81 lines
2.9 KiB
Markdown
81 lines
2.9 KiB
Markdown
# 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
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`INDIVIDUAL_ACKNOWLEDGE`
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In some cases you can afford to lose messages in event of failure, so it
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would make sense to acknowledge the message on the server *before*
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delivering it to the client.
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This extra mode is supported by Apache ActiveMQ Artemis and will call it
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*pre-acknowledge* mode.
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The disadvantage of acknowledging on the server before delivery is that
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the message will be lost if the system crashes *after* acknowledging the
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message on the server but *before* it is delivered to the client. In
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that case, the message is lost and will not be recovered when the system
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restart.
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Depending on your messaging case, `preAcknowledgement` mode can avoid
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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
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update messages. With these messages it might be reasonable to lose a
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message in event of crash, since the next price update message will
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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
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> transactional semantics for messages being consumed, since clearly
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> they are being acknowledged first on the server, not when you commit
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> the transaction. This may be stating the obvious but we like to be
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> clear on these things to avoid confusion!
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## Using PRE_ACKNOWLEDGE
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This can be configured by setting the boolean URL parameter `preAcknowledge`
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to `true`.
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Alternatively, when using the JMS API, create a JMS Session with the
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`ActiveMQSession.PRE_ACKNOWLEDGE` constant.
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```java
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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
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to have your own scheduling and you don't know when your message
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processing will be finished. You should prefer having one consumer per
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thread worker but this is not possible in some circumstances depending
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on how complex is your processing. For that you can use the individual
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acknowledgement.
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You basically setup Individual ACK by creating a session with the
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acknowledge mode with `ActiveMQJMSConstants.INDIVIDUAL_ACKNOWLEDGE`.
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Individual ACK inherits all the semantics from Client Acknowledge, with
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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
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> ACKNOWLEDGE is not supported through MDBs (or the inbound resource
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> adapter). this is because you have to finish the process of your
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> message inside the MDB.
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## Example
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See the [Pre-acknowledge Example](examples.md#pre-acknowledge) which shows how
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to use pre-acknowledgement mode with JMS.
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