single_instance

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= SingleInstance

SingleInstance creates a system-wide lock that prevents multiple copies of the same code to run at the same time. We use it to make sure that cron jobs do not start if an earlier invocation is still running.

# in Rakefile
task :build_models do |t, args|
  SingleInstance.exclusive_non_blocking(:build_models) do
    # ...
  end
end

Optionally, the block can be allowed to react to the fact that another process is running:

SingleInstance.exclusive_non_blocking(:build_models) do |blocker|
  if blocker
    puts "Cannot run; pid #{blocker.pid} is running since #{blocker.started_at}"
  else
    # ...
  end
end

Run without a block, it creates a lock that lasts as long as the process is running:

# in Rakefile
task :all => [:exclusive, :this, :that, :whatever]
task :exclusive do
  SingleInstance.exclusive_non_blocking(:build_models) || exit
end

There is also a simple binary (called single_instance) that you can use to wrap non-Ruby tasks:

# in crontab
*/10 * * * * single_instance /usr/local/bin/long-running-task

== Note on Patches/Pull Requests

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

== Copyright

Copyright (c) 2010-11 ICE House AB. See LICENSE for details.