Quarkus - Asynchronous messages between beans
Quarkus allows different beans to interact using asynchronous messages, enforcing loose-coupling. The messages are sent to virtual addresses. It offers 3 types of delivery mechanism:
-
point-to-point - send the message, one consumer receives it. If several consumers listen to the address, a round robin is applied;
-
publish/subscribe - publish a message, all the consumers listening to the address are receiving the message;
-
request/reply - send the message and expect a response. The receiver can respond to the message in an asynchronous-fashion
All these delivery mechanism are non-blocking, and are providing one of the fundamental brick to build reactive applications.
| The asynchronous message passing feature allows replying to messages which is not supported by Reactive Messaging. However, it is limited to single-event behavior (no stream) and to local messages. |
Installing
This mechanism uses the Vert.x EventBus, so you need to enable the vertx extension to use this feature.
If you are creating a new project, set the extensions parameter are follows:
mvn io.quarkus:quarkus-maven-plugin:0.16.1:create \
-DprojectGroupId=org.acme \
-DprojectArtifactId=vertx-quickstart \
-Dextensions="vertx"
If you have an already created project, the vertx extension can be added to an existing Quarkus project with
the add-extension command:
./mvnw quarkus:add-extension -Dextensions="vertx"
Otherwise, you can manually add this to the dependencies section of your pom.xml file:
<dependency>
<groupId>io.quarkus</groupId>
<artifactId>quarkus-vertx</artifactId>
</dependency>
Consuming events
To consume events, use the io.quarkus.vertx.ConsumeEvent annotation:
package org.acme.vertx;
import io.quarkus.vertx.ConsumeEvent;
import javax.enterprise.context.ApplicationScoped;
@ApplicationScoped
public class GreetingService {
@ConsumeEvent (1)
public String consume(String name) { (2)
return name.toUpperCase();
}
}
| 1 | If not set, the address is the fully qualified name of the bean, for instance, in this snippet it’s org.acme.vertx.GreetingService. |
| 2 | The method parameter is the message body. If the method returns something it’s the message response. |
|
By default, the code consuming the event must be non-blocking, as it’s called on the Vert.x event loop.
If your processing is blocking, use the
|
Configuring the address
The @ConsumeEvent annotation can be configured to set the address:
@ConsumeEvent("greeting") (1)
public String consume(String name) {
return name.toUpperCase();
}
| 1 | Receive the messages sent to the greeting address |
Replying
The return value of a method annotated with @ConsumeEvent is used as response to the incoming message.
For instance, in the following snippet, the returned String is the response.
@ConsumeEvent("greeting")
public String consume(String name) {
return name.toUpperCase();
}
You can also return a CompletionStage<T> to handle asynchronous reply:
@ConsumeEvent("greeting")
public CompletionStage<String> consume2(String name) {
return CompletableFuture.supplyAsync(name::toUpperCase, executor);
}
Implementing fire and forget interactions
You don’t have to reply to received messages.
Typically for a fire and forget interaction, the messages are consumed and the sender does not need to know about it.
To implement this, your consumer method just returns void
@ConsumeEvent("greeting")
public void consume(String event) {
// Do something with the event
}
Sending messages
Ok, we have seen how to receive messages, let’s now switch to the other side: the sender. Sending and publishing messages use the Vert.x event bus:
package org.acme;
import io.vertx.axle.core.eventbus.EventBus;
import io.vertx.axle.core.eventbus.Message;
import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import java.util.concurrent.CompletionStage;
@Path("/async")
public class EventResource {
@Inject
EventBus bus; (1)
@GET
@Path("/{name}")
public CompletionStage<String> hello(String name) {
return bus.<String>send("greeting", name) (2)
.thenApply(Message::body);
}
}
| 1 | Inject the Event bus |
| 2 | Send a message to the address greeting. Message payload is name |
The EventBus object provides methods to:
-
senda message to a specific address - one single consumer receives the message. -
publisha message to a specific address - all consumers receive the messages. -
senda message and expect reply
// Case 1
bus.send("address", "hello");
// Case 2
bus.publish("address", "hello");
// Case 3
bus.send("address", "hello, how are you?").thenAccept(message -> {
// reponse
});
Putting things together - bridging HTTP and messages
Let’s revisit a greeting HTTP endpoint and use asynchronous message passing to delegate the call to a separated bean. It uses the request/reply dispatching mechanism. Instead of implementing the business logic inside the JAX-RS endpoint, we are sending a message. This message is consumed by another bean and the response is sent using the reply mechanism.
First create a new project using:
mvn io.quarkus:quarkus-maven-plugin:0.16.1:create \
-DprojectGroupId=org.acme \
-DprojectArtifactId=vertx-quickstart \
-Dextensions="vertx"
You can already start the application in dev mode using mvn compile quarkus:dev.
Then, creates a new JAX-RS resource with the following content:
package org.acme.vertx;
import io.vertx.axle.core.eventbus.EventBus;
import io.vertx.axle.core.eventbus.Message;
import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import java.util.concurrent.CompletionStage;
@Path("/hello")
public class EventResource {
@Inject EventBus bus;
@GET
@Path("/async/{name}")
public CompletionStage<String> hello(@PathParam("name") String name) {
return bus.<String>send("greeting", name) (1)
.thenApply(Message::body); (2)
}
}
| 1 | send the name to the greeting address |
| 2 | when we get the reply, extract the body and send this as response to the user |
If you call this endpoint, you will wait and get a timeout. Indeed, no one is listening.
So, we need a consumer listening on the greeting address. Create a GreetingService bean with the following content:
package org.acme.vertx;
import io.quarkus.vertx.ConsumeEvent;
import javax.enterprise.context.ApplicationScoped;
@ApplicationScoped
public class GreetingService {
@ConsumeEvent("greeting")
public String greeting(String name) {
return "Hello " + name;
}
}
This bean receives the name, and returns the greeting message.
Now, open your browser to http://localhost:8080/async/Quarkus, and you should see:
Hello Quarkus
To better understand, let’s detail how the HTTP request/response has been handled:
-
The request is received by the
hellomethod -
a message containing the name is sent to the event bus
-
Another bean receives this message and computes the response
-
This response is sent back using the reply mechanism
-
Once the reply is received by the sender, the content is written to the HTTP response
This application can be packaged using:
./mvnw clean package
You can also compile it as a native executable with:
./mvnw clean package -Pnative