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Subscription Query UI Streaming

This demo application aims to showcase how to stream the results from a Subscription Query to the UI. To that end, this module contains two main packages:

  1. src/main/java - contains the Axon-based Spring Boot application called SubscriptionQueryUiStreamingApplication, using Java 11
  2. src/main/ui - contains a basic Angular UI that on start up invokes an endpoint of the java application

For more detail on the application, read this and this section for the Java and Angular portion respectively. If you want to test this sample application, the How to Run section explains how to run it.

Java App

The Java application consists out of three mains parts.

First of these is the EventPublisher in the commandmodel package. The EventPublisher periodically publishes a StreamUpdatedEvent to spoof an active applications. It fills the StreamUpdatedEvent with UUIDs.

Secondly, the ModelProjector in the querymodel package is in charge of handling the StreamUpdatedEvent. It adds th e contents to a List of strings, and emits an update through the QueryUpdateEmitter. Next to that, a @QueryHandler annotated method is present for the ModelQuery. This query handler returns the entire list of updates. The combination of this query handler, and the update emission on the event handler provide an entry point for a subscription query.

Thirdly, the QueryController in the ui package provides an endpoint on /app/updates. This endpoint returns a Flux of ServerSentEvents. TheServerSentEvents are filled with the result of a subscription query on the ModelQuery. As an addition, the controller combines the update stream with a heartbeat stream, ensuring a application shutdown correctly closes the stream.

Angular App

How to run

To run this application, both the Java app and the Angular app should be started. The Java app will require Axon Server as a backing event store.

To run Axon Server, Docker can be used:

docker run -d -p 8024:8024 -p 8124:8124 --name axonserver axoniq/axonserver

If desired, Axon Server can also be downloaded in a zip here. The zip file contains a runnable jar that will start Axon Server for you.

With Axon Server in place, the Java application SubscriptionQueryUiStreamingApplication can be started. To start the application, the following command can be run from the main folder:

mvn spring-boot:run

This starts the application using port 8080. At this stage the application is already publishing events. The endpoint returning the stream of server sent events can already be tested at http://localhost:8080/app/updates.

To run the Angular application, the following command should be executed from the src/main/ui folder:

npm start

This starts the UI and http://localhost:4200/. Opening the URL we prompt you with the text App is running!. Below this, a new entry is added everytime a new ServerSentEvent is received.