Question

When deploying Rails via Passenger or Mongrel you have multiple instances of the application running. What is the best practice or pattern to establish a mutex on shared resources such as a writing to a local file or to a remote file. I want to ensure two processes are not writing to the same resource at the same time.

Answer 1

If you just need to prevent multiple writers from working with a file simultaneously, you can use the File#flock method to request an exclusive write lock from each process:

fh = File.new("/some/file/path")
begin
  fh.flock(File::LOCK_EX)
  # ... write to the file here, or perform some other critical operation
ensure
  fh.flock(File::LOCK_UN)
end

Note: putting the unlock call in an ensure block is important to prevent deadlock if an uncaught exception is thrown after you’ve locked the file.