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Thread-ed Background Workers on top of JRuby::Rack
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README.md

JRuby Rack Worker

Thread based workers on top of jruby-rack.

With out of the box JRuby "adapters" for:

... but one can easily write/adapt his own worker loop.

Motivation

Ruby attempts to stay pretty close to UNIX and most popular workers have been modeled the spawn a background process way. JRuby brings Java to the table, where "Young Java Knights" are thought to use threads whenever in a need to compute something parallel while serving requests.

There's no right or wrong way of doing this. If you do expect chaos like Resque proclaims - have long running jobs that consume a lot of memory they have trouble releasing (e.g. due C extensions) run a separate process for sure. But otherwise (after all C exts usually have a native Java alternative on JRuby) having predictable thread-safely written workers, one should be fine with running them concurrently as part of the application in a daemon thread.

This does have the advantage of keeping the deployment simple and saving some precious memory (most notably with threadsafe! mode) that would have been eaten by the separate process. Besides, your application might warm up faster and start benefiting from JRuby's runtime optimalizations slightly sooner ...

On the other hand your jobs should be fairly simple and complete "fast" (in a rate of seconds rather than several minutes or hours) as they will live and restart with the lifecycle of the deployed application and application server.

Setup

Copy the jruby-rack-worker.jar into the lib folder or the directory being mapped to WEB-INF/lib e.g. lib/java.

Configure the worker in web.xml, you'll need to add a servlet context listener that will start threads when your application boots and a script to be executed (should be an "endless" loop-ing script). Sample configuration :

<context-param>
  <param-name>jruby.worker.script</param-name>
  <param-value>
    require 'delayed/jruby_worker'
    Delayed::JRubyWorker.new.start
  </param-value>
</context-param>

<listener>
  <listener-class>org.kares.jruby.rack.WorkerContextListener</listener-class>
</listener>

The WorkerContextListener needs to be executed (and thus configured) after the RailsServletContextListener/RackServletContextListener as it expects the jruby-rack environment to be available.

Sample deployment descriptor including optional parameters: web.xml.

Warbler

If you're using Warbler to assemble your application you might simply declare a gem dependency with Bundler as your gems will be scanned for jars and packaged correctly:

gem 'jruby-rack-worker', :platform => :jruby, :require => nil

Otherwise copy the jar into your warble.rb configured config.java_libs.

Warbler checks for a config/web.xml.erb thus configure the worker there, e.g. :

<!DOCTYPE web-app PUBLIC
  "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
  "http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
<% webxml.context_params.each do |k,v| %>
  <context-param>
    <param-name><%= k %></param-name>
    <param-value><%= v %></param-value>
  </context-param>
<% end %>

  <filter>
    <filter-name>RackFilter</filter-name>
    <filter-class>org.jruby.rack.RackFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>RackFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>

  <listener>
    <listener-class><%= webxml.servlet_context_listener %></listener-class>
  </listener>

<% if webxml.jndi then [webxml.jndi].flatten.each do |jndi| %>
  <resource-ref>
    <res-ref-name><%= jndi %></res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
  </resource-ref>
<% end; end %>

  <!-- jruby-rack-worker setup using the built-in libraries support : -->

  <context-param>
    <param-name>jruby.worker</param-name>
    <param-value>delayed_job</param-value> <!-- or resque or navvy -->
  </context-param>

  <listener>
    <listener-class>org.kares.jruby.rack.WorkerContextListener</listener-class>
  </listener>

</web-app>

If you're deploying a Rails application on JRuby it's highly recommended to uncomment config.threadsafe!. Otherwise, if unsure or you're code is not thread-safe yet you'll end up polling several JRuby runtimes in a single process, in this case however each worker thread will use (block) an application runtime from the pool (consider it while setting jruby.min.runtimes and jruby.max.runtimes parameters).

Custom Workers

Worker Migration

There are a few gotchas to keep in mind when migrating a worker such as Delayed::Job to JRuby, You'll most probably need to start by looking at the current worker spawning script (such as script/delayed_job) :

  • avoid native gems such as daemons (in DJ's case this means avoiding the whole Delayed::Command implementation)

  • remove command line processing - all your configuration should happen in an application initializer or the web.xml

  • make sure the worker code is thread-safe in case your application is running in threadsafe! mode (make sure no global state is changing by the worker or class variables are not being used to store worker state)

  • refactor your worker's exit code from a (process oriented) signal based trap to at_exit - which respects better the JRuby environment your workers are going to run in

See the Delayed::Job JRuby "adapted" worker code for inspiration.

Build

JRuby 1.5+ is required to build the project. The build is performed by rake which should be part of your JRuby installation, if you're experiencing conflicts with another Ruby and it's rake executable use jruby -S rake instead of the bare rake command. Besides you'll to need ant installed for the Java part.

Build the jruby-rack-worker.jar using :

rake jar

Build the gem (with the jar packaged) :

rake gem
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