How the java client in cumulocity listens to events?

Question

Aim is to build a java client that subscribe and listen to a channel. Then process the arriving events from the cumulocity server to hadoop. First it was difficult to connect(subscribe) to the cumulocity server using the java client. However, now we have subscriber in place( as we are able to get some value against it as explained in the code comments). Next we want is the subscriber listen to the channel that we have defined in cumulocity server. But we could not get any method or anything useful in the cumulocity java docs that will help to achieve this step. Here is the code. I have anonymized the credentials and the server url.

        package c8y.example.hello_agent;

    import c8y.IsDevice;
    import com.cumulocity.model.authentication.CumulocityCredentials;
    import com.cumulocity.rest.representation.inventory.ManagedObjectRepresentation;
    import com.cumulocity.sdk.client.Platform;
    import com.cumulocity.sdk.client.PlatformImpl;
    import com.cumulocity.sdk.client.inventory.InventoryApi;

    import com.cumulocity.sdk.client.PlatformParameters;
    import com.cumulocity.sdk.client.SDKException;
    import com.cumulocity.sdk.client.notification.*;
    import com.cumulocity.sdk.client.ClientConfiguration;
    import com.cumulocity.*;

    import c8y.example.hello_agent.cred;

    public class CepCustomNotificationsSubscriber implements Subscriber<String, Object> {

        public static final String CEP_CUSTOM_NOTIFICATIONS_URL = "test/sendTemperature";

        private final Subscriber<String, Object> subscriber;

        public CepCustomNotificationsSubscriber(PlatformParameters parameters) {
            subscriber = createSubscriber(parameters);
        }

        private Subscriber<String, Object> createSubscriber(PlatformParameters parameters) {
            // @formatter:off
            return SubscriberBuilder.<String, Object>anSubscriber()
                        .withParameters(parameters)
                        .withEndpoint(CEP_CUSTOM_NOTIFICATIONS_URL)
                        .withSubscriptionNameResolver(new Identity())
                        .withDataType(Object.class)
                        .build();
            // @formatter:on
        }



        public Subscription<String> subscribe(final String channelID, final SubscriptionListener<String, Object> handler) throws SDKException {
            return subscriber.subscribe(channelID, handler);
        }

        public void disconnect() {
            subscriber.disconnect();
        }

        private static final class Identity implements SubscriptionNameResolver<String> {
            @Override
            public String apply(String id) {
                return id;
            }
        }

        public static void main( String[] args )
        {
           cred crede = new cred(); 
           String uRl = "https://xxx.cumulocityiox.com"; 
           CumulocityCredentials rC = new CumulocityCredentials(crede.name,crede.pass);
           PlatformParameters parameters = new PlatformParameters(uRl,rC, new ClientConfiguration());
           CepCustomNotificationsSubscriber t = new CepCustomNotificationsSubscriber(parameters);


           System.out.println(t.toString() + " - " + t.CEP_CUSTOM_NOTIFICATIONS_URL.toString()); // It prints an integer number corresponding to the subscriber t.
// Now how to listen to the events on the channel and get the desired data.                       
        }

    }

Since we are able to get some integer value which validate the connectivity to the server. But Now the next point is how to listen to the channel for events and get those events. Any help will be highly appreciated.

Answer 1

I supposed you have a cep module created like

insert into
  SendNotification
select
  e.event as payload,
  "customevent/" || e.event.source.value as channelName
from
  EventCreated e;

you will be able to get those events if you create a subscription to the channel "customevent/cumulocity-system-id"

In your Java code just add the lines below (Using a lambda expression)

t.subscribe("/customevent/1191201", new  SubscriptionListener (){

        @Override
        public void onNotification(Subscription s, Object r) {
            // here come the notification of the desired channel. The Object r is the event desired.
            System.out.println(r);

        }

        @Override
        public void onError(Subscription s, Throwable thrwbl) {
            // errors will come here
        }


    });

and

System.out.println(r);

will print something like

{creationTime=2017-01-26T19:00:15.837+01:00, c8y_Position=Position 
[lat=4, lng=-71.80, alt=67, accuracy=null],    
self=http://yourTenant.cumulocity.com/event/events/1202018, 
time=2017-01-    26T13:00:15.000-05:00, id=1202018, source={name=Lancer 
UBL142,
self=http://yourTenant.cumulocity.com/inventory/managedObjects/1191201, 
id=1191201}, text=Estado,Idle mode (Parking), type=c8y_LocationUpdate}

Please note that "/customevent/1191201" is your desire channelId.

Hope this helps!