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Chapter 27. Pre-Acknowledge Mode

JMS specifies three acknowledgment modes:
  • AUTO_ACKNOWLEDGE
  • CLIENT_ACKNOWLEDGE
  • DUPS_OK_ACKNOWLEDGE
However there is another case which is not supported by JMS. In some cases you can afford to lose messages in event of failure, so it would make sense to acknowledge the message on the server before delivering it to the client.
This extra mode is referred to in HornetQ as pre-acknowledge mode.
The disadvantage of acknowledging on the server before delivery is that the message will be lost if the system crashes after acknowledging the message on the server but before it is delivered to the client.
Depending on your messaging case, pre-acknowledgement mode can avoid extra network traffic and CPU at the cost of coping with message loss.
An example of a use case for pre-acknowledgment is for stock price update messages. With these messages it might be reasonable to lose a message in event of crash, since the next price update message will arrive soon, overriding the previous price.

Note

Please note, that if you use pre-acknowledge mode, then you will lose transactional semantics for messages being consumed. This is because they are being acknowledged first on the server, not when you commit the transaction.

27.1. Using PRE_ACKNOWLEDGE
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This can be configured in the JBOSS_DIST/jboss-as/server/<PROFILE>/deploy/hornetq/hornetq-jms.xml file on the connection factory like this: 
<connection-factory name="NettyConnectionFactory">
   <connectors>
      <connector-ref connector-name="netty-connector"/>
   </connectors>
   <entries>
      <entry name="/ConnectionFactory"/>
   </entries>
   <pre-acknowledge>true</pre-acknowledge>
</connection-factory>
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Alternatively, to use pre-acknowledgment mode using the JMS API, create a JMS Session with the HornetQSession.PRE_ACKNOWLEDGE constant. 
// messages will be acknowledge on the server *before* being delivered to the client
Session session = connection.createSession(false, HornetQSession.PRE_ACKNOWLEDGE);
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Or you can set pre-acknowledge directly on the HornetQConnectionFactory instance using the setter method.
To use pre-acknowledgment mode using the core API you can set it directly on the ClientSessionFactory instance using the setter method.
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