RabbitMQ is an open-source message-broker software that originally implemented the Advanced Message Queuing Protocol (AMQP) and has since been extended with a plug-in architecture to support Streaming Text Oriented Messaging Protocol (STOMP), Message Queuing Telemetry Transport (MQTT), and other protocols.

2. What you will need

To complete this guide, you will need the following:

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the Application

Let’s describe the microservices you will build through the guide.

  • bookcatalogue - This returns a list of books. It uses a domain consisting of a book name and an ISBN.

  • bookinventory - This exposes an endpoint to check whether a book has sufficient stock to fulfill an order. It uses a domain consisting of a stock level and an ISBN.

  • bookrecommendation - This consumes previous services and exposes an endpoint that recommends book names that are in stock.

Setup IntelliJ IDEA to develop Micronaut Applications.

5. Test Resources

When the application is started locally — either under test or by running the application — resolution of the rabbitmq.uri property is detected and the Test Resources service will start a local RabbitMQ docker container, and inject the properties required to use this as the message broker.

When running under production, you should replace this property with the location of your production message broker via an environment variable.

RABBITMQ_URI=amqp://production-server:5672

For more information, see the RabbitMQ section of the Test Resources documentation.

5.1. Catalogue microservice

Create the bookcatalogue microservice using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app --features=rabbitmq,graalvm example.micronaut.bookcatalogue --build=gradle --lang=java
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

If you use Micronaut Launch, select Micronaut Application as application type and add the rabbitmq and graalvm features.

The previous command creates a directory named bookcatalogue and a Micronaut application inside it with default package example.micronaut.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features, and apply those changes to your application.

Diff

5.1.1. Create RabbitMQ exchange, queue, and binding

Before being able to send and receive messages using RabbitMQ, it is necessary to define the exchange, queue, and binding. One option is to create them directly in the RabbitMQ Admin UI available on http://localhost:15672. Use guest for both username and password.

Another option is to create them programmatically. Create the class ChannelPoolListener.java:

bookcatalogue/src/main/java/example/micronaut/ChannelPoolListener.java
package example.micronaut;

import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.Channel;
import io.micronaut.rabbitmq.connect.ChannelInitializer;
import jakarta.inject.Singleton;

import java.io.IOException;

@Singleton
public class ChannelPoolListener extends ChannelInitializer {

    @Override
    public void initialize(Channel channel, String name) throws IOException {
        channel.exchangeDeclare("micronaut", BuiltinExchangeType.DIRECT, true); (1)

        channel.queueDeclare("inventory", true, false, false, null); (2)
        channel.queueBind("inventory", "micronaut", "books.inventory"); (3)

        channel.queueDeclare("catalogue", true, false, false, null); (4)
        channel.queueBind("catalogue", "micronaut", "books.catalogue"); (5)
    }
}
1 Define an exchange named micronaut. From the producer point of view, everything is sent to the exchange with the appropriate routing key.
2 Define a queue named inventory. The consumer will listen for messages in that queue.
3 Define a binding between the exchange and the queue using the routing key books.inventory.
4 Define a queue named catalogue. The consumer will listen for messages in that queue.
5 Define a binding between the exchange and the queue using the routing key books.catalogue.
In this Catalogue Microservice, the only necessary element is the catalogue queue, but it is a good practice to define all the elements in the same file and share the file between all the projects.

5.1.2. Create consumer

Create a BookCatalogueService class to handle incoming RPC requests into the bookcatalogue microservice:

bookcatalogue/src/main/java/example/micronaut/BookCatalogueService.java
package example.micronaut;

import io.micronaut.rabbitmq.annotation.Queue;
import io.micronaut.rabbitmq.annotation.RabbitListener;

import java.util.Arrays;
import java.util.List;

@RabbitListener (1)
public class BookCatalogueService {

    @Queue("catalogue") (2)
    List<Book> listBooks() {
        Book buildingMicroservices = new Book("1491950358", "Building Microservices");
        Book releaseIt = new Book("1680502395", "Release It!");
        Book cidelivery = new Book("0321601912", "Continuous Delivery:");

        return Arrays.asList(buildingMicroservices, releaseIt, cidelivery);
    }
}
1 Annotate the class with @RabbitListener to indicate that this bean will consume messages from RabbitMQ.
2 Annotate the method with @Queue. This listener will listen to messages in the catalogue queue.

The previous service responds a List<Book>. Create the Book POJO:

bookcatalogue/src/main/java/example/micronaut/Book.java
package example.micronaut;

import io.micronaut.serde.annotation.Serdeable;

import java.util.Objects;

@Serdeable
public class Book {

    private final String isbn;
    private final String name;

    public Book(String isbn, String name) {
        this.isbn = isbn;
        this.name = name;
    }

    public String getIsbn() {
        return isbn;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Book{" +
                "isbn='" + isbn + '\'' +
                ", name='" + name + '\'' +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Book book = (Book) o;
        return Objects.equals(isbn, book.isbn) &&
                Objects.equals(name, book.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(isbn, name);
    }
}

5.2. Inventory microservice

Create the bookinventory microservice using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app --features=rabbitmq,graalvm example.micronaut.bookinventory --build=gradle --lang=java
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

If you use Micronaut Launch, select Micronaut Application as application type and add the rabbitmq and graalvm features.

