When you writing a distributed application or webservice, sometimes you need to synchronize processes on different machines. For example, you may need to run some cron job once on a random machine.

If you are using MySQL, than you can ise it's GET_LOCK function, to accomplish this task. I wrote this helper to create global lock in python, using with statement:

import contextlib
@contextlib.contextmanager
@write_session
def global_lock(name, timeout, session):
    result = session.execute('SELECT GET_LOCK(%s, %s)', (name, timeout)).fetchall()
    result = len(result) == 1 and result[0][0]
    try:
        yield result and True or False
    finally:
        if result:
            session.execute('DO RELEASE_LOCK(%s)', (name, ))

Here write_session is decorator, which passes a session argument to a function. Here I use sqlalchemy's session, but you could use MySQLdb's connection or something like that.

And here is one of usecases for function:

def cron():
    with global_lock('myapp.cron_lock', 0) as acquired:
        if acquired:
            do_something_significant()
        else:
            exit_without_doing_any_job()

That's it. But make sure, to use unique name for the lock, because you may get in trouble if some other application uses same name.