Introduction

Climax is a Ruby gem designed to speed-up the development of command line applications. It provides a number of features that nearly all long-running cli application need such as logging, running as a daemon, processing command line arguments, and something else unexpected. When your application uses climax, a control DRb runs with your application, allowing you to manipulate your application as it runs in the background. For instance, if your application is running, you can remotely debug the application at any time. This is great for long-running processes in production. Something strange going on that you'd like to investigate? Use the control drb to change the log level from "info" to "debug" to get more info. Still not sure what's happening? Attach a debugger to the running process.

You get all of these features for free when your application uses climax.

Installation

To install climax simply install the climax gem: gem install climax or alternatively place gem "climax" in your application's Gemfile and run bundler.

Getting Started

It's easy to get started. Just include the Climax::Application module into your application class:

require 'climax'

class MyApplication
  include Climax::Application

  def main
    log.info "Hello World"
    return 0
  end
end

MyApplication.new(ARGV).run()

The above example is about as simple as you can get. Here we define an application and give it a main method. The main method will be called REPEATEDLY until it returns a value other than nil. In the example above main will only be called once because it returns 0.

If you save the above example in a file and run it with --help you will get a list of default command line options provided by climax:

Usage: my_application [options]
    -d, --daemon            Fork application and run in background
        --log_level         Set to debug, info, warn, error, or fatal.  Default: info.
        --log_file          File to log output to.  By default logs to stdout.
        --control_port      Override the port for the control DRb to listen on.  Default is 7249
    -h, --help              Display this help message.

As you can see climax by default provides some options free of charge. If you would like to modify how climax behaves, such as by adding more command line options or maybe removing the option to fork your application, you can provide these configurations by defining a method in your application class named configure. The configure method is called by climax before it parses command line options, before it sets up logging, basically before it does anything. This means that you cannot use climax facilities such as logging in this method because climax has not yet bootstrapped.

After the configure method is run climax bootstraps the environment for your application. It parses the arguments passed from the command line, it sets up logging, forks your application if asked to, and starts the control drb unless your application requests otherwise.

The climax framework then runs your application as follows:

• Calls the pre_main method of your application class if it exists. This is an excellent place to put code that should run once before your main method.

• Calls the main method of your application class. If this method does not exist climax will raise an exception. The main method will be called repeatedly until it returns a value other than nil. If the main method returns an integer then your application will exit with that integer as the status code. If the main method returns a string then your application will abort with that string as the message.

• Calls the post_main method of your application class if it exists. This is a good place to put code that should run once when your application is exiting.

Here is a slightly larger example that shows more of climax's features:

require 'climax'

class MyApplication
  include Climax::Application

  def configure
    options do
      on 'v', 'verbose', 'More verbose'
    end
  end
  
  def pre_main
    log.debug "App has initialized and is about to run."
  end
  
  def main
    puts "Hello World!"
    sleep 3
    return nil
  end

  def post_main
    log.debug "App has finished running and is about to exit."
  end
end

The above example is a simple application that adds an extra command line option --verbose in the configure method. The pre_main method is simple and just writes a debug log statement. As you can see logging has been setup at this point. Then the main method is called. Because the main method in this example always returns nil, this application will run forever until it is stopped externally (Ctrl-C, kill, or using the Control DRb). Using Ctrl-C and kill -INT (i.e., sending an Interrupt signal) will cause the application to exit gracefully. The current iteration of main will finish and then post_main will be run. You can send another Interrupt signal, for example by hitting Ctrl-C twice, to exit the application immediately. Likewise if the Control DRb is used then the application will exit in an orderly fashion and the post_main method will be called. Notice that for free you can fork this application, change the log level with the --log-level option, write the logs to a file using the --log-file option, and you can start a debugger at any time while this application is running using the Control DRb. The Control DRb has a lot of power. We'll talk about it more below.

Climax Event Queue

An important concept to understand is that climax handles events that come in from the Control DRb. DRb's by necessity run in a separate thread. However, climax makes absolutely no assumptions about the thread-safeness of your application. You do not need to write thread safe applications. Climax is written in such a way that it simply places events into a thread safe queue. Every time your main method completes, any existing events in the queue are processed. Your main method is then called again. This process goes on until the main method returns a value other than nil or an :exit or :quit event is placed in the event queue.

Your application can send events to the climax event queue with the climax_send_event method. You must pass climax_send_event the event type (e.g., :exit or :start_remote_debugger) and you may also optionally pass a payload (i.e., extra data) as a second parameter.

For example, if you wish for your application to exit in an orderly fashion after the current iteration of main has had a chance to finish, you can accomplish this by placing an :exit event onto the event queue and letting your main method finish its work. When your main method is finished the events on the queue will be processed by climax. When climax reads the :exit event off of the queue it will call your post_main method and perform other cleanup tasks and then exit.

require 'climax'

class MyApplication
  include Climax::Application
  
  def main
    climax_send_event(:exit) if about_to_meet_work_quota?
    work = get_some_work
    do_work(work)
    return nil
  end

  # ...

end

A consequence of this is that when you issue commands to the Control DRb, your command will not be processed until the current iteration of main has had a chance to complete. Therefore it is a good idea to keep each iteration of main as short as possible, although this is certainly not a requirement. In other words, if you wish to enter the remote debugger for a running process, the debugger will not begin until the current iteration of main has completed.

