On 16-12-23 22:14, Kent Borg wrote:
Hello, a newbie here!
Doing some playing with Python and librados, and it is mostly easy to
use, but I am confused about atomic operations. The documentation
isn't clear to me, and Google isn't giving me obvious answers either...
I would like to do some locking. The data structures I am playing with
in RADOS should work great if I don't accidentally fire up more than
one instance of myself. So I would like to drop an attribute on a
central object saying it's mine, all mine--but only if another copy of
myself hasn't done so already.
Is there some sample code to show me the safe way to do this?
I was using this approach when I needed to ensure that only one instance
of script is running. It would grab a lock and do what it needs to do,
you'll need to adjust it to work with your case:
def break_lock():
try:
ceph_ioctx.lock_exclusive(key=lock_obj_name + '_breaker',
name=lock_obj_name + '_lock', cookie='cookie1', duration=60)
ceph_ioctx.remove_object(key=lock_obj_name)
ceph_ioctx.unlock(key=lock_obj_name + '_breaker',
name=lock_obj_name + '_lock', cookie='cookie1')
ceph_ioctx.remove_object(key=lock_obj_name + '_breaker')
logger.info('Broke lock')
except:
logger.error('Failed to break lock')
return False
return True
def try_locking():
try:
ceph_ioctx.lock_exclusive(key=lock_obj_name, name=lock_obj_name
+ '_lock', cookie='cookie1', duration=72000)
ceph_ioctx.write_full(lock_obj_name, str(date_now))
got_lock = True
logger.info('Locked')
except:
got_lock = False
logger.warning("Didn't lock")
return got_lock
def aquire_lock():
# Let's aquire lock and update objects timestamp
if not try_locking():
logger.warning('Failed to lock, preping for retry')
lock_date = ceph_ioctx.read(lock_obj_name)
lock_age = int((date_now -
dateutil.parser.parse(lock_date)).total_seconds() / 60 / 60)
logger.debug('Lock age: %i' % (lock_age))
if lock_age > 20 and break_lock():
logger.warning('Retrying locking')
try_locking()
else:
logger.error("Failed lock breaking, bailing out")
return False
return True
def release_lock():
logger.info('Releasing lock')
ceph_ioctx.unlock(key=lock_obj_name, name=lock_obj_name + '_lock',
cookie='cookie1')
ceph_ioctx.remove_object(key=lock_obj_name)
Thanks,
-kb, the Kent who is reluctant to just play around with locking to see
what works...because it might look like it is working, yet still have
a race susceptibility.
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