Live ISS Position

A very simple project using The Open Notify API and Leaflet to show where the International Space Station is, right now.

The Open Notify is a pure (and free, with no auth needed, thanks mates ! that's awesome for a demo) JSON API sending the coordinates of the ISS when you make the call.

The API is located here.

Launching

You should first install vertx-sse to your local maven repository. Navigate to the root of this project and type :

./gradlew install

Then, from this directory, simply type :

./gradlew run

Then point your browser at : http://localhost:9001

Explanations

You can see there's just a single Verticle (ISSVerticle) which :

• starts an http server on port 9001
• uses a vertx-web router to define static files
• reroutes / to /static/index.html

But that's not really what we're interested about in this example.

The most interesting parts are :

sse.connectHandler(connection -> {
    connection.forward(EB_ADDRESS);
});
router.get("/iss/position").handler(sse);

Which means that every EventSource connection made to /iss/position will receive messages sent over the event-bus on the EB_ADDRESS address.

Then, a periodic task fetches the ISS Position using the Open Notify API every 10 seconds.

client.getNow("/iss-now.json", resp -> {
    if (resp.statusCode() != 200) {
        LOG.error("Could not fetch ISS position {}", resp.statusCode());
        return;
    }
    resp.bodyHandler(buff -> {
        JsonObject json = buff.toJsonObject();
        if (!"success".equals(json.getString("message"))) {
            LOG.error("Could not fetch ISS position {}", json.toString());
            return;
        }
        JsonObject position = json.getJsonObject("iss_position");
        vertx.eventBus().publish(EB_ADDRESS, position);
    });
});

When the position is received, it's published over the event-bus on EB_ADDRESS.

Which means, thanks to the forward usage, that every EventSource connected will receive the position.

Let's have a look at client side : webroot/iss.js

var es = new EventSource('/iss/position');
es.onmessage = function(msg) {
    moveISS(JSON.parse(msg.data));
};

Simply creates a new EventSource, connected to /iss/position and calls the moveISS methods whenever it receives a new message. The moveISS is nothing but displaying a marker on a leaflet map.