The previous command creates a directory named bookinventory and a Micronaut application inside it with default package example.micronaut.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features, and apply those changes to your application.

Diff

The previous command creates a directory named bookinventory and a Micronaut application inside it with default package example.micronaut.

5.2.1. Create RabbitMQ exchange, queue and binding

Copy the ChannelPoolListener class you created in the bookcatalogue microservice to bookinventory/src/main/java/example/micronaut/bookcatalogue.

5.2.2. Create consumer

Create a BookInventoryService class to handle incoming RPC requests into the bookinventory microservice:

bookinventory/src/main/java/example/micronaut/BookInventoryService.java
package example.micronaut;

import io.micronaut.rabbitmq.annotation.Queue;
import io.micronaut.rabbitmq.annotation.RabbitListener;

import jakarta.validation.constraints.NotBlank;
import java.util.Optional;

@RabbitListener (1)
public class BookInventoryService {

    @Queue("inventory") (2)
    public Boolean stock(@NotBlank String isbn) {
        return bookInventoryByIsbn(isbn).map(bi -> bi.getStock() > 0).orElse(null);
    }

    private Optional<BookInventory> bookInventoryByIsbn(String isbn) {
        if (isbn.equals("1491950358")) {
            return Optional.of(new BookInventory(isbn, 4));
        }
        if (isbn.equals("1680502395")) {
            return Optional.of(new BookInventory(isbn, 0));
        }
        return Optional.empty();
    }
}
1 Annotate the class with @RabbitListener to indicate that this bean will consume messages from RabbitMQ.
2 Annotate the method with @Queue. This listener will listen to messages in inventory queue.

The previous service uses BookInventory POJO. Create it:

bookinventory/src/main/java/example/micronaut/BookInventory.java
package example.micronaut;

import io.micronaut.serde.annotation.Serdeable;

import java.util.Objects;

@Serdeable
public class BookInventory {

    private final String isbn;
    private final Integer stock;

    public BookInventory(String isbn, Integer stock) {
        this.isbn = isbn;
        this.stock = stock;
    }

    public String getIsbn() {
        return isbn;
    }

    public Integer getStock() {
        return stock;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        BookInventory that = (BookInventory) o;
        return Objects.equals(isbn, that.isbn) &&
                Objects.equals(stock, that.stock);
    }

    @Override
    public int hashCode() {
        return Objects.hash(isbn, stock);
    }
}

5.3. Recommendation microservice

Create the bookrecommendation microservice using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app --features=rabbitmq,reactor,graalvm example.micronaut.bookrecommendation --build=gradle --lang=java
If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

If you use Micronaut Launch, select Micronaut Application as application type and add the rabbitmq, reactor, and graalvm features.

The previous command creates a directory named bookrecommendation and a Micronaut application inside it with default package example.micronaut.

If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features, and apply those changes to your application.

Diff

5.3.1. Create RabbitMQ exchange, queue and binding

Copy the ChannelPoolListener class you created in the bookcatalogue microservice to bookrecommendation/src/main/java/example/micronaut/bookcatalogue.

5.3.2. Create clients

Let’s create two interfaces to send messages to RabbitMQ. The Micronaut framework will implement the interfaces at compilation time. Create CatalogueClient:

bookrecommendation/src/main/java/example/micronaut/CatalogueClient.java
package example.micronaut;

import io.micronaut.rabbitmq.annotation.Binding;
import io.micronaut.rabbitmq.annotation.RabbitClient;
import io.micronaut.rabbitmq.annotation.RabbitProperty;
import org.reactivestreams.Publisher;

import java.util.List;

@RabbitClient("micronaut") (1)
@RabbitProperty(name = "replyTo", value = "amq.rabbitmq.reply-to") (2)
public interface CatalogueClient {

    @Binding("books.catalogue") (3)
    Publisher<List<Book>> findAll(byte[] data); (4)
}
1 Send the messages to exchange micronaut.
2 Set the replyTo property to amq.rabbitmq.reply-to. This is a special queue that always exists and does not need to be created. That is why we did not create the queue in the ChannelInitializer. RabbitMQ uses that queue in a special way, and setting the value of the property replyTo to that queue will enable this call as an RPC one. RabbitMQ will create a temporary queue for the callback.
3 Set the routing key.
4 Define the method that will "mirror" the one in the consumer. Keep in mind that in the consumer, it is not possible to return a reactive type, but on the client side it is. Also, it is necessary to send something, even if it’s not used in the consumer.

