32 lines
2.0 KiB
Markdown
32 lines
2.0 KiB
Markdown
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# Dead Letter Example
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To run the example, simply type **mvn verify** from this directory, or **mvn -PnoServer verify** if you want to start and create the broker manually.
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This example shows you how to define and deal with dead letter messages.
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Messages can be delivered unsuccessfully (e.g. if the transacted session used to consume them is rolled back). Such a message goes back to the JMS destination ready to be redelivered. However, this means it is possible for a message to be delivered again and again without any success and remain in the destination, clogging the system.
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To prevent this, messaging systems define dead letter messages: after a specified unsuccessful delivery attempts, the message is removed from the destination and instead routed to a _dead letter address_ where they can be consumed for further investigation.
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The example will show how to configure ActiveMQ Artemis to route a message to a dead letter address after 3 unsuccessful delivery attempts.
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The example will send 1 message to a queue. We will deliver the message 3 times and rollback the session every time.
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On the 4th attempt, there won't be any message to consume: it will have been moved to a _dead letter address_.
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We will then consume this dead letter message.
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## Example setup
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_Dead letter addresses_ and _maximum delivery attempts_ are defined in the configuration file [broker.xml](src/main/resources/activemq/server0/broker.xml):
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<address-setting match="exampleQueue">
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<dead-letter-address>deadLetterQueue</dead-letter-address>
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<max-delivery-attempts>3</max-delivery-attempts>
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</address-setting>
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This configuration will moved dead letter messages from `exampleQueue` to the `deadLetterQueue`.
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ActiveMQ Artemis allows to specify either an address or a queue. In this example, we will use a queue to hold the dead letter messages.
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The maximum attempts of delivery is `3`. Once this figure is reached, a message is considered a dead letter message and is moved to the `deadLetterQueue`.
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