Database semaphore mechanism used for example to guarantee that only one instance of a job will run

database_semaphore.rb

# Please refer to http://ctoinsights.wordpress.com/2011/10/17/running-distributed-cron-jobs-in-the-cloud/
class DatabaseSemaphore < ActiveRecord::Base
  validates_presence_of :name, :message => "can't be blank"

  def self.open?(name, lock_duration = 600)
  # only one requestor can get open semaphore at a time
  # sempahore can be locked in a closed position for lock_duration in seconds
    semaphore_open = false
    now = Time.now
    # insert record if it does not exist yet
    DatabaseSemaphore.create(:name => name, :locked_at => now - lock_duration) if !DatabaseSemaphore.find_by_name(name)
    DatabaseSemaphore.transaction do
      semaphore = DatabaseSemaphore.find_by_name(name, :lock => "LOCK IN SHARE MODE")
      if semaphore and semaphore.locked_at <= now - lock_duration
        semaphore.locked_at = now
        semaphore_open = true if semaphore.save
      end
    end
    return semaphore_open
  rescue ActiveRecord::StatementInvalid => e
    # deadlock
    return false
  end
end

class CreateDatabaseSemaphores < ActiveRecord::Migration
  def self.up
    create_table :database_semaphores do |t|
      t.string :name
      t.datetime :locked_at

      t.timestamps
    end
    add_index :database_semaphores, :name, :unique => true
  end

  def self.down
    drop_table :database_semaphores
  end
end