Do you have a business requirement in which you need ActiveMQ to deploy your Camel routes but you have come to realize that in a Master/Slave configuration the Camel Context is always started on the slave broker?

In this example I will show you how you can configure ActiveMQ to deploy Camel routes as well as how to control when these routes should be started.  In this example we have a master broker with routes that start when the broker is started.  Additionally, we will have a slave broker which will have routes that we only want to start when the slave becomes the master.

I am currently using the apache-activemq-5.5.1-fuse-04-01, which is the latest release of ActiveMQ from FuseSource at the time of this writing.  You can grab the binaries/source from the following link: apache-activemq-5.5.1-fuse-04-01

So you may be wondering how you might be able to accomplish this, right?  Well luckily, we can easily have something working with just a little code and a little configuration.

Code

The code we need to implement is rather simple.  We just need to create a class that implements the ActiveMQ Service interface.  Below is the simple example I created to demonstrate how this works:

package com.fusesource.example;  
 import org.apache.activemq.Service;  
 import org.apache.camel.spring.SpringCamelContext;  
 import org.slf4j.Logger;  
 import org.slf4j.LoggerFactory;  
 /**  
  * Example used to start and stop the camel context using the ActiveMQ Service interface  
  *  
  */  
 public class CamelContextService implements Service  
 {  
   private final Logger LOG = LoggerFactory.getLogger(CamelContextService.class);  
   SpringCamelContext camel;  
   @Override  
   public void start() throws Exception {  
     try {  
       camel.start();  
     } catch (Exception e) {  
       LOG.error("Unable to start camel context: " + camel);  
       e.printStackTrace();  
     }  
   }  
   @Override  
   public void stop() throws Exception {  
     try {  
       camel.stop();  
     } catch (Exception e) {  
       LOG.error("Unable to stop camel context: " + camel);  
       e.printStackTrace();  
     }  
   }  
   public SpringCamelContext getCamel() {  
     return camel;  
   }  
   public void setCamel(SpringCamelContext camel) {  
     this.camel = camel;  
   }  
 } 

The magic behind all this is in the Service interface.  When this class is registered as a service with the broker, the start method will be called when the broker is fully initialized.  Remember to copy the jar file to the lib directory of the broker where you want this code to be invoked.

Configuration

First let's have a look at how we deploy Camel routes from ActiveMQ's broker configuration file.  In the installation directory of the broker you will find the conf directory which holds multiple examples of different broker configuration files and such.  One such file is called camel.xml which defines a simple route.  In our example we will import this file in our broker's activemq.xml as follows which will start the camel context and associated route.

 <import resource="camel.xml"/>  

  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camel) is starting  
  INFO | JMX enabled. Using ManagedManagementStrategy.  
  INFO | Found 3 packages with 14 @Converter classes to load  
  INFO | Loaded 163 core type converters (total 163 type converters)  
  INFO | Loaded 3 @Converter classes  
  INFO | Loaded additional 4 type converters (total 167 type converters) in 0.006 seconds  
  WARN | Broker localhost not started so using broker1 instead  
  INFO | Connector vm://localhost Started  
  INFO | Route: route1 started and consuming from: Endpoint[activemq://example.A]  
  INFO | Total 1 routes, of which 1 is started.  
  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camel) started in 0.532 seconds  

So now that we have the master broker up and running with a Camel route deployed we are going to do the same to the slave broker, but this time we are going to edit the camel.xml slightly as follows to set the Camel Context id to camelBackup and most importantly we are going add an attribute autoStartup and set it to false to prevent the route from starting when the Camel Context is discovered:

<camelContext id="camelBackup" xmlns="http://camel.apache.org/schema/spring" autoStartup="false">

One last thing we need to do to the slave broker is configure the new service we created from the above code.  Copy the following configuration to your slave broker's activemq.xml:

 <services>  
   <bean xmlns="http://www.springframework.org/schema/beans" class="com.fusesource.example.CamelContextService">  
      <property name="camel" ref="camelBackup"/>  
   </bean>  
 </services>  

From the above configuration and sample code, you can see Spring is being used to inject the camel property into our Service class.  Additionally, notice the ref has been set to camelBackup which is the id we used for the CamelContext in the slave's camel.xml file.

Additionally, the broker has been configured as a slave so the broker will only be fully initialized when the master broker fails.  If you want more information on configuring ActiveMQ Master/Slave brokers, take at look at one of my early posts on Master/Slave Broker Configuration.

If you haven't done it all ready, copy the jar that was created from packaging up the code from the example above to the slave broker's lib directory.

Note:  In a production system you might want to configure this on both the master and slave broker to keep the configurations mirrored, as the routes will be started on the master as well once it is fully initialized. In this post I am keeping the required configuration to the slave broker just to demonstrate the behavior.

Test Run

Now that we have the code and configuration done, let's give this a test run by starting up the slave broker.

  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camelBackup) is starting  
  INFO | JMX enabled. Using ManagedManagementStrategy.  
  INFO | Found 3 packages with 14 @Converter classes to load  
  INFO | Loaded 163 core type converters (total 163 type converters)  
  INFO | Loaded 3 @Converter classes  
  INFO | Loaded additional 4 type converters (total 167 type converters) in 0.007 seconds  
  INFO | Total 1 routes, of which 0 is started.  
  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camelBackup) started in 0.512 seconds

Looking at the output from the slave broker you can see the CamelContext is still started, however the route is not (remember we set autoStartup="false).  Now, in the terminal where the master broker is running, issue a kill to stop the broker.

If you have a look at the slave broker's output again, you can see the connector was started, openwire in this case, and the Camel route is now started.

  ERROR | Network connection between vm://broker2#0 and tcp://localhost/127.0.0.1:61616 shutdown: null  
 java.io.EOFException  
      at java.io.DataInputStream.readInt(DataInputStream.java:375)  
      at org.apache.activemq.openwire.OpenWireFormat.unmarshal(OpenWireFormat.java:275)  
      at org.apache.activemq.transport.tcp.TcpTransport.readCommand(TcpTransport.java:222)  
      at org.apache.activemq.transport.tcp.TcpTransport.doRun(TcpTransport.java:214)  
      at org.apache.activemq.transport.tcp.TcpTransport.run(TcpTransport.java:197)  
      at java.lang.Thread.run(Thread.java:680)  
  WARN | Master Failed - starting all connectors  
  INFO | Listening for connections at: tcp://macbookpro-251a.home:62616  
  INFO | Connector openwire Started  
  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camelBackup) is starting  
  WARN | Broker localhost not started so using broker2 instead  
  INFO | Connector vm://localhost Started  
  INFO | Route: route1 started and consuming from: Endpoint[activemq://example.A]  
  INFO | Total 1 routes, of which 1 is started.  
  INFO | Apache Camel 2.8.0-fuse-04-01 (CamelContext: camelBackup) started in 0.036 seconds 

Conclusion

That's all there is to it, and I think you would agree starting a route from ActiveMQ slave is rather simple and easy to implement.  I'd also like to thank Hiram Chirino, an Apache ActiveMQ Founder, for pointing me in the direction of using the ActiveMQ Service Interface.