Our connected world is full of events that are triggered or received by different software services. One of the big issues is that event publishers tend to describe events differently and in ways that are mostly incompatible with each other.
To address this, the Serverless Working Group from the Cloud Native Computing Foundation (CNCF) recently announced version 0.2 of the CloudEvents specification. The specification aims to describe event data in a common, standardized way. To some degree, a CloudEvent is an abstract envelope with some specified attributes that describe a concrete event and its data.
Working with CloudEvents is simple. This article shows how to use the powerful JVM toolkit provided by Vert.x to either generate or receive and process CloudEvents.
SDKs for working with CloudEvents
In addition to the specification, the CloudEvents team from the Serverless Working Group is working on different SDKs for various platforms, such as JavaScript, Golang, C Sharp, Java, and Python. This article will give a quick overview of the Java SDK and how it can be used inside an application built with Eclipse Vert.x.
The API is very simple and contains a generic CloudEvent class as well as a builder to create an instance of a CloudEvent:
final CloudEvent<MyCustomEvent> cloudEvent = new CloudEventBuilder<MyCustomEvent>()
.data(new MyCustomEvent(...))
.type("My.Cloud.Event.Type")
.id(UUID.randomUUID().toString();)
.source(URI.create("/trigger");)
.build();
Above, we use the CloudEventBuilder to create a very simple CloudEvent instance. However, in isolation, the API does not show its strength.
Eclipse Vert.x
Eclipse Vert.x is a toolkit for building reactive applications on the JVM. It is event-driven and nonblocking, which means applications can handle a lot of concurrency using a small number of kernel threads. See the resources below for more info on Vert.x. Fortun
Fortunately, support for Eclipse Vert.x is included in the CloudEvents Java SDK:
<dependency>
<groupId>io.cloudevents</groupId>
<artifactId>http-vertx</artifactId>
<version>0.2.0</version>
</dependency>Sending a CloudEvent to a remote service
Now that we have our CloudEvent object, capturing our event data, we want to send it to a remote cloud service, which will then process it:
final HttpClientRequest request = vertx.createHttpClient().post(8080, "localhost", "/");
// add a client response handler
request.handler(resp -> {
// react on the server response
});
// write the CloudEvent to the given HTTP Post request object
VertxCloudEvents.create().writeToHttpClientRequest(cloudEvent, request);
request.end();
After creating an HTTP Post request, we set up an async handler to deal with the future response of the server. Finally, the writeToHttpClientRequest of our VertxCloudEvents utility is used to serialize the actual CloudEvent object to the given HttpClientRequest.
Receiving CloudEvents with Vert.x
The VertxCloudEvents utility also contains a different function to receive a CloudEvent inside an Eclipse Vert.x HTTP server application:
import io.cloudevents.http.reactivex.vertx.VertxCloudEvents;
import io.vertx.core.http.HttpHeaders;
import io.vertx.reactivex.core.AbstractVerticle;
public class CloudEventVerticle extends AbstractVerticle {
public void start() {
vertx.createHttpServer()
.requestHandler(req -> VertxCloudEvents.create().rxReadFromRequest(req)
.subscribe((receivedEvent, throwable) -> {
if (receivedEvent != null) {
// I got a CloudEvent object:
System.out.println("The event type: " + receivedEvent.getType())
}
}))
.rxListen(8080)
.subscribe(server -> {
System.out.println("Server running!");
});
}
}
Above, we start a simple HTTPServer, using the Vert.x API for RxJava 2. Inside the reactive request handler, we invoke the rxReadFromRequest() method and subscribe to the CloudEvents it returns for further processing. Now we can work with the CloudEvent object inside our own server-side framework!
Conclusion and Outlook
Working with CloudEvents is simple and Vert.x provides a powerful JVM toolkit to either generate or receive and process CloudEvents in our system. CloudEvents are being adopted by more and more tools and frameworks such as Knative, which uses CloudEvents to exchange data between different components and services in a standardized format.
The CloudEvent specification is in its early stages with its current 0.2 version. However, even in such infancy, it is generating traction and proving an increasingly useful specification to allow interoperability between applications.
Additonal Resources
- Vert.x:
- Building Reactive Microservices in Java: Asynchronous and Event-Based Application Design, a free ebook
- Introduction to Vert.x article series by Clement Escoffier
- CloudEvents:
Last updated: August 21, 2019