EventNotification

Notification mechanism is introduced in Hcatalog so that a user can be notified of certain events occurring in Metastore if she desires so. Events are of six types: add_database, add_table, add_partition, drop_partition, drop_table, drop_database. When any of these events occur on Metastore message is sent to a Message Bus. Any user who wish to get notification for these messages can subscribe to a message bus. Once subscribed, message bus will deliver the messages to the subscriber.

To start receiving message you first need to create a connection to messagebus as demonstrated below:

ConnectionFactory connFac = new ActiveMQConnectionFactory(amqurl);
Connection conn = connFac.createConnection();
conn.start();

Then subscribe to a topic you are interested in. While subscribing on a message bus, user need to subscribe to a particular topic to receive the messages which are being delivered on that topic. The topic name corresponding to a particular table is stored in table properties and can be retrieved using following piece of code:

HiveMetaStoreClient msc = new HiveMetaStoreClient(hiveConf);
String topicName = msc.getTable("mydb", "myTbl").getParameters().get(HCatConstants.HCAT_MSGBUS_TOPIC_NAME);

Use the topic name to subscribe to a topic as follows:

Session session = conn.createSession(true, Session.SESSION_TRANSACTED);
Destination hcatTopic = session.createTopic(topicName);
MessageConsumer consumer = session.createConsumer(hcatTopic);
consumer.setMessageListener(this);

To start receiving messages user need to implement a jms interface MessageListener which will make you implement a method onMessage(Message msg). This method will be called whenever a new message arrives on MessageBus. The message contains a partition object representing the corresponding partition, which can be retrieved as following:

@Override
public void onMessage(Message msg)

You need to have jms jar in your classpath to make this work. You additionally need to have a jms provider’s jar in your classpath as well. Hcatalog uses ActiveMQ as a jms provider. In principle any JMS provider can be used in client side. However, ActiveMQ is recommended. It can be obtained from: http://activemq.apache.org/activemq-550-release.html