Create InventoryClient.java:

bookrecommendation/src/main/java/example/micronaut/InventoryClient.java
package example.micronaut;

import io.micronaut.rabbitmq.annotation.Binding;
import io.micronaut.rabbitmq.annotation.RabbitClient;
import io.micronaut.rabbitmq.annotation.RabbitProperty;
import reactor.core.publisher.Mono;

@RabbitClient("micronaut") (1)
@RabbitProperty(name = "replyTo", value = "amq.rabbitmq.reply-to") (2)
public interface InventoryClient {

    @Binding("books.inventory") (3)
    Mono<Boolean> stock(String isbn); (4)
}
1 Send the messages to exchange micronaut.
2 Set the replyTo property to amq.rabbitmq.reply-to.
3 Set the routing key.
4 Define the method that will "mirror" the one in the consumer. As we did with CatalogueClient, we use a reactive type to wrap the result.

5.3.3. Create the controller

Create a Controller that injects both clients.

bookrecommendation/src/main/java/example/micronaut/BookController.java
package example.micronaut;

import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Flux;

@Controller("/books") (1)
public class BookController {

    private final CatalogueClient catalogueClient; (2)
    private final InventoryClient inventoryClient; (2)

    public BookController(CatalogueClient catalogueClient, InventoryClient inventoryClient) { (2)
        this.catalogueClient = catalogueClient;
        this.inventoryClient = inventoryClient;
    }

    @Get (3)
    public Publisher<BookRecommendation> index() {
        return Flux.from(catalogueClient.findAll(null))
                .flatMap(Flux::fromIterable)
                .flatMap(book -> Flux.from(inventoryClient.stock(book.getIsbn()))
                        .filter(Boolean::booleanValue)
                        .map(response -> book))
                .map(book -> new BookRecommendation(book.getName()));
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /books.
2 Clients are injected via constructor injection
3 The @Get annotation maps the index method to an HTTP GET request on /books.

The previous controller returns a Publisher<BookRecommendation>. Create the BookRecommendation POJO:

bookrecommendation/src/main/java/example/micronaut/BookRecommendation.java
package example.micronaut;

import io.micronaut.serde.annotation.Serdeable;

import java.util.Objects;

@Serdeable
public class BookRecommendation {

    private final String name;

    public BookRecommendation(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        BookRecommendation that = (BookRecommendation) o;
        return Objects.equals(name, that.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }
}

6. RabbitMQ and the Micronaut Framework

As mentioned above a shared dockerized instance of RabbitMQ will be started automatically when you run the application, however if you want to start your own instance of RabbitMQ, you have a few options:

6.1. Install RabbitMQ via Docker

The fastest way to start using RabbitMQ is via Docker:

docker run --rm -it \
        -p 5672:5672 \
        -p 15672:15672 \
        rabbitmq:3.8.12-management

6.2. Alternative methods

Alternatively you can install and run a local RabbitMQ instance.

7. Running the Application

Configure bookinventory to run on port 8082:

Since Micronaut Framework 4.0, to use YAML configuration, you have to add the YAML dependency.
bookinventory/src/main/resources/application.yml
micronaut:
  server:
    port: 8082 (1)

Run bookinventory microservice:

bookinventory
./gradlew run
13:30:22.426 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 742ms. Server Running: 1 active message listeners.

Configure bookcatalogue to run on port 8081:

bookcatalogue/src/main/resources/application.yml
micronaut:
  server:
    port: 8081 (1)

Run bookcatalogue microservice:

bookcatalogue
./gradlew run
13:31:19.887 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 949ms. Server Running: 1 active message listeners.

Configure bookrecommendation to run on port 8080:

bookrecommendation/src/main/resources/application.yml
micronaut:
  server:
    port: 8080 (1)
8080 is the default port if you don’t specify micronaut.server.port property

Run bookrecommendation microservice:

bookrecommendation
./gradlew run
13:32:06.045 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 959ms. Server Running: http://localhost:8080

You can run a curl command to test the whole application:

curl http://localhost:8080/books
[{"name":"Building Microservices"}]

8. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, an advanced JDK with ahead-of-time Native Image compilation, to generate a native executable of this Micronaut application.

Compiling Micronaut applications ahead of time with GraalVM significantly improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

8.1. GraalVM Installation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 21
sdk install java 21.0.5-graal

For installation on Windows, or for a manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.

Alternatively, you can use the GraalVM Community Edition:

Java 21
sdk install java 21.0.2-graalce

8.2. Native Executable Generation

To generate a native executable using Gradle, run:

./gradlew nativeCompile

The native executable is created in build/native/nativeCompile directory and can be run with build/native/nativeCompile/micronautguide.

It is possible to customize the name of the native executable or pass additional parameters to GraalVM:

build.gradle
graalvmNative {
    binaries {
        main {
            imageName.set('mn-graalvm-application') (1)
            buildArgs.add('-Ob') (2)
        }
    }
}
1 The native executable name will now be mn-graalvm-application
2 It is possible to pass extra build arguments to native-image. For example, -Ob enables the quick build mode.

Start the native executables for the two microservices and run the same curl request as before to check that everything works with GraalVM.

9. Next Steps

Read more about RabbitMQ RPC Support in the Micronaut framework.

10. License

All guides are released with an Apache license 2.0 license for the code and a Creative Commons Attribution 4.0 license for the writing and media (images…​).