Class: CloudCrowd::WorkUnit

Inherits:
ActiveRecord::Base
  • Object
show all
Includes:
ModelStatus
Defined in:
lib/cloud_crowd/models/work_unit.rb

Overview

A WorkUnit is an atomic chunk of work from a job, processing a single input through a single action. The WorkUnits are run in parallel, with each worker daemon processing one at a time. The splitting and merging stages of a job are each run as a single WorkUnit.

Constant Summary

MAX_RESERVATION =

We use a random number in (0…MAX_RESERVATION) to reserve work units. The size of the maximum signed integer in MySQL – SQLite has no limit.

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RESERVATION_LIMIT =

We only reserve a certain number of WorkUnits in a single go, to avoid reserving the entire table.

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Class Method Summary (collapse)

Instance Method Summary (collapse)

Methods included from ModelStatus

#complete?, #display_status, #failed?, included, #incomplete?, #merging?, #processing?, #splitting?, #succeeded?

Class Method Details

+ (Object) cancel_all_reservations

Cancels all outstanding WorkUnit reservations for all processes. (Useful in the console for debugging.)



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# File 'lib/cloud_crowd/models/work_unit.rb', line 93

def self.cancel_all_reservations
  WorkUnit.update_all('reservation = null')
end

+ (Object) cancel_reservations(reservation)

Cancels all outstanding WorkUnit reservations for this process.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 87

def self.cancel_reservations(reservation)
  WorkUnit.reserved(reservation).update_all('reservation = null')
end

+ (Object) distribute_to_nodes

Attempt to send a list of WorkUnits to nodes with available capacity. A single central server process stops the same WorkUnit from being distributed to multiple nodes by reserving it first. The algorithm used should be lock-free.

We reserve WorkUnits for this process in chunks of RESERVATION_LIMIT size, and try to match them to Nodes that are capable of handling the Action. WorkUnits get removed from the availability list when they are successfully sent, and Nodes get removed when they are busy or have the action in question disabled.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 40

def self.distribute_to_nodes
  reservation = nil
  loop do

    # Find the available nodes, and determine what actions we're capable
    # of running at the moment.
    available_nodes   = NodeRecord.available
    available_actions = available_nodes.map {|node| node.actions }.flatten.uniq
    filter            = "action in (#{available_actions.map{|a| "'#{a}'"}.join(',')})"

    # Reserve a handful of available work units.
    WorkUnit.cancel_reservations(reservation) if reservation
    return unless reservation = WorkUnit.reserve_available(:limit => RESERVATION_LIMIT, :conditions => filter)
    work_units = WorkUnit.reserved(reservation)

    # Round robin through the nodes and units, sending the unit if the node
    # is able to process it.
    work_units.each do |unit|
      available_nodes.each do |node|
        if node.actions.include? unit.action
          if node.send_work_unit unit
            work_units.delete unit
            available_nodes.delete node if node.busy?
            break
          end
        end
      end
    end

    # If we still have units at this point, or we're fresh out of nodes,
    # that means we're done.
    return if work_units.any? || available_nodes.empty?
  end
ensure
  WorkUnit.cancel_reservations(reservation) if reservation
end

+ (Object) find_by_worker_name(name)

Look up a WorkUnit by the worker that's currently processing it. Specified by pid@host.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 99

def self.find_by_worker_name(name)
  pid, host = name.split('@')
  node = NodeRecord.find_by_host(host)
  node && node.work_units.find_by_worker_pid(pid)
end

+ (Object) reserve_available(options = {})

Reserves all available WorkUnits for this process. Returns false if there were none available.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 79

def self.reserve_available(options={})
  reservation = ActiveSupport::SecureRandom.random_number(MAX_RESERVATION)
  conditions = "reservation is null and node_record_id is null and status in (#{INCOMPLETE.join(',')}) and #{options[:conditions]}"
  any = WorkUnit.update_all("reservation = #{reservation}", conditions, options) > 0
  any && reservation
end

+ (Object) start(job, action, input, status)

Convenience method for starting a new WorkUnit.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 106

def self.start(job, action, input, status)
  input = input.to_json unless input.is_a? String
  self.create(:job => job, :action => action, :input => input, :status => status)
end

Instance Method Details

- (Object) assign_to(node_record, worker_pid)

When a Node checks out a WorkUnit, establish the connection between WorkUnit and NodeRecord and record the worker_pid.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 166

def assign_to(node_record, worker_pid)
  update_attributes!(:node_record => node_record, :worker_pid => worker_pid)
end

- (Object) cancel_reservation

If the node can't process the unit, cancel it's reservation.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 160

def cancel_reservation
  update_attributes!(:reservation => nil)
end

- (Object) fail(output, time_taken)

Mark this unit as having failed. May attempt a retry.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 136

def fail(output, time_taken)
  tries = self.attempts + 1
  return try_again if tries < CloudCrowd.config[:work_unit_retries]
  update_attributes({
    :status         => FAILED,
    :node_record    => nil,
    :worker_pid     => nil,
    :attempts       => tries,
    :output         => output,
    :time           => time_taken
  })
  job && job.check_for_completion
end

- (Object) finish(result, time_taken)

Mark this unit as having finished successfully. Splitting work units are handled differently (an optimization) – they immediately fire off all of their resulting WorkUnits for processing, without waiting for the rest of their splitting cousins to complete.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 115

def finish(result, time_taken)
  if splitting?
    [parsed_output(result)].flatten.each do |new_input|
      WorkUnit.start(job, action, new_input, PROCESSING)
    end
    self.destroy
    job.set_next_status if job && job.done_splitting?
  else
    update_attributes({
      :status         => SUCCEEDED,
      :node_record    => nil,
      :worker_pid     => nil,
      :attempts       => attempts + 1,
      :output         => result,
      :time           => time_taken
    })
    job && job.check_for_completion
  end
end

- (Object) parsed_output(out = self.output)

All output needs to be wrapped in a JSON object for consistency (unfortunately, JSON.parse needs the top-level to be an object or array). Convenience method to provide the parsed version.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 173

def parsed_output(out = self.output)
  JSON.parse(out)['output']
end

- (Object) to_json

The JSON representation of a WorkUnit shares the Job's options with all its cousin WorkUnits.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 179

def to_json
  {
    'id'        => self.id,
    'job_id'    => self.job_id,
    'input'     => self.input,
    'attempts'  => self.attempts,
    'action'    => self.action,
    'options'   => JSON.parse(self.job.options),
    'status'    => self.status
  }.to_json
end

- (Object) try_again

Ever tried. Ever failed. No matter. Try again. Fail again. Fail better.



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# File 'lib/cloud_crowd/models/work_unit.rb', line 151

def try_again
  update_attributes({
    :node_record  => nil,
    :worker_pid   => nil,
    :attempts     => self.attempts + 1
  })
end