Asynchronous Client Creation in Ditto Java Client 1.3.0

Before Ditto Java Client 1.3.0, a client object connects to a configured Ditto back-end during its creation.

// create a client object and block until it connects to the Ditto back-end.
final DittoClient client = DittoClients.newInstance(messagingProvider);

There are several problems with the approach.

1. The calling thread blocks waiting for IO, namely the authentication process and establishment of a websocket.
2. If the client is configured to reconnect, then an incorrect end-ponit configuration makes the factory method block forever.
3. If the client is not configured to reconnect, then the factory method will throw an exception. But it is not possible to give the client reference to the exception handler, since the client creation did not complete. Consequently the exception handler has no simple way to free all resources allocated for the client.

1.3.0 addresses these problems by introducing an asynchronous client creation interface.

public final class DittoClients {

    public static DisconnectedDittoClient newDisconnectedInstance(MessagingProvider mp);
}

public interface DisconnectedDittoClient {

    CompletionStage<DittoClient> connect();

    void destroy();
}

The method DittoClients.newDisconnectedInstance(MessagingProvider) creates a DisconnectedDittoClient object. The DisconnectedDittoClient has references to all resources allocated for the client and can free them via the destroy() method. The DisconnectedDittoClient object offers no method to interact with the Ditto API. By calling connect(), one obtains a future that yields a familiar DittoClient object upon completion. One might use the asynchronous client creation interface thus:

final DisconnectedDittoClient disconnectedClient =
    DittoClients.newDisconnectedInstance(messagingProvider);

disconnectedClient.connect()
    .thenAccept(this::startUsingDittoClient)
    .exceptionally(exception -> {
        this.handleConnectionFailure(exception);
        disconnectedClient.destroy();
        return null;
    });

The asynchronous client creation interface has the following advantages.

1. The calling thread does not block.

2. Even if configured to reconnect, the user can receive connection errors via the connection error handler in MessagingConfiguration and shut down the client at will.

3. When initial reconnection is disabled, the method DisconnectedDittoClient.connect() returns a failed future on connection error. It is possible to reference the DittoDisconnectedClient object in the future’s error handler, where the client can be destroyed.

–
The Eclipse Ditto team