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Chapter 13. 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.

13.1. 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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13.2. 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.

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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Before the broker moves an expired or undelivered AMQP message to an expiry or dead letter queue that you have configured, the broker applies annotations and properties to the message. A client can create a filter based on the properties or annotations, to select particular messages to consume from the expiry or dead letter queue.

Note

The properties that the broker applies are internal properties These properties are are not exposed to clients for regular use, but can be specified by a client in a filter.

Shown below are examples of filters based on message properties and annotations. Filtering based on properties is the recommended approach, when possible, because this approach requires less processing by the broker.

Filter based on message properties

ConnectionFactory factory = new JmsConnectionFactory("amqp://localhost:5672");
Connection connection = factory.createConnection();
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
connection.start();
javax.jms.Queue queue = session.createQueue("my_DLQ");
MessageConsumer consumer = session.createConsumer(queue, "_AMQ_ORIG_ADDRESS='original_address_name'");
Message message = consumer.receive();
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Filter based on message annotations

ConnectionFactory factory = new JmsConnectionFactory("amqp://localhost:5672");
Connection connection = factory.createConnection();
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
connection.start();
javax.jms.Queue queue = session.createQueue("my_DLQ");
MessageConsumer consumer = session.createConsumer(queue, "\"m.x-opt-ORIG-ADDRESS\"='original_address_name'");
Message message = consumer.receive();
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Note

When consuming AMQP messages based on an annotation, the client must include append a m. prefix to the message annotation, as shown in the preceding example.

Additional resources

13.4. Filtering XML Messages
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AMQ Broker provides a way of filtering Text messages that contain an XML body using XPath. XPath (XML Path Language) is a query language for selecting nodes from an XML document.

Note

Only text based messages are supported. Filtering large messages is not supported.

To filter text based messages, you need to create a Message Selector of the form XPATH '<xpath-expression>.

An example of a Message Body

<root>
    <a key='first' num='1'/>
    <b key='second' num='2'>b</b>
</root>
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Filter based on an XPath query

PATH 'root/a'
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Warning

Since XPath applies to the body of the message and requires parsing of XML, filtering can be significantly slower than normal filters.

XPath filters are supported with and between producers and consumers using the following protocols:

  • OpenWire JMS
  • Core (and Core JMS)
  • STOMP
  • AMQP

Configuring the XML Parser

By default the XML Parser used by the Broker is the Platform default DocumentBuilderFactory instance used by the JDK.

The XML parser used for XPath default configuration includes the following settings:

However, in order to deal with any implementation-specific issues the features can be customized by configuring System properties in the artemis.profile configuration file.

org.apache.activemq.documentBuilderFactory.feature:prefix

Example feature configuration

-Dorg.apache.activemq.documentBuilderFactory.feature:http://xml.org/sax/features/external-general-entities=true
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