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Chapter 15. Filtering Messages

AMQ Broker provides a powerful filter language based on a subset of the SQL 92 expression syntax. The filter language uses the same syntax as used for JMS selectors, but the predefined identifiers are different. The table below lists the identifiers that apply to a AMQ Broker message.

Expand
IdentifierAttribute

AMQPriority

The priority of a message. Message priorities are integers with valid values from 0 through 9. 0 is the lowest priority and 9 is the highest.

AMQExpiration

The expiration time of a message. The value is a long integer.

AMQDurable

Whether a message is durable or not. The value is a string. Valid values are DURABLE or NON_DURABLE.

AMQTimestamp

The timestamp of when the message was created. The value is a long integer.

AMQSize

The value of the encodeSize property of the message. The value of encodeSize is the space, in bytes, that the message takes up in the journal. Because the broker uses a double-byte character set to encode messages, the actual size of the message is half the value of encodeSize.

Any other identifiers used in core filter expressions are assumed to be properties of the message. For documentation on selector syntax for JMS Messages, see the Java EE API.

15.1. Configuring a Queue to Use a Filter
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You can add a filter to the queues you configure in BROKER_INSTANCE_DIR/etc/broker.xml. Only messages that match the filter expression enter the queue.

Procedure

  • Add the filter element to the desired queue and include the filter you want to apply as the value of the element. In the example below, the filter NEWS='technology' is added to the queue technologyQueue. 

    <configuration>
  <core>
    ...
    <addresses>
        <address name="myQueue">
           <anycast>
              <queue name="myQueue">
                <filter string="NEWS='technology'"/>
              </queue>
           </anycast>
        </address>
     </addresses>
   </core>
</configuration>
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15.2. Filtering JMS Message Properties
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The JMS specification states that a String property must not be converted to a numeric type when used in a selector. For example, if a message has the age property set to the String value 21, the selector age > 18 must not match it. This restriction limits STOMP clients because they can send only messages with String properties.

The JMS specification also states that hyphens cannot be used as property identifiers, making them unavailable for use in filters. However, this constraint can be overcome by using the hyphenated_props: prefix.

Configuring a Filter to Convert a String to a Number

To convert String properties to a numeric type, add the prefix convert_string_expressions: to the value of the filter.

Procedure

  • Edit BROKER_INSTANCE_DIR/etc/broker.xml by applying the prefix convert_string_expressions: to the desired filter. The example below edits the filter value from age > 18 to convert_string_expressions:age > 18. 

    <configuration>
  <core>
    ...
    <addresses>
        <address name="myQueue">
           <anycast>
              <queue name="myQueue">
                <filter string="convert_string_expressions='age > 18'"/>
              </queue>
           </anycast>
        </address>
     </addresses>
   </core>
</configuration>
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Enabling a Filter to Use Hyphens

To enable the use of hyphens when filtering JMS properties, add the prefix hyphenated_props: to the value of the filter.

Procedure

  • Edit BROKER_INSTANCE_DIR/etc/broker.xml by applying the prefix hyphenated_props: to the desired filter. In the example below, a filter is edited so that it can select for the hyphenated property foo-bar. 

    <configuration>
  <core>
    ...
    <addresses>
        <address name="myQueue">
           <anycast>
              <queue name="myQueue">
                <filter string="hyphenated_props='foo-bar = 0'"/>
              </queue>
           </anycast>
        </address>
     </addresses>
   </core>
</configuration>
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