Chapter 11. Flow Control

The maximum size of messages held in the client-side buffer is determined by its window size. The default size of the window for AMQ Broker clients is 1 MiB, or 1024 * 1024 bytes. The default is fine for most use cases. For other cases, finding the optimal value for the window size might require benchmarking your system. AMQ Broker allows you to set the buffer window size if you need to change the default.

Setting the Window Size

The following examples demonstrate how to set the consumer window size parameter when using a Core JMS client. Each example sets a consumers window size to 300000 bytes.

Fast consumers can process messages as fast as they consume them. If you are confident that the consumers in your messaging system are that fast, consider setting the window size to -1. This setting allows for unbounded message buffering on the client side. Use this setting with caution, however. It can overflow client-side memory if the consumer is not able to process messages as fast as it receives them.

Setting the Window Size for Fast Consumers

Slow consumers take significant time to process each message. In these cases, it is recommended to not buffer messages on the client side. Messages remain on the broker side ready to be consumed by other consumers instead. One benefit of turning off the buffer is that it provides deterministic distribution between multiple consumers on a queue. To handle slow consumers by disabling the client-side buffer, set the window size to 0.

Setting the Window Size for Slow Consumers

You can regulate the rate at which a consumer can consume messages. Also known as "throttling", regulating the rate of consumption ensures that a consumer never consumes messages at a rate faster than configuration allows.

The rate must be a positive integer to enable this functionality and is the maximum desired message consumption rate specified in units of messages per second. Setting the rate to -1 disables rate-limited flow control. The default value is -1.

Setting the Rate of Consuming Messages

The window size is negotiated between the broker and producer on the basis of credits, one credit for each byte in the window. As messages are sent and credits are used, the producer must request, and be granted, credits from the broker before it can send more messages. The exchange of credits between producer and broker regulates the flow of data between them.

Setting the Window Size

The following examples demonstrate how to set the producer window size to 1024 bytes when using Core JMS clients.

Because more than one producer can be associated with the same address, it is possible for the broker to allocate more credits across all producers than what is actually available. However, you can set a maximum size on any address that prevents the broker from sending more credits than are available.

In the default configuration, a global maximum size of 100Mb is used for each address. When the address is full, the broker writes further messages to the paging journal instead of routing them to the queue. Instead of paging, you can block the sending of more messages on the client side until older messages are consumed. Blocking producer flow control in this way prevents the broker from running out of memory due to producers sending more messages than can be handled at any one time.

In the configuration, blocking producer flow control is managed on a per address-setting basis. The configuration applies to all queues registered to an address. In other words, the total memory for all queues bound to that address is capped by the value given for max-size-bytes.

Configuring the Maximum Size for an Address

To configure the broker to block messages if they are larger than the set maximum number of bytes, add a new addres-setting configuration element to BROKER_INSTANCE_DIR/etc/broker.xml.

<configuration>
  <core>
    ...
    <address-settings>
       <address-setting match="my.blocking.queue"> 1
          <max-size-bytes>300000</max-size-bytes>  2
          <address-full-policy>BLOCK</address-full-policy> 3
       </address-setting>
    </address-settings>
  </core>
</configuration>

As explained earlier in this chapter the Core protocol uses a producer window-size flow control system. In this system, credits represent bytes and are allocated to producers. If a producer wants to send a message, it must wait until it has sufficient credits to accommodate the size of a message before sending it.

AMQP flow control credits are not representative of bytes, however, but instead represent the number of messages a producer is permitted to send, regardless of the message size. It is therefore possible in some scenarios for an AMQP client to significantly exceed the max-size-bytes of an address.

To manage this situation, add the element max-size-bytes-reject-threshold to the address-setting to specify an upper bound on an address size in bytes. Once this upper bound is reached, the broker rejects AMQP messages. By default, max-size-bytes-reject-threshold is set to -1, or no limit.

Configuring the Broker to Block AMQP Messages

To configure the broker to block AMQP messages if they are larger than the set maximum number of bytes, add a new addres-setting configuration element to BROKER_INSTANCE_DIR/etc/broker.xml.

AMQ Broker can also limit the rate a producer can emit messages. The producer rate is specified in units of messages per second. Setting it to -1, the default, disables rate-limited flow control.

Setting the Rate of Sending Messages

The examples below demonstrate how to set the rate of sending messages when the producer is using a Core JMS client. Each example sets the maximum rate of sending messages to 10 per second.