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This component provides a server which is able to handle connections, communication and messages exchange with remote MQTT clients.

Its API provides a bunch of events related to raw protocol messages received by clients and exposes some features in order to send messages to them.

It’s not a fully featured MQTT broker but can be used for building something like that or for protocol translation.

Warning
this module has the tech preview status, this means the API can change between versions.

Using Vert.x MQTT server

This component had officially released in the Vert.x stack, just following dependency to the dependencies section of your build descriptor:

  • Maven (in your pom.xml):

<dependency>
    <groupId>io.vertx</groupId>
    <artifactId>vertx-mqtt-server</artifactId>
    <version>3.4.2</version>
</dependency>
  • Gradle (in your build.gradle file):

compile io.vertx:vertx-mqtt-server:3.4.2

Getting Started

Handling client connection/disconnection

This example shows how it’s possible to handle the connection request from a remote MQTT client. First, an MqttServer instance is created and the endpointHandler method is used to specify the handler called when a remote client sends a CONNECT message for connecting to the server itself. The MqttEndpoint instance, provided as parameter to the handler, brings all main information related to the CONNECT message like client identifier, username/password, "will" information, clean session flag, protocol version and the "keep alive" timeout. Inside that handler, the endpoint instance provides the accept method for replying to the remote client with the corresponding CONNACK message : in this way, the connection is established. Finally, the server is started using the listen method with the default behavior (on localhost and default MQTT port 1883). The same method allows to specify an handler in order to check if the server is started properly or not.

MqttServer mqttServer = MqttServer.create(vertx);
mqttServer.endpointHandler(endpoint -> {

  // shows main connect info
  System.out.println("MQTT client [" + endpoint.clientIdentifier() + "] request to connect, clean session = " + endpoint.isCleanSession());

  if (endpoint.auth() != null) {
    System.out.println("[username = " + endpoint.auth().userName() + ", password = " + endpoint.auth().password() + "]");
  }
  if (endpoint.will() != null) {
    System.out.println("[will topic = " + endpoint.will().willTopic() + " msg = " + endpoint.will().willMessage() +
      " QoS = " + endpoint.will().willQos() + " isRetain = " + endpoint.will().isWillRetain() + "]");
  }

  System.out.println("[keep alive timeout = " + endpoint.keepAliveTimeSeconds() + "]");

  // accept connection from the remote client
  endpoint.accept(false);

})
  .listen(ar -> {

    if (ar.succeeded()) {

      System.out.println("MQTT server is listening on port " + ar.result().actualPort());
    } else {

      System.out.println("Error on starting the server");
      ar.cause().printStackTrace();
    }
  });

The same endpoint instance provides the disconnectHandler for specifying the handler called when the remote client sends a DISCONNECT message in order to disconnect from the server; this handler takes no parameters.

endpoint.disconnectHandler(v -> {

  System.out.println("Received disconnect from client");
});

Handling client connection/disconnection with SSL/TLS support

The server has the support for accepting connection requests through the SSL/TLS protocol for authentication and encryption. In order to do that, the MqttServerOptions class provides the setSsl method for setting the usage of SSL/TLS (passing 'true' as value) and some other useful methods for providing server certificate and related private key (as Java key store reference, PEM or PFX format). In the following example, the setKeyCertOptions method is used in order to pass the certificates in PEM format. This method requires an instance of the possible implementations of the KeyCertOptions interface and in this case the PemKeyCertOptions class is used in order to provide the path for the server certificate and the private key with the correspondent setCertPath and setKeyPath methods. The MQTT server is started passing the Vert.x instance as usual and the above MQTT options instance to the creation method.

