graviton-worker-base-java

What is it

A base library to implement Java based Graviton RabbitMQ queue workers.

What does it do

This library provides two abstracts:

• WorkerAbstract
  An abstract you can extend from to implement general purpose workers.
• FileWorkerAbstract
  An abstract you can extend from to implement /file orientated workers that provide some more convenience functions that deal with the /file API.

It also contains some models (POJOs) that help when you deal with a Graviton backend (package com.github.libgraviton.workerbase.model).

When you extend from one of those abstracts, the worker base takes care of:

• connecting to the queue
• registering the worker on /event/worker
• deserializing the Queue message to a POJO
• calling your worker function
• status tracking (working / done / failed)
• Error handling. It also provides the class WorkerException that you can throw inside your logic function. Any exception thrown (and not catched) inside your logic function will turn the /event/status entry to failed.

Configuration

There are some basic settings needed in order for the library to work. These are defined in the file src/main/resources/default.properties. Additionally, you may want to have own properties and/or overrides of defaults in your worker implementation.

You may specify a path to an additional parsed properties file using the propFile system property (i.e. by command line).

Thus, once your worker implementing the base library is usable, you may specify an additional properties file like so:

java -DpropFile=path/to/additional.properties <yourCommand>

Using the library

Due to frustrations with Maven central deployment, this library is now hosted at our own bintray repository.

Add this to your pom.xml:

<repository>
    <snapshots>
        <enabled>false</enabled>
    </snapshots>
    <id>bintray-libgraviton-maven</id>
    <name>bintray</name>
    <url>https://dl.bintray.com/libgraviton/maven</url>
</repository>

You can use this library in your project by including this in your pom.xml:

<dependencies>
	<dependency>
		<groupId>com.github.libgraviton</groupId>
		<artifactId>worker-base</artifactId>
		<version>0.15.0</version>
	</dependency>
</dependencies>	

make sure that version points to the newest release on Maven central (see badge above).

RabbitMQ integration

RabbitMQ is integrated as Work Queues (aka: Task Queues). The Howto page explains the implementation details. See (https://www.rabbitmq.com/tutorials/tutorial-two-java.html).

With this setup, we create a persistent, separate message queue for each graviton.workerId and can be sure each worker gets only the messages it needs. Additionally we have round-robin load balancing for the case when there are more than one worker running with matching graviton.workerId. Thanks to the message acknowledgment we also have a fail-over mechanism whenever a worker loses its connection to the message queue. In that case the queue will requeue the message for another available worker.

Aside from the graviton.workerId, there is also the queue.prefetchCount that can be configured. Each running worker will take as many messages at a time as configured.

graviton.workerId=myWorkerId
queue.prefetchCount=2

Cloudfoundry support

This library comes with basic Cloudfoundry VCAP_SERVICES support. If this environment variable is found with an rabbitmq-3.0 element, those connection credentials will automatically override whatever you specify via properties.

Deploying a release

bash deploy.sh