53 lines
2.0 KiB
Markdown
53 lines
2.0 KiB
Markdown
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# Non-Destructive Queues
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When a consumer attaches to a queue, the normal behaviour is that messages are
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sent to that consumer are acquired exclusively by that consumer, and when the
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consumer acknowledges them, the messages are removed from the queue.
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Another common pattern is to have queue "browsers" which send all messages to
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the browser, but do not prevent other consumers from receiving the messages,
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and do not remove them from the queue when the browser is done with them. Such
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a browser is an instance of a "non-destructive" consumer.
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If every consumer on a queue is non destructive then we can obtain some
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interesting behaviours. In the case of a [last value
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queue](last-value-queues.md) then the queue will always contain the most up to
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date value for every key.
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A queue can be created to enforce all consumers are non-destructive using the
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following queue configuration:
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```xml
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<address name="foo.bar">
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<multicast>
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<queue name="orders1" non-destructive="true" />
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</multicast>
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</address>
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```
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Or on auto-create when using the JMS client by using address parameters when
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creating the destination used by the consumer.
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```java
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Queue queue = session.createQueue("my.destination.name?non-destructive=true");
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Topic topic = session.createTopic("my.destination.name?non-destructive=true");
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```
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Also the default for all queues under and address can be defaulted using the
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`address-setting` configuration:
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```xml
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<address-setting match="nonDestructiveQueue">
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<default-non-destructive>true</default-non-destructive>
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</address-setting>
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```
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By default, `default-non-destructive` is `false`.
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## Limiting the Size of the Queue
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For queues other than last-value queues, having only non-destructive consumers
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could mean that messages would never get deleted, leaving the queue to grow
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without constraint. To prevent this you can use the ability to set a default
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`expiry-delay`. See [expiry-delay](message-expiry.md#configuring-expiry-delay)
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for more details on this. You could also use a [ring queue](ring-queues.md).
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