# 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.

    # Copyright (C) 2020 Red Hat, Inc.
    #
    # Licensed under the Apache License, Version 2.0 (the "License");
    # you may not use this file except in compliance with the License.
    # You may obtain a copy of the License at
    #
    #    http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
    # implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.

    import time
    import uuid

    from tooz import coordination

    ALIVE_TIME = 1
    coordinator = coordination.get_coordinator('zake://', b'host-1')
    coordinator.start()

    # Create a group
    group = bytes(str(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.

this page last updated: 2025-02-03 10:42:00

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