Kestrel

Since Camel 2.6

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.

The kestrel project is inactive and the Camel team regard this components as 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

The Kestrel component supports 2 options, which are listed below.

Name Description Default Type

configuration (advanced)

To use a shared configured configuration as base for creating new endpoints.

KestrelConfiguration

resolveProperty Placeholders (advanced)

Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders.

true

boolean

The Kestrel endpoint is configured using URI syntax:

kestrel:addresses/queue

with the following path and query parameters:

Path Parameters (2 parameters):

Name Description Default Type

addresses

The address(es) on which kestrel is running

localhost:22133

String[]

queue

Required The queue we are polling

String

Query Parameters (6 parameters):

Name Description Default Type

concurrentConsumers (common)

How many concurrent listeners to schedule for the thread pool

1

int

waitTimeMs (common)

How long a given wait should block (server side), in milliseconds

100

int

bridgeErrorHandler (consumer)

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN or ERROR level and ignored.

false

boolean

exceptionHandler (consumer)

To let the consumer use a custom ExceptionHandler. Notice if the option bridgeErrorHandler is enabled then this option is not in use. By default the consumer will deal with exceptions, that will be logged at WARN or ERROR level and ignored.

ExceptionHandler

exchangePattern (consumer)

Sets the exchange pattern when the consumer creates an exchange.

ExchangePattern

synchronous (advanced)

Sets whether synchronous processing should be strictly used, or Camel is allowed to use asynchronous processing (if supported).

false

boolean

Spring Boot Auto-Configuration

When using Spring Boot make sure to use the following Maven dependency to have support for auto configuration:

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-kestrel-starter</artifactId>
  <version>x.x.x</version>
  <!-- use the same version as your Camel core version -->
</dependency>

The component supports 5 options, which are listed below.

Name Description Default Type

camel.component.kestrel.configuration.addresses

The address(es) on which kestrel is running

String[]

camel.component.kestrel.configuration.concurrent-consumers

How many concurrent listeners to schedule for the thread pool

1

Integer

camel.component.kestrel.configuration.wait-time-ms

How long a given wait should block (server side), in milliseconds

100

Integer

camel.component.kestrel.enabled

Enable kestrel component

true

Boolean

camel.component.kestrel.resolve-property-placeholders

Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders.

true

Boolean

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.

Warning: Limitations

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.
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