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Chapter 91. Kestrel

Kestrel Component

The Kestrel component allows messages to be sent to a Kestrel queue, or messages to be consumed from a Kestrel queue. This component uses the spymemcached client for memcached protocol communication with Kestrel servers.
Warning
The Kestrel project is inactive and this component is therefore deprecated.

URI format

kestrel://[addresslist/]queuename[?options]
Where queuename is the name of the queue on Kestrel. The addresslist part of the URI may include one or more host:port pairs. For example, to connect to the queue foo on kserver01:22133, use: 
kestrel://kserver01:22133/foo
If the addresslist is omitted, localhost:22133 is assumed, i.e.: 
kestrel://foo
Likewise, if a port is omitted from a host:port pair in addresslist, the default port 22133 is assumed, i.e.: 
kestrel://kserver01/foo
Here is an example of a Kestrel endpoint URI used for producing to a clustered queue: 
kestrel://kserver01:22133,kserver02:22133,kserver03:22133/massive
Here is an example of a Kestrel endpoint URI used for consuming concurrently from a queue: 
kestrel://kserver03:22133/massive?concurrentConsumers=25&waitTimeMs=500

Options

You can configure properties on each Kestrel endpoint individually by specifying them in the ?parameters portion of the endpoint URI. Any ?parameters that are omitted will default to what is configured on the KestrelComponent's base KestrelConfiguration. The following properties may be set on KestrelConfiguration and/or each individual endpoint:
Option Default Value Description
concurrentConsumers 1 Specifies the number of concurrent consumer threads.
waitTimeMs 100 Specifies the /t=... wait time passed to Kestrel on GET requests.
NOTE: If waitTimeMs is set to zero (or negative), the /t=... specifier does not get passed to the server on GET requests. When a queue is empty, the GET call returns immediately with no value. In order to prevent "tight looping" in the polling phase, this component will do a Thread.sleep(100) whenever nothing is returned from the GET request (only when nothing is returned). You are highly encouraged to configure a positive non-zero value for waitTimeMs.

Configuring the Kestrel component using Spring XML

The simplest form of explicit configuration is as follows: 
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">

  <bean id="kestrel" class="org.apache.camel.component.kestrel.KestrelComponent"/>

  <camelContext xmlns="http://camel.apache.org/schema/spring">
  </camelContext>

</beans>
That will enable the Kestrel component with all default settings, i.e. it will use localhost:22133, 100ms wait time, and a single non-concurrent consumer by default.
To use specific options in the base configuration (which supplies configuration to endpoints whose ?properties are not specified), you can set up a KestrelConfiguration POJO as follows: 
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">

  <bean id="kestrelConfiguration" class="org.apache.camel.component.kestrel.KestrelConfiguration">
    <property name="addresses" value="kestrel01:22133"/>
    <property name="waitTimeMs" value="100"/>
    <property name="concurrentConsumers" value="1"/>
  </bean>

  <bean id="kestrel" class="org.apache.camel.component.kestrel.KestrelComponent">
    <property name="configuration" ref="kestrelConfiguration"/>
  </bean>

  <camelContext xmlns="http://camel.apache.org/schema/spring">
  </camelContext>

</beans>

Usage Examples

Example 1: Consuming

from("kestrel://kserver02:22133/massive?concurrentConsumers=10&waitTimeMs=500")
  .bean("myConsumer", "onMessage");

public class MyConsumer {
    public void onMessage(String message) {
        ...
    }
}

Example 2: Producing

public class MyProducer {
    @EndpointInject(uri = "kestrel://kserver01:22133,kserver02:22133/myqueue")
    ProducerTemplate producerTemplate;

    public void produceSomething() {
        producerTemplate.sendBody("Hello, world.");
    }
}

Example 3: Spring XML Configuration

  <camelContext xmlns="http://camel.apache.org/schema/spring">
    <route>
      <from uri="kestrel://ks01:22133/sequential?concurrentConsumers=1&waitTimeMs=500"/>
      <bean ref="myBean" method="onMessage"/>
    </route>
    <route>
      <from uri="direct:start"/>
      <to uri="kestrel://ks02:22133/stuff"/>
    </route>
  </camelContext>

public class MyBean {
    public void onMessage(String message) {
        ...
    }
}

Dependencies

The Kestrel component has the following dependencies:
  • spymemcached 2.5 (or greater)

spymemcached

You must have the spymemcached jar on your classpath. Here is a snippet you can use in your pom.xml: 
<dependency>
  <groupId>spy</groupId>
  <artifactId>memcached</artifactId>
  <version>2.5</version>
</dependency>
Alternatively, you can download the jar directly.
Limitations
NOTE: The spymemcached client library does not work properly with kestrel when JVM assertions are enabled. There is a known issue with spymemcached when assertions are enabled and a requested key contains the /t=... extension (i.e. if you're using the waitTimeMs option on an endpoint URI, which is highly encouraged).
Fortunately, JVM assertions are disabled by default, unless you explicitly enable them, so this should not present a problem under normal circumstances.
Something to note is that Maven's Surefire test plugin enables assertions. If you're using this component in a Maven test environment, you may need to set enableAssertions to false. Please refer to the surefire:test reference for details.