Chapter 34. 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.
34.1. 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");
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.
34.2. URI format
master:namespace:endpoint[?options]
Where endpoint is any Camel endpoint you want to run in master/slave mode.
34.3. Configuring Options
Camel components are configured on two separate levels:
- component level
- endpoint level
34.3.1. 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.
34.3.2. 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 as a type safe way of configuring endpoints.
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.
34.4. 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 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 |
34.5. Endpoint Options
The Master endpoint is configured using URI syntax:
master:namespace:delegateUri
with the following path and query parameters:
34.5.1. 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 |
34.5.2. Query Parameters (3 parameters)
| Name | Description | Default | Type |
|---|---|---|---|
| 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:
| ExchangePattern |
34.6. 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
34.7. Implementations
Camel provides the following ClusterService implementations:
- camel-consul
- camel-file
- camel-infinispan
- camel-jgroups-raft
- camel-jgroups
- camel-kubernetes
- camel-zookeeper
34.8. Spring Boot Auto-Configuration
When using master with 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>
The component supports 5 options, which are listed below.
| Name | Description | Default | Type |
|---|---|---|---|
| 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 |
