Leader Election¶
Each group can elect its own leader. There can be only one leader at a time in a group. Only members that are running for the election can be elected. As soon as one of leader steps down or dies, a new member that was running for the election will be elected.
import time
import uuid
import six
from tooz import coordination
ALIVE_TIME = 1
coordinator = coordination.get_coordinator('zake://', b'host-1')
coordinator.start()
# Create a group
group = six.binary_type(six.text_type(uuid.uuid4()).encode('ascii'))
request = coordinator.create_group(group)
request.get()
# Join a group
request = coordinator.join_group(group)
request.get()
def when_i_am_elected_leader(event):
# event is a LeaderElected event
print(event.group_id, event.member_id)
# Propose to be a leader for the group
coordinator.watch_elected_as_leader(group, when_i_am_elected_leader)
start = time.time()
while time.time() - start < ALIVE_TIME:
coordinator.heartbeat()
coordinator.run_watchers()
time.sleep(0.1)
coordinator.stop()
The method
tooz.coordination.CoordinationDriver.watch_elected_as_leader() allows
to register for a function to be called back when the member is elected as a
leader. Using this function indicates that the run is therefore running for
the election. The member can stop running by unregistering all its callbacks
with tooz.coordination.CoordinationDriver.unwatch_elected_as_leader().
It can also temporarily try to step down as a leader with
tooz.coordination.CoordinationDriver.stand_down_group_leader(). If
another member is in the run for election, it may be elected instead.
To retrieve the leader of a group, even when not being part of the group,
the method tooz.coordination.CoordinationDriver.get_leader() can be
used.
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