MqttServerOptions options = new MqttServerOptions()
  .setPort(8883)
  .setKeyCertOptions(new PemKeyCertOptions()
    .setKeyPath("./src/test/resources/tls/server-key.pem")
    .setCertPath("./src/test/resources/tls/server-cert.pem"))
  .setSsl(true);

MqttServer mqttServer = MqttServer.create(vertx, options);
mqttServer.endpointHandler(endpoint -> {

  // shows main connect info
  System.out.println("MQTT client [" + endpoint.clientIdentifier() + "] request to connect, clean session = " + endpoint.isCleanSession());

  if (endpoint.auth() != null) {
    System.out.println("[username = " + endpoint.auth().userName() + ", password = " + endpoint.auth().password() + "]");
  }
  if (endpoint.will() != null) {
    System.out.println("[will topic = " + endpoint.will().willTopic() + " msg = " + endpoint.will().willMessage() +
      " QoS = " + endpoint.will().willQos() + " isRetain = " + endpoint.will().isWillRetain() + "]");
  }

  System.out.println("[keep alive timeout = " + endpoint.keepAliveTimeSeconds() + "]");

  // accept connection from the remote client
  endpoint.accept(false);

})
  .listen(ar -> {

    if (ar.succeeded()) {

      System.out.println("MQTT server is listening on port " + ar.result().actualPort());
    } else {

      System.out.println("Error on starting the server");
      ar.cause().printStackTrace();
    }
  });

All the other stuff related to handle endpoint connection and related disconnection is managed in the same way without SSL/TLS support.

Handling client subscription/unsubscription request

After a connection is established between client and server, the client can send a subscription request for a topic using the SUBSCRIBE message. The MqttEndpoint interface allows to specify an handler for the incoming subscription request using the subscribeHandler method. Such handler receives an instance of the MqttSubscribeMessage interface which brings the list of topics with related QoS levels as desired by the client. Finally, the endpoint instance provides the subscribeAcknowledge method for replying to the client with the related SUBACK message containing the granted QoS levels.

endpoint.subscribeHandler(subscribe -> {

  List<MqttQoS> grantedQosLevels = new ArrayList<>();
  for (MqttTopicSubscription s: subscribe.topicSubscriptions()) {
    System.out.println("Subscription for " + s.topicName() + " with QoS " + s.qualityOfService());
    grantedQosLevels.add(s.qualityOfService());
  }
  // ack the subscriptions request
  endpoint.subscribeAcknowledge(subscribe.messageId(), grantedQosLevels);

});

In the same way, it’s possible to use the unsubscribeHandler method on the endpoint in order to specify the handler called when the client sends an UNSUBSCRIBE message. This handler receives an instance of the MqttUnsubscribeMessage interface as parameter with the list of topics to unsubscribe. Finally, the endpoint instance provides the unsubscribeAcknowledge method for replying to the client with the related UNSUBACK message.

endpoint.unsubscribeHandler(unsubscribe -> {

  for (String t: unsubscribe.topics()) {
    System.out.println("Unsubscription for " + t);
  }
  // ack the subscriptions request
  endpoint.unsubscribeAcknowledge(unsubscribe.messageId());
});

Handling client published message

In order to handle incoming messages published by the remote client, the MqttEndpoint interface provides the publishHandler method for specifying the handler called when the client sends a PUBLISH message. This handler receives an instance of the MqttPublishMessage interface as parameter with the payload, the QoS level, the duplicate and retain flags.

If the QoS level is 0 (AT_MOST_ONCE), there is no need from the endpoint to reply the client.

If the QoS level is 1 (AT_LEAST_ONCE), the endpoind needs to reply with a PUBACK message using the available publishAcknowledge method.

