Blog: Redis as a mutex service, Code: redis_spinlock

gistfile1.rb

# CAVEAT EMPTOR: This is excerpted code that I pulled out of context,
# re-arranged, renamed the variables of, and otherwise dicked around with
# a lot without having put it back into a Ruby interpreter, so it's
# possible that there's some gnarly typo or bug in here. In short, don't
# cut-and-paste this code.

class AlreadyLockedException < Exception; end

def redis_spinlock(redis, lock_handle, opts = {}, &critical_section)
  lock_set = opts[:lock_set] || 'named_locks'
  sleep_time = opts[:sleep] || 0.1
  spins = 0

  begin
    # Attempt to acquire the lock by adding our lock handle to the set of
    # locks. If the number of elements added to the set isn't 1, then
    # another process has already acquired the lock.
    #
    # Syntax subtlety: in somewhat obsequious fashion, the Ruby Redis
    # client returns bool(true) rather than int(1) if the element was
    # successfully inserted and was not passed as a single-element
    # array.
    added = redis.sadd lock_set, lock_handle
    raise AlreadyLockedException unless added === 1 || added === true

    begin
      # Here, we'll perform the operation that originally required the
      # lock. We'll pass the number of times we had to spin on the lock
      # to the received block of code. In most cases, you'll probably
      # only care whether the spin count was zero or non-zero, i.e.,
      # whether you actually had to compete with another process for the
      # lock.
      critical_section.call(spins)
    ensure
      # Unlock, even if we run into errors
      redis.srem lock_set, lock_handle
    end
  rescue AlreadyLockedException
    spins += 1
    sleep(sleep_time) if sleep_time > 0
    retry
  end
end