THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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Notification
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mechanism
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is
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introduced
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in
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Hcatalog
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so
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that
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a
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user
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can
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be
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notified
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of
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certain
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events
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occurring
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in
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Metastore
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if
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she
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desires
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so.
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Events
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are
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of
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six
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types:
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add_database,
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add_table,
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add_partition,
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drop_partition,
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drop_table,
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drop_database.
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When
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any
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of
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these
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events
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occur
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on
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Metastore
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message
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is
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sent
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to
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a
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Message
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Bus.
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Any
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user
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who
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wish
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to
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get
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notification
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for
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these
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messages
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can
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subscribe
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to
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a
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message
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bus.
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Once
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subscribed,
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message
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bus
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will
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deliver
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the
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messages
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to
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the
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subscriber.
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To
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start
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receiving
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message
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you
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first
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need
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to
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create
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a
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connection
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to
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messagebus
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as
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demonstrated
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below:
| Code Block |
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ConnectionFactory connFac = new ActiveMQConnectionFactory(amqurl); Connection conn = connFac.createConnection(); conn.start(); |
Then
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subscribe
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to
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a
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topic
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you
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are
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interested
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in.
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While
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subscribing
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on
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a
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message
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bus,
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user
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need
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to
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subscribe
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to
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a
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particular
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topic
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to
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receive
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the
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messages
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which
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are
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being
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delivered
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on
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that
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topic.
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The
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topic
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name
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corresponding
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to
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a
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particular
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table
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is
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stored
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in
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table
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properties
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and
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can
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be
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retrieved
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using
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following
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piece
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of
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code:
| Code Block |
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HiveMetaStoreClient msc = new HiveMetaStoreClient(hiveConf); String topicName = msc.getTable("mydb", "myTbl").getParameters().get(HCatConstants.HCAT_MSGBUS_TOPIC_NAME); |
Use
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the
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topic
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name
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to
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subscribe
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to
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a
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topic
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as
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follows:
| Code Block |
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Session session = conn.createSession(true, Session.SESSION_TRANSACTED); Destination hcatTopic = session.createTopic(topicName); MessageConsumer consumer = session.createConsumer(hcatTopic); consumer.setMessageListener(this); |
To
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start
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receiving
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messages
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user
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need
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to
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implement
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a
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jms
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interface
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MessageListener
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which
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will
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make
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you
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implement
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a
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method
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onMessage(Message
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msg).
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This
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method
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will
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be
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called
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whenever
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a
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new
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message
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arrives
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on
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MessageBus.
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The
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message
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contains
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a
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partition
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object
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representing
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the
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corresponding
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partition,
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which
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can
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be
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retrieved
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as
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following:
| Code Block |
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@Override public void onMessage(Message @Override public void onMessage(Message msg) { Object obj = msg){ // We are interested in only add_partition events on this table. // So, check message type first. if(msg.getStringProperty(HCatConstants.HCAT_EVENT).equals(HCatConstants.HCAT_ADD_PARTITION_EVENT)){ //Get the actual partition object which got added. Partition partition = (Partition)(((ObjectMessage)msg).getObject()); } } |
You
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need
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to
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have
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jms
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jar
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in
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your
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classpath
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to
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make
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this
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work.
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You
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additionally
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need
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to
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have
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a
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jms
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provider’s
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jar
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in
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your
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classpath
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as
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well.
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Hcatalog
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uses
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ActiveMQ
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as
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a
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jms
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provider.
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In
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principle
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any
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JMS
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provider
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can
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be
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used
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in
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client
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side.
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However,
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ActiveMQ
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is
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recommended.
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It
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can
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be
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obtained
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from:
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http://activemq.apache.org/activemq-550-release.html
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