If the QoS level is 2 (EXACTLY_ONCE), the endpoint needs to reply with a PUBREC message using the available publishReceived method; in this case the same endpoint should handle the PUBREL message received from the client as well (the remote client sends it after receiving the PUBREC from the endpoint) and it can do that specifying the handler through the publishReleaseHandler method. In order to close the QoS level 2 delivery, the endpoint can use the publishComplete method for sending the PUBCOMP message to the client.

endpoint.publishHandler(message -> {

  System.out.println("Just received message [" + message.payload().toString(Charset.defaultCharset()) + "] with QoS [" + message.qosLevel() + "]");

  if (message.qosLevel() == MqttQoS.AT_LEAST_ONCE) {
    endpoint.publishAcknowledge(message.messageId());
  } else if (message.qosLevel() == MqttQoS.EXACTLY_ONCE) {
    endpoint.publishRelease(message.messageId());
  }

}).publishReleaseHandler(messageId -> {

  endpoint.publishComplete(messageId);
});

Publish message to the client

The endpoint can publish a message to the remote client (sending a PUBLISH message) using the publish method which takes the following input parameters : the topic to publish, the payload, the QoS level, the duplicate and retain flags.

If the QoS level is 0 (AT_MOST_ONCE), the endpoint won’t receiving any feedback from the client.

If the QoS level is 1 (AT_LEAST_ONCE), the endpoint needs to handle the PUBACK message received from the client in order to receive final acknowledge of delivery. It’s possible using the publishAcknowledgeHandler method specifying such an handler.

If the QoS level is 2 (EXACTLY_ONCE), the endpoint needs to handle the PUBREC message received from the client. The publishReceivedHandler method allows to specify the handler for that. Inside that handler, the endpoint can use the publishRelease method for replying to the client with the PUBREL message. The last step is to handle the PUBCOMP message received from the client as final acknowledge for the published message; it’s possible using the publishCompleteHandler for specifying the handler called when the final PUBCOMP message is received.

endpoint.publish("my_topic",
  Buffer.buffer("Hello from the Vert.x MQTT server"),
  MqttQoS.EXACTLY_ONCE,
  false,
  false);

// specifing handlers for handling QoS 1 and 2
endpoint.publishAcknowledgeHandler(messageId -> {

  System.out.println("Received ack for message = " +  messageId);

}).publishReceivedHandler(messageId -> {

  endpoint.publishRelease(messageId);

}).publishCompleteHandler(messageId -> {

  System.out.println("Received ack for message = " +  messageId);
});

Be notified by client keep alive

The underlying MQTT keep alive mechanism is handled by the server internally. When the CONNECT message is received, the server takes care of the keep alive timeout specified inside that message in order to check if the client doesn’t send messages in such timeout. At same time, for every PINGREQ received, the server replies with the related PINGRESP.

Even if there is no need for the high level application to handle that, the MqttEndpoint interface provides the pingHandler method for specifying an handler called when a PINGREQ message is received from the client. It’s just a notification to the application that the client isn’t sending meaningful messages but only pings for keeping alive; in any case the PINGRESP is automatically sent by the server internally as described above.

endpoint.pingHandler(v -> {

  System.out.println("Ping received from client");
});

Closing the server

The MqttServer interface provides the close method that can be used for closing the server; it stops to listen for incoming connections and closes all the active connections with remote clients. This method is asynchronous and one overload provides the possibility to specify a complention handler that will be called when the server is really closed.

mqttServer.close(v -> {

  System.out.println("MQTT server closed");
});

Automatic clean-up in verticles

If you’re creating MQTT servers from inside verticles, those servers will be automatically closed when the verticle is undeployed.

Scaling : sharing MQTT servers

The handlers related to the MQTT server are always executed in the same event loop thread. It means that on a system with more cores, only one instance is deployed so only one core is used. In order to use more cores, it’s possible to deploy more instances of the MQTT server.

It’s possible to do that programmatically:

for (int i = 0; i < 10; i++) {

  MqttServer mqttServer = MqttServer.create(vertx);
  mqttServer.endpointHandler(endpoint -> {
    // handling endpoint
  })
    .listen(ar -> {

      // handling start listening
    });

}

or using a verticle specifying the number of instances:

DeploymentOptions options = new DeploymentOptions().setInstances(10);
vertx.deployVerticle("com.mycompany.MyVerticle", options);

What’s really happen is that even only MQTT server is deployed but as incoming connections arrive, Vert.x distributes them in a round-robin fashion to any of the connect handlers executed on different cores.