Chapter 209. Master Component

Available as of Camel version 2.20

The camel-master: endpoint provides a way to ensure only a single consumer in a cluster consumes from a given endpoint; with automatic failover if that JVM dies.

This can be very useful if you need to consume from some legacy back end which either doesn’t support concurrent consumption or due to commercial or stability reasons you can only have a single connection at any point in time.

209.1. Using the master endpoint

Just prefix any camel endpoint with master:someName: where someName is a logical name and is used to acquire the master lock. e.g.

from("master:cheese:jms:foo").to("activemq:wine");

The above simulates the [Exclusive Consumers](http://activemq.apache.org/exclusive-consumer.html) type feature in ActiveMQ; but on any third party JMS provider which maybe doesn’t support exclusive consumers.

209.2. URI format

master:namespace:endpoint[?options]

Where endpoint is any Camel endpoint you want to run in master/slave mode.

209.3. Options

The Master component supports 3 options which are listed below.

NameDescriptionDefaultType

service (advanced)

Inject the service to use.

 

CamelClusterService

serviceSelector (advanced)

Inject the service selector used to lookup the CamelClusterService to use.

 

Selector

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 Master endpoint is configured using URI syntax:

master:namespace:delegateUri

with the following path and query parameters:

209.3.1. Path Parameters (2 parameters):

NameDescriptionDefaultType

namespace

Required The name of the cluster namespace to use

 

String

delegateUri

Required The endpoint uri to use in master/slave mode

 

String

209.3.2. Query Parameters (4 parameters):

NameDescriptionDefaultType

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 options 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

209.4. Example

You can protect a clustered Camel application to only consume files from one active node.

// the file endpoint we want to consume from
String url = "file:target/inbox?delete=true";

// use the camel master component in the clustered group named myGroup
// to run a master/slave mode in the following Camel url
from("master:myGroup:" + url)
    .log(name + " - Received file: ${file:name}")
    .delay(delay)
    .log(name + " - Done file:     ${file:name}")
    .to("file:target/outbox");

The master component leverages CamelClusterService you can configure using

  • Java

    ZooKeeperClusterService service = new ZooKeeperClusterService();
    service.setId("camel-node-1");
    service.setNodes("myzk:2181");
    service.setBasePath("/camel/cluster");
    
    context.addService(service)
  • Xml (Spring/Blueprint)

    <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="cluster" class="org.apache.camel.component.zookeeper.cluster.ZooKeeperClusterService">
        <property name="id" value="camel-node-1"/>
        <property name="basePath" value="/camel/cluster"/>
        <property name="nodes" value="myzk:2181"/>
      </bean>
    
      <camelContext xmlns="http://camel.apache.org/schema/spring" autoStartup="false">
        ...
      </camelContext>
    
    </beans>
  • Spring boot

    camel.component.zookeeper.cluster.service.enabled   = true
    camel.component.zookeeper.cluster.service.id        = camel-node-1
    camel.component.zookeeper.cluster.service.base-path = /camel/cluster
    camel.component.zookeeper.cluster.service.nodes     = myzk:2181

209.5. Implementations

Camel provide the following ClusterService implementations:

  • camel-atomix
  • camel-consul
  • camel-file
  • camel-kubernetes
  • camel-zookeeper

209.6. See Also

  • Configuring Camel
  • Component
  • Endpoint
  • Getting Started