Master

Since Camel 2.20

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.

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");

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.

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

URI format

master:namespace:endpoint[?options]

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

Configuring Options

Camel components are configured on two separate levels:

  • component level

  • endpoint level

Configuring Component Options

The component level is the highest level which holds general and common configurations that are inherited by the endpoints. For example a component may have security settings, credentials for authentication, urls for network connection and so forth.

Some components only have a few options, and others may have many. Because components typically have pre configured defaults that are commonly used, then you may often only need to configure a few options on a component; or none at all.

Configuring components can be done with the Component DSL, in a configuration file (application.properties|yaml), or directly with Java code.

Configuring Endpoint Options

Where you find yourself configuring the most is on endpoints, as endpoints often have many options, which allows you to configure what you need the endpoint to do. The options are also categorized into whether the endpoint is used as consumer (from) or as a producer (to), or used for both.

Configuring endpoints is most often done directly in the endpoint URI as path and query parameters. You can also use the Endpoint DSL and DataFormat DSL as a type safe way of configuring endpoints and data formats in Java.

A good practice when configuring options is to use Property Placeholders, which allows to not hardcode urls, port numbers, sensitive information, and other settings. In other words placeholders allows to externalize the configuration from your code, and gives more flexibility and reuse.

The following two sections lists all the options, firstly for the component followed by the endpoint.

Component Options

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

Name Description Default Type

bridgeErrorHandler (consumer)

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions (if possible) occurred while the Camel consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. Important: This is only possible if the 3rd party component allows Camel to be alerted if an exception was thrown. Some components handle this internally only, and therefore bridgeErrorHandler is not possible. In other situations we may improve the Camel component to hook into the 3rd party component and make this possible for future releases. 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

Endpoint Options

The Master endpoint is configured using URI syntax:

master:namespace:delegateUri

with the following path and query parameters:

Path Parameters (2 parameters)

Name Description Default Type

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

Query Parameters (3 parameters)

Name Description Default Type

bridgeErrorHandler (consumer (advanced))

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions (if possible) occurred while the Camel consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. Important: This is only possible if the 3rd party component allows Camel to be alerted if an exception was thrown. Some components handle this internally only, and therefore bridgeErrorHandler is not possible. In other situations we may improve the Camel component to hook into the 3rd party component and make this possible for future releases. 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

ExchangePattern

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

Implementations

Camel provides the following ClusterService implementations:

  • camel-consul

  • camel-file

  • camel-infinispan

  • camel-jgroups-raft

  • camel-jgroups

  • camel-kubernetes

  • camel-zookeeper