May 22, 2017

JMS ActiveMQ and the failover protocol

ActiveMQ is a open source message oriented middleware (MOM) from the Apache Software Foundation. ActiveMQ is written in Java and allows communication between systems using the JMS (Java Message Service) specification.
The most important advantage of using a MOM is to loosen the coupling between applications. A loosely coupled architecture makes the comunication asynchronous, in fact the calls from either application have no bearing on
one another. Pointing the concept we can say this:
– there is no interdependence or timing requirements
– senders know nothing about receivers and receivers know nothing about senders.
– the MOM ActveMQ is delegated to guarantee message delivery

ActiveMQ provides a JMS compliant API for sending and receiving messages. The JMS specification defines many concepts and artifacts related to messaging such as the JMS-client paradigm. A JMS client is a pure Java application which uses the JMS API for interacting with the JMS provider (ActiveMQ). JMS defines two important objects:

MessageProducer class for sending messages to a destination.
MessageConsumer class for consuming messages from a destination.

Producer and Consumer can process messages in two main styles or domains as they’re referred in the JMS specification:

point-to-point domain
publish/subscribe domain

I will focus on a point-to-point domain context, you can find further information here and here.

The point-to-point (PTP) messaging domain uses destinations known as queues:
– a producer sent messages to the queue.
– each message received on the queue is delivered (synchronously or asynchronously) to a single consumer.
In this article I will show a PTP asynchronous messaging example using the failover protocol in a Master Slave ActiveMQ configuration (Shared File System). For this purpose I will configure the following components:
– Apache ActiveMQ installed into two separate machines in a Master/Slave configuration sharing a NFS file system
– a Producer class to send messages to queue
– a Consumer class for registering
– a Listener class to consume messages from queues in an asynchronous manner

Follows the high-level architecture.

activemq-master-slave-high-level

We start with the configuration of the two Master/Slave machines. To do this we use the “shared storage” approach, where multiple ActiveMQ brokers can connect to the shared message store but only one broker will be active at a time. The shared storage is a shared NFS file system (an alternative is to using a shared database). The two amq01 and amq02 machines mount a shared NFS directory where is running the default KahaDB persistence layer. A important note: the shared file system requires the support to a distributed shared file lock, so I use the NFSv4 to prevent issues related to files corruption. Here the steps to configuration.

Now we continue with the two ActiveMQ configurations. Open the file activemq.xml in the “conf” directory in the ActiveMQ installation path and set the NFS shared directory for data.

Complete these changes on both the amq01 and amq02 machines then start the amq01 message broker. We also start the second ActiveMQ, if no errors occurs we should see the message of waiting for the database to be unlocked on the amq02 output log.

log-activemq-master-slave

Now the amq01 machine is the master entity and the amq02 is the slave. But If amq01 shuts down or fails, the amq02 slave will grabs the exclusive lock on the sahred file system and commences becoming the master entity. Now we move to the Producer and Consumer implementation.

I’m a Maven enthusiast so I create a simple java project using the “maven-archetype-quickstart” archetype. Both Producer and Consumer use a failover URL to create a connection. With the failover protocol, the ActiveMQ client will instantiate a periodic keepalive protocol, so that it can detect whether the broker is no longer reachable  (connection or broker lost). When it detects that the broker is no longer available, it will randomly select another broker from the list provided at startup. The connection string sounds like this: “failover:(tcp://broker_host01:61616,tcp://broker_host02:61616)

In the Producer class I use a “main” method to send messages to the ActiveMQ instance. The sendMessage method creates a randomly destination on coda1 and coda2 queues for the sent messages. Here an example.

Similary the Consumer class create a jms MessageConsumer and set a MessageListener on the queues to consume asynchronously the sent messages.

I also use a jms MessageListener implementation to print some message info to the standard output.

After running the maven “clean install” goal the two jms clients Producer and Consumer can be started asynchronously. I’ve run 3 use case using two dedicated maven profiles to easily start Producer and Consumer asynchronously.

USE CASE 1

UC1.a –  The Producer start
UC1.b – The Producer sends all messages to queues

activemq-uc1

UC1.c – The Consumer start
UC1.d –The Consumer immediately receives and consumes all messages on queues

activemq-uc1-consumer

USE CASE 2

UC2.a – The Consumer start
UC2.b – The Consumer is listening on the two queues and is waiting to receive messages

activemq-uc2-consumer1

UC2.c -The Producer start
UC2.d -The Producer sends all messages to queues
UC2.e -The Consumer immediately receives and consumes all messages on queues

activemq-uc2-consumer2

USE CASE 3

UC3.a -The Producer start creating a connection to the Master broker instance (AMQ01)
UC3.b -The Producer sends all messages to queues

activemq-uc3-producer1

UC3.c -The AMQ01 stops
UC3.d – In a interval of 10 seconds the Slave instance AMQ02 becames the new Master instance

activemq-uc3-producer2

UC3.e -The Consumer starts creating a connection to the new Master broker instance (AMQ02)
UC3.f – The Consumer immediately receives and consumes all messages on queues

activemq-uc3-consumer

Well, the failover protocol works!!!

 

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