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Chapter 83. Master

Only consumer is supported

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.

83.1. Dependencies
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When using master with Red Hat build of Camel Spring Boot make sure to use the following Maven dependency to have support for auto configuration: 

<dependency>
  <groupId>org.apache.camel.springboot</groupId>
  <artifactId>camel-master-starter</artifactId>
</dependency>
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83.2. Using the master endpoint
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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");
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In this example, there master component ensures that the route is only active in one node, at any given time, in the cluster. So if there are 8 nodes in the cluster, then the master component will elect one route to be the leader, and only this route will be active, and hence only this route will consume messages from jms:foo. In case this route is stopped or unexpected terminated, then the master component will detect this, and re-elect another node to be active, which will then become active and start consuming messages from jms:foo.

Note

Apache ActiveMQ 5.x has such feature out of the box called Exclusive Consumers.

83.3. URI format
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master:namespace:endpoint[?options]
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Where endpoint is any Camel endpoint you want to run in master/slave mode.

83.4. Configuring Options
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Camel components are configured on two levels:

  • Component level
  • Endpoint level

83.4.1. Component Level Options
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The component level is the highest level. The configurations you define at this level are inherited by all the endpoints. For example, a component can have security settings, credentials for authentication, urls for network connection, and so on.

Since components typically have pre-configured defaults for the most common cases, you may need to only configure a few component options, or maybe none at all.

You can configure components with Component DSL in a configuration file (application.properties|yaml), or directly with Java code.

83.4.2. Endpoint Level Options
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At the Endpoint level you have many options, which you can use to configure what you want the endpoint to do. The options are categorized according to whether the endpoint is used as a consumer (from) or as a producer (to) or used for both.

You can configure endpoints directly in the endpoint URI as path and query parameters. You can also use Endpoint DSL and DataFormat DSL as type safe ways of configuring endpoints and data formats in Java.

When configuring options, use Property Placeholders for urls, port numbers, sensitive information, and other settings.

Placeholders allows you to externalize the configuration from your code, giving you more flexible and reusable code.

83.5. Component Options
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The Master component supports 4 options, which are listed below.

Expand
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

autowiredEnabled (advanced)

Whether autowiring is enabled. This is used for automatic autowiring options (the option must be marked as autowired) by looking up in the registry to find if there is a single instance of matching type, which then gets configured on the component. This can be used for automatic configuring JDBC data sources, JMS connection factories, AWS Clients, etc.

true

boolean

service (advanced)

Inject the service to use.

 

CamelClusterService

serviceSelector (advanced)

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

 

Selector

83.6. Endpoint Options
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The Master endpoint is configured using URI syntax: 

master:namespace:delegateUri
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with the following path and query parameters:

83.6.1. Path Parameters (2 parameters)
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Expand
NameDescriptionDefaultType

namespace (consumer)

Required The name of the cluster namespace to use.

 

String

delegateUri (consumer)

Required The endpoint uri to use in master/slave mode.

 

String

83.6.2. Query Parameters (3 parameters)
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Expand
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 (advanced))

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 (advanced))

Sets the exchange pattern when the consumer creates an exchange.

Enum values:

  • InOnly
  • InOut
  • InOptionalOut
 

ExchangePattern

83.7. Example
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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");
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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)
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  • 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>
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  • 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
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83.8. Implementations
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Camel provides the following ClusterService implementations:

  • camel-consul
  • camel-file
  • camel-infinispan
  • camel-jgroups-raft
  • camel-jgroups
  • camel-kubernetes
  • camel-zookeeper

83.9. Spring Boot Auto-Configuration
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The component supports 5 options, which are listed below.

Expand
NameDescriptionDefaultType

camel.component.master.autowired-enabled

Whether autowiring is enabled. This is used for automatic autowiring options (the option must be marked as autowired) by looking up in the registry to find if there is a single instance of matching type, which then gets configured on the component. This can be used for automatic configuring JDBC data sources, JMS connection factories, AWS Clients, etc.

true

Boolean

camel.component.master.bridge-error-handler

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

camel.component.master.enabled

Whether to enable auto configuration of the master component. This is enabled by default.

 

Boolean

camel.component.master.service

Inject the service to use. The option is a org.apache.camel.cluster.CamelClusterService type.

 

CamelClusterService

camel.component.master.service-selector

Inject the service selector used to lookup the CamelClusterService to use. The option is a org.apache.camel.cluster.CamelClusterService.Selector type.

 

CamelClusterService$Selector

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