This is exactly how graceful exiting is accomplished with climax. When you send an Interrupt signal (via Ctrl-C or by sending a kill -INT to your application), climax intercepts this signal and places a :exit event onto the event queue. If climax intercepts a second Interrupt signal it will halt execution immediately.

This is excellent for stopping a long running process without interrupting its work. By sending an :exit or :quit event, whether through the Control DRb, or by sending an Interrupt signal, or from your application itself, your application will be allowed to finish processing its current unit of work before exiting.

For your convenience there is also a 'climax_has_event?' method that returns true only if there are events waiting on the event queue. Your application can use this to determine if main should give up control temporarily to allow climax to handle events on the queue.

Generating a New Application

To help you get started quickly, you can easily generate a new application using climax by running the following command:

climax create <project-name>

Climax will then create a new project for you that is ready to rock and roll. Here is the file structure generated for a new project named "my_project":

my_project/
  Gemfile
  LICENSE.txt
  README.md
  Rakefile
  bin/
    my_project*
  features/
    step_definitions/
    support/
      io.rb
      env.rb
  lib/
    my_project.rb
    my_project/
      version.rb
  my_project.gemspec
  pkg

Your new application has the boilerplate code necessary to start running your application immediately:

cd my_project
./bin/my_project --help
Usage: my_project [options]
    -d, --daemon            Fork application and run in background
        --log_level         Set to debug, info, warn, error, or fatal.  Default: info.
        --log_file          File to log output to.  By default logs to stdout.
        --control_port      Override the port for the control DRb to listen on.  Default is 7249
    -h, --help              Display this help message.

Running the application will simply print "Hello World" until you hit Ctrl-C.

The three files you will want to modify are README.md (with info about your application), <project-name>.gemspec and lib/<project-name>.rb.

You can bundle your application with gem bundle *.gemspec.

Control DRb

The Control DRb provides a way to interact with your running application. In future releases expect new functionality to be added to the Control DRb and expect new ways of interacting with your long running jobs.

You control your application with climax control. Simply pass climax control the name of the command you wish to send to your application and any optional arguments. If your application is using a different port than the default (7249) you can tell climax which port to connect on with the -p <port> option:

climax control -p 1234 start_debugger

Starting the Debugger

To attach to your running application with a debugger, simply make sure your application is running and then execute:

climax control start_debugger

When you are finished type quit to exit the debugger and resume your application.

Extending the Control DRb with Custom Functionality

Extending the Control DRb is a key aspect of climax. For instance it is a common idiom for web applications to delegate difficult tasks to background jobs. Let's take a common scenario and see how we might implement this scenario with climax.

In this example there is a web application that allows users to upload images. The image is stored in a temporary directory by the web application and then a request is sent to a background job asking it to process the image and store it in a more permanent location. Let's say the web application simply wants to send the command process_image and pass as an argument the path of the image that was uploaded.

Let's see how we can extend the Control DRb with a process_image method. We'll also see a possible workflow for how the background job might tackle its work.

require 'climax'
require 'fileutils'

module Climax
  class ControlServer
    def process_image (path)
      app.climax_send_event(:process_image, path)
    end
  end
end

class ImageProcessor
  include Climax::Application

  def configure
    options do
      on 't', 'target-directory=', 'Destination directory to save processed images to.', :default => '/var/saved/images'
    end
  end

  def pre_main
    FileUtils.mkdir_p(opts[:'target-directory'])
    @processor_queue = []
  end

  def main
    if @processor_queue.empty?
      sleep 0.5
      return nil
    end

    image_path = @processor_queue.pop
    log.info "Processing image #{image_path}"
    do_work(image_path)
    return nil
  end

  def process_image (path)
    @processor_queue.push(path)
  end

  def do_work(path)
    # process image at path
    # save to target directory
  end
end

Adding your own commands to the Control DRb couldn't be easier. Simply extend the Climax::ControlServer class with your own methods that push events onto the climax event queue. When climax processes the event it will call a method in your application instance with the same name as the event. In the example above we send a :process_image event with an argument. When climax processes this event it will attempt to call a process_image method in the application instance, passing along any arguments.

In this case we simply add the work to a simple queue and allow main to process the jobs from the queue.

Notice that when you are extending Climax::ControlServer you have access to your application instance via the app method. From here you can call any publicly available methods on your application instance. But be careful. The DRb is running in a separate thread. This is why we simply send events! It is always best to only send events from your custom ControlServer methods. Doing so allows you to never need worry about making your application thread-safe. If you're a cowboy and decide to call various public methods on your app instance directly from your custom ControlServer methods you may end up with some strange results. So unless you are familiar with thread-safe applications it is suggested that you only place events onto the queue from your custom ControlServer methods.

Now the web application can simply connect to your application's DRb and call the process_image method, passing it an image path.

Something interesting about this is that non-ruby applications can also easily tell our image processor to process images.

For instance you can send commands to your application using the climax CLI interface:

climax control process_image /tmp/uploaded-images/tmp-d8ej2.jpg

This means any application, even just simple shell scripts, can easily send commands to our background job.

Of course it is best to give each of your climax applications a unique Control DRb port. You can then specify which port to send commands to with the -p option to climax control:

climax control -p $IMAGE_PROCESSOR_PORT process_image /tmp/uploaded-images/tmp-d8ej2.jpg