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# Copyright 2021 The Kubernetes Authors.
#
# 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 datetime
import sys
import time
import json
import threading
from .leaderelectionrecord import LeaderElectionRecord
import logging
# if condition to be removed when support for python2 will be removed
if sys.version_info > (3, 0):
from http import HTTPStatus
else:
import httplib
logging.basicConfig(level=logging.INFO)
"""
This package implements leader election using an annotation in a Kubernetes object.
The onstarted_leading function is run in a thread and when it returns, if it does
it might not be safe to run it again in a process.
At first all candidates are considered followers. The one to create a lock or update
an existing lock first becomes the leader and remains so until it keeps renewing its
lease.
"""
class LeaderElection:
def __init__(self, election_config):
if election_config is None:
sys.exit("argument config not passed")
# Latest record observed in the created lock object
self.observed_record = None
# The configuration set for this candidate
self.election_config = election_config
# Latest update time of the lock
self.observed_time_milliseconds = 0
# Point of entry to Leader election
def run(self):
# Try to create/ acquire a lock
if self.acquire():
logging.info("{} successfully acquired lease".format(self.election_config.lock.identity))
# Start leading and call OnStartedLeading()
threading.daemon = True
threading.Thread(target=self.election_config.onstarted_leading).start()
self.renew_loop()
# Failed to update lease, run OnStoppedLeading callback
self.election_config.onstopped_leading()
def acquire(self):
# Follower
logging.info("{} is a follower".format(self.election_config.lock.identity))
retry_period = self.election_config.retry_period
while True:
succeeded = self.try_acquire_or_renew()
if succeeded:
return True
time.sleep(retry_period)
def renew_loop(self):
# Leader
logging.info("Leader has entered renew loop and will try to update lease continuously")
retry_period = self.election_config.retry_period
renew_deadline = self.election_config.renew_deadline * 1000
while True:
timeout = int(time.time() * 1000) + renew_deadline
succeeded = False
while int(time.time() * 1000) < timeout:
succeeded = self.try_acquire_or_renew()
if succeeded:
break
time.sleep(retry_period)
if succeeded:
time.sleep(retry_period)
continue
# failed to renew, return
return
def try_acquire_or_renew(self):
now_timestamp = time.time()
now = datetime.datetime.fromtimestamp(now_timestamp)
# Check if lock is created
lock_status, old_election_record = self.election_config.lock.get(self.election_config.lock.name,
self.election_config.lock.namespace)
# create a default Election record for this candidate
leader_election_record = LeaderElectionRecord(self.election_config.lock.identity,
str(self.election_config.lease_duration), str(now), str(now))
# A lock is not created with that name, try to create one
if not lock_status:
# To be removed when support for python2 will be removed
if sys.version_info > (3, 0):
if json.loads(old_election_record.body)['code'] != HTTPStatus.NOT_FOUND:
logging.info("Error retrieving resource lock {} as {}".format(self.election_config.lock.name,
old_election_record.reason))
return False
else:
if json.loads(old_election_record.body)['code'] != httplib.NOT_FOUND:
logging.info("Error retrieving resource lock {} as {}".format(self.election_config.lock.name,
old_election_record.reason))
return False
logging.info("{} is trying to create a lock".format(leader_election_record.holder_identity))
create_status = self.election_config.lock.create(name=self.election_config.lock.name,
namespace=self.election_config.lock.namespace,
election_record=leader_election_record)
if create_status is False:
logging.info("{} Failed to create lock".format(leader_election_record.holder_identity))
return False
self.observed_record = leader_election_record
self.observed_time_milliseconds = int(time.time() * 1000)
return True
# A lock exists with that name
# Validate old_election_record
if old_election_record is None:
# try to update lock with proper annotation and election record
return self.update_lock(leader_election_record)
if (old_election_record.holder_identity is None or old_election_record.lease_duration is None
or old_election_record.acquire_time is None or old_election_record.renew_time is None):
# try to update lock with proper annotation and election record
return self.update_lock(leader_election_record)
# Report transitions
if self.observed_record and self.observed_record.holder_identity != old_election_record.holder_identity:
logging.info("Leader has switched to {}".format(old_election_record.holder_identity))
if self.observed_record is None or old_election_record.__dict__ != self.observed_record.__dict__:
self.observed_record = old_election_record
self.observed_time_milliseconds = int(time.time() * 1000)
# If This candidate is not the leader and lease duration is yet to finish
if (self.election_config.lock.identity != self.observed_record.holder_identity
and self.observed_time_milliseconds + self.election_config.lease_duration * 1000 > int(now_timestamp * 1000)):
logging.info("yet to finish lease_duration, lease held by {} and has not expired".format(old_election_record.holder_identity))
return False
# If this candidate is the Leader
if self.election_config.lock.identity == self.observed_record.holder_identity:
# Leader updates renewTime, but keeps acquire_time unchanged
leader_election_record.acquire_time = self.observed_record.acquire_time
return self.update_lock(leader_election_record)
def update_lock(self, leader_election_record):
# Update object with latest election record
update_status = self.election_config.lock.update(self.election_config.lock.name,
self.election_config.lock.namespace,
leader_election_record)
if update_status is False:
logging.info("{} failed to acquire lease".format(leader_election_record.holder_identity))
return False
self.observed_record = leader_election_record
self.observed_time_milliseconds = int(time.time() * 1000)
logging.info("leader {} has successfully acquired lease".format(leader_election_record.holder_identity))
return True
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