Introduction

The purpose of this demo is to show how to use websockets to publish and subscribe events (useful in an Event Driven Architecture - EDA - based system).

WebSocket is a computer communications protocol, providing full-duplex communication channels over a single TCP connection (see https://en.wikipedia.org/wiki/WebSocket).

In this demo, we use 3 concepts: event (and its eventCode), publication, subscription.

• an event is a message sent from a publication to multiple subscriptions. An event has an eventCode which is the category of the event. An eventCode is for example FRAUD_DETECTED or TRANSFER_COMPLETED or EUR_USD_UPDATED... An event has only one eventCode but multiple events can be sent with the same eventCode.

• a publication sends events. A publication sends events of a given category (events with a giveneventCode). A publisher - a component of the system - can use multiple publications, with one publication for each category of events. Publications do not know subscriptions. The ony thing that publications and subscriptions share are the event categories (the eventCode).

• a subscription receives events. A subscription subscribes and receives events of a given category (events with a giveneventCode). A subscriber - a component of the system - can use multiple subscriptions, with one subscriptions for each category of events. Subscriptions do not know publications. The ony thing that publications and subscriptions share are the event categories (the eventCode).

This demo uses Quarkus (www.quarkus.io) (and not SpringBoot). No special reason for that; it's just because I wanted to play with Quarkus !

In a nutshell

Build and start the broker:

cd broker
../mvnw clean package
java -jar target/events-websockets-quarkus-broker-1.0-SNAPSHOT-runner.jar

In a terminal, start a publication for events of category eventA:

wscat -c localhost:8080/events/eventA/publications

In another terminal, start a first subscription for events of category eventA:

wscat -c localhost:8080/events/eventA/subscriptions

In another terminal, start a second subscription for events of category eventA:

wscat -c localhost:8080/events/eventA/subscriptions

In another terminal, start a publication for events of category eventB:

wscat -c localhost:8080/events/eventB/publications

In another terminal, start a first subscription for events of category eventB:

wscat -c localhost:8080/events/eventB/subscriptions

In another terminal, start a second subscription for events of category eventB:

wscat -c localhost:8080/events/eventB/subscriptions

In the terminal of the publication for events of category eventA, type a message. Check the message is displayed in the terminals of the two subscriptions for events of category eventA.

In the terminal of the publication for events of category eventB, type a message. Check the message is displayed in the terminals of the two subscriptions for events of category eventB.

Pre-requisites

Install WSCat

wscat is a NodeJS utility used to send and display messages sent/received via websockets (and not regular HTTP REST/JSON API).

To install wscat, run:

sudo npm install -g wscat

To run wscat, run:

wscat -c ws://echo.websocket.org

Replace ws://echo.websocket.org by the actual endpoint of you websocket

Broker

Build and start

The broker runs on the port 8080

Build and start in dev mode (with hot reload)

cd broker
../mvnw compile quarkus:dev

Build and start as a jar

cd broker
../mvnw clean package
java -jar target/events-websockets-quarkus-broker-1.0-SNAPSHOT-runner.jar

Test

In a terminal, start a publication for events of category eventA:

wscat -c localhost:8080/events/eventA/publications

In another terminal, start a subscription for events of category eventA:

wscat -c localhost:8080/events/eventA/subscriptions

Type some message in the terminal of the publication and check that the message is diplayed in the terminal of the subscription.

Publisher

Build and start

The publisher runs on the port 8081

Build and start in dev mode (with hot reload)

cd publisher
../mvnw compile quarkus:dev

Build and start as a jar

cd publisher
../mvnw clean package
java -jar target/events-websockets-quarkus-publisher-1.0-SNAPSHOT-runner.jar

Test

Publish an event using curl:

curl -d '{"eventCode":"eventA", "payload":"helloA"}' -H "Content-Type: application/json" -X POST http://localhost:8081/events

Publish an event using a browser (from the navigation bar of the browser):

http://localhost:8081/events/eventA/publications?payload=helloA

After publishing the event, look at the logs displayed in the terminal(s) of the subscription(s).

You should start the subscriber before the publisher otherwise you will not be able to see the event in the logs of the subscriber !

Subscriber

Build and start

The subscriber runs on the port 8082

Build and start in dev mode (with hot reload)

cd subscriber
../mvnw compile quarkus:dev

Build and start as a jar

cd subscriber
../mvnw clean package
java -jar target/events-websockets-quarkus-subscriber-1.0-SNAPSHOT-runner.jar

Test

Subscribe to an event category using curl:

curl -X POST http://localhost:8082/events/eventA/subscriptions

Subscribe to an event category using a browser (from the navigation bar of the browser):

http://localhost:8082/events/eventA/subscriptions