29 lines
2.1 KiB
Markdown
29 lines
2.1 KiB
Markdown
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# No Consumer Buffering 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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By default, ActiveMQ Artemis consumers buffer messages from the broker in a client side buffer before actual delivery actually occurs.
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This improves performance since otherwise every time you called receive() or had processed the last message in a MessageListener onMessage() method, the ActiveMQ Artemis client would have to go the broker to request the next message involving a network round trip for every message reducing performance.
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Therefore, by default, ActiveMQ Artemis pre-fetches messages into a buffer on each consumer. The total maximum size of messages in bytes that will be buffered on each consumer is determined by the `consumerWindowSize` parameter on the connection URL.
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In some cases it is not desirable to buffer any messages on the client side consumer.
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An example would be an order queue which had multiple consumers that processed orders from the queue. Each order takes a significant time to process, but each one should be processed in a timely fashion.
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If orders were buffered in each consumer, and a new consumer was added that consumer would not be able to process orders which were already in the client side buffer of another consumer.
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To turn off client side buffering of messages, set `consumerWindowSize` to zero.
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With ActiveMQ Artemis you can specify a maximum consume rate at which a JMS MessageConsumer will consume messages. This can be specified when configuring the connection URL.
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## Example step-by-step
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In this example we specify a `consumerWindowSize` of `0` bytes on the connection URL in `jndi.properties`.
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We create a consumer on a queue and send 10 messages to it. We then create another consumer on the same queue.
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We then consume messages from each consumer in a semi-random order. We note that the messages are consumed in the order they were sent.
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If the messages had been buffered in each consumer they would not be available to be consumed in an order determined after delivery.
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