63 lines
2.9 KiB
Plaintext
63 lines
2.9 KiB
Plaintext
= Extra Acknowledge Modes
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JMS specifies 3 acknowledgement modes:
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* `AUTO_ACKNOWLEDGE`
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* `CLIENT_ACKNOWLEDGE`
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* `DUPS_OK_ACKNOWLEDGE`
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Apache ActiveMQ Artemis supports two additional modes: `PRE_ACKNOWLEDGE` and `INDIVIDUAL_ACKNOWLEDGE`
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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.
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This extra mode is supported by Apache ActiveMQ Artemis and will call it _pre-acknowledge_ mode.
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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.
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In that case, the message is lost and will not be recovered when the system restart.
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Depending on your messaging case, `preAcknowledgement` mode can avoid extra network traffic and CPU at the cost of coping with message loss.
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An example of a use case for pre-acknowledgement is for stock price update messages.
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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.
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[NOTE]
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====
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Please note, that if you use pre-acknowledge mode, then you will lose transactional semantics for messages being consumed, since clearly they are being acknowledged first on the server, not when you commit the transaction.
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This may be stating the obvious but we like to be clear on these things to avoid confusion!
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====
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== Using PRE_ACKNOWLEDGE
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This can be configured by setting the boolean URL parameter `preAcknowledge` to `true`.
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Alternatively, when using the JMS API, create a JMS Session with the `ActiveMQSession.PRE_ACKNOWLEDGE` constant.
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[,java]
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----
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// messages will be acknowledge on the server *before* being delivered to the client
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Session session = connection.createSession(false, ActiveMQJMSConstants.PRE_ACKNOWLEDGE);
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----
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== Individual Acknowledge
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A valid use-case for individual acknowledgement would be when you need to have your own scheduling and you don't know when your message processing will be finished.
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You should prefer having one consumer per thread worker but this is not possible in some circumstances depending on how complex is your processing.
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For that you can use the individual acknowledgement.
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You basically setup Individual ACK by creating a session with the acknowledge mode with `ActiveMQJMSConstants.INDIVIDUAL_ACKNOWLEDGE`.
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Individual ACK inherits all the semantics from Client Acknowledge, with the exception the message is individually acked.
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[NOTE]
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====
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Please note, that to avoid confusion on MDB processing, Individual ACKNOWLEDGE is not supported through MDBs (or the inbound resource adapter).
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this is because you have to finish the process of your message inside the MDB.
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====
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== Example
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See the xref:examples.adoc#pre-acknowledge[Pre-acknowledge Example] which shows how to use pre-acknowledgement mode with JMS.
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