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.

Defined Under Namespace

Classes: Serializer

Constant Summary collapse

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.

2147483647
RESERVATION_LIMIT =

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

25

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

.cancel_all_reservations ⇒ Object

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



91
92
93
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 91

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

.cancel_reservations(reservation) ⇒ Object

Cancels all outstanding WorkUnit reservations for this process.



85
86
87
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 85

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

.distribute_to_nodes ⇒ Object

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.



38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 38

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.to_a
    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).to_a

    # Round robin through the nodes and units, sending the unit if the node
    # is able to process it.
    while (unit = work_units.shift) and available_nodes.any? do
      while node = available_nodes.shift do
        if node.actions.include?(unit.action) and node.send_work_unit(unit)
          available_nodes.push(node) unless node.busy?
          break
        end
      end
      work_units.push(unit) unless unit.assigned?
    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

.find_by_worker_name(name) ⇒ Object

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



97
98
99
100
101
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 97

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

.reserve_available(options = {}) ⇒ Object

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



75
76
77
78
79
80
81
82
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 75

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

.start(job, action, input, status) ⇒ Object

Convenience method for starting a new WorkUnit.



104
105
106
107
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 104

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

#active_model_serializer ⇒ Object 



187
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 187

def active_model_serializer; Serializer; end

#assign_to(node_record, worker_pid) ⇒ Object

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



164
165
166
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 164

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

#assigned? ⇒ Boolean

Returns:



168
169
170
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 168

def assigned?
  !!(node_record_id && worker_pid)
end

#cancel_reservation ⇒ Object

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



158
159
160
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 158

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

#fail(output, time_taken) ⇒ Object

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



134
135
136
137
138
139
140
141
142
143
144
145
146
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 134

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

#finish(result, time_taken) ⇒ Object

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.



113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 113

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

#parsed_output(out = self.output) ⇒ Object

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.



175
176
177
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 175

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

#to_json ⇒ Object 



188
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 188

def to_json; Serializer.new(self).to_json; end

#try_again ⇒ Object

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



149
150
151
152
153
154
155
 
# File 'lib/cloud_crowd/models/work_unit.rb', line 149

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