9.6. Selective Consumer
Overview
The selective consumer pattern, shown in Figure 9.5, “Selective Consumer Pattern”, describes a consumer that applies a filter to incoming messages, so that only messages meeting specific selection criteria are processed.
Figure 9.5. Selective Consumer Pattern
You can implement the selective consumer pattern in Apache Camel using one of the following approaches:
JMS selector
A JMS selector is a predicate expression involving JMS headers and JMS properties. If the selector evaluates to
true, the JMS message is allowed to reach the consumer, and if the selector evaluates to false, the JMS message is blocked. For example, to consume messages from the queue, selective, and select only those messages whose country code property is equal to US, you can use the following Java DSL route:
from("jms:selective?selector=" + java.net.URLEncoder.encode("CountryCode='US'","UTF-8")).
to("cxf:bean:replica01");
Where the selector string,
CountryCode='US', must be URL encoded (using UTF-8 characters) to avoid trouble with parsing the query options. This example presumes that the JMS property, CountryCode, is set by the sender. For more details about JMS selectors, see the section called “JMS selectors”.
Note
If a selector is applied to a JMS queue, messages that are not selected remain on the queue and are potentially available to other consumers attached to the same queue.
JMS selector in ActiveMQ
You can also define JMS selectors on ActiveMQ endpoints. For example:
from("acivemq:selective?selector=" + java.net.URLEncoder.encode("CountryCode='US'","UTF-8")).
to("cxf:bean:replica01");
For more details, see ActiveMQ: JMS Selectors and ActiveMQ Message Properties.
Message filter
If it is not possible to set a selector on the consumer endpoint, you can insert a filter processor into your route instead. For example, you can define a selective consumer that processes only messages with a US country code using Java DSL, as follows:
from("seda:a").filter(header("CountryCode").isEqualTo("US")).process(myProcessor);
The same route can be defined using XML configuration, as follows:
<camelContext id="buildCustomProcessorWithFilter" xmlns="http://camel.apache.org/schema/spring">
<route>
<from uri="seda:a"/>
<filter>
<xpath>$CountryCode = 'US'</xpath>
<process ref="#myProcessor"/>
</filter>
</route>
</camelContext>
For more information about the Apache Camel filter processor, see Message Filter.
Warning
Be careful about using a message filter to select messages from a JMS queue. When using a filter processor, blocked messages are simply discarded. Hence, if the messages are consumed from a queue (which allows each message to be consumed only once—see Section 9.4, “Competing Consumers”), then blocked messages are not processed at all. This might not be the behavior you want.
