Use ZooKeeper to elect the leader and share data with a Python script.
Create leader_example.py Python script.
#!/usr/bin/env python3
from kazoo.client import KazooClient
from datetime import datetime
import time
import argparse
class ApplicationNode(object):
def __init__(self, server_name, server_data, chroot, zookeeper_hosts=''):
self.zookeeper = KazooClient(hosts=zookeeper_hosts)
self.server_name = server_name
self.server_data = server_data
self.patch_chroot = chroot
self.path_nodes = "/nodes"
self.path_data = "/data"
self.connect()
self.chroot()
self.register()
self.watch_application_nodes()
self.watch_application_data()
def connect(self):
self.zookeeper.start()
def chroot(self):
self.zookeeper.ensure_path(self.patch_chroot)
self.zookeeper.chroot = self.patch_chroot
def register(self):
self.zookeeper.create("{0}/{1}_".format(self.path_nodes, self.server_name),
ephemeral=True, sequence=True, makepath=True)
def watch_application_data(self):
self.zookeeper.ensure_path(self.path_data)
self.zookeeper.DataWatch(path=self.path_data, func=self.check_application_data)
def watch_application_nodes(self):
self.zookeeper.ensure_path(self.path_nodes)
self.zookeeper.ChildrenWatch(path=self.path_nodes, func=self.check_application_nodes)
def check_application_nodes(self, children):
application_nodes = [{"node": i[0], "sequence": i[1]} for i in (i.split("_") for i in children)]
current_leader = min(application_nodes, key=lambda x: x["sequence"])["node"]
self.display_server_information(application_nodes, current_leader)
if current_leader == self.server_name:
self.update_shared_data()
def check_application_data(self, data, stat):
print(
"Data change detected on {0}:\nData: {1}\nStat: {2}".format((datetime.now()).strftime("%B %d, %Y %H:%M:%S"),
data, stat))
print()
def update_shared_data(self):
if not self.zookeeper.exists(self.path_data):
self.zookeeper.create(self.path_data,
bytes("name: {0}\ndata: {1}".format(self.server_name, self.server_data), "utf8"),
ephemeral=True, sequence=False, makepath=True)
def display_server_information(self, application_nodes, current_leader):
print("Datetime: {0}".format((datetime.now()).strftime("%B %d, %Y %H:%M:%S")))
print("Server name: {0}".format(self.server_name))
print("Nodes:")
for i in application_nodes:
print(" - {0} with sequence {1}".format(i["node"], i["sequence"]))
print("Role: {0}".format("leader" if current_leader == self.server_name else "follower"))
print()
def __del__(self):
self.zookeeper.close()
if __name__ == '__main__':
parser = argparse.ArgumentParser(description='ZooKeeper example application')
parser.add_argument('--server')
parser.add_argument('--data')
parser.add_argument('--chroot')
parser.add_argument('--zookeeper')
args = parser.parse_args()
ApplicationNode(server_name=args.server, server_data=args.data, chroot=args.chroot, zookeeper_hosts=args.zookeeper)
while True:
time.sleep(10)
Use it to play with leader election and watches.
$ python3 leader_example.py --server server1 --data "mariadb://172.16.0.111:3306" --chroot /application --zookeeper 172.16.0.111:2181,172.16.0.112:2181,172.16.0.113:2181
Datetime: June 15, 2021 23:38:38 Server name: server1 Nodes: - server1 with sequence 0000000000 Role: leader Data change detected on June 15, 2021 23:38:38: Data: b'name: server1\ndata: mariadb://172.16.0.111:3306' Stat: ZnodeStat(czxid=4294973701, mzxid=4294973701, ctime=1623718347592, mtime=1623718347592, version=0, cversion=0, aversion=0, ephemeralOwner=72057819866923296, dataLength=47, numChildren=0, pzxid=4294973701) Datetime: June 15, 2021 23:38:50 Server name: server1 Nodes: - server2 with sequence 0000000001 - server1 with sequence 0000000000 Role: leader Datetime: June 15, 2021 23:38:59 Server name: server1 Nodes: - server3 with sequence 0000000002 - server2 with sequence 0000000001 - server1 with sequence 0000000000 Role: leader ^C
$ python3 leader_example.py --server server2 --data "mariadb://172.16.0.112:3306" --chroot /application --zookeeper 172.16.0.111:2181,172.16.0.112:2181,172.16.0.113:2181
Datetime: June 15, 2021 23:38:50 Server name: server2 Nodes: - server2 with sequence 0000000001 - server1 with sequence 0000000000 Role: follower Data change detected on June 15, 2021 23:38:50: Data: b'name: server1\ndata: mariadb://172.16.0.111:3306' Stat: ZnodeStat(czxid=4294973701, mzxid=4294973701, ctime=1623718347592, mtime=1623718347592, version=0, cversion=0, aversion=0, ephemeralOwner=72057819866923296, dataLength=47, numChildren=0, pzxid=4294973701) Datetime: June 15, 2021 23:38:59 Server name: server2 Nodes: - server3 with sequence 0000000002 - server2 with sequence 0000000001 - server1 with sequence 0000000000 Role: follower Datetime: June 15, 2021 23:39:33 Server name: server2 Nodes: - server3 with sequence 0000000002 - server2 with sequence 0000000001 Role: leader Data change detected on June 15, 2021 23:39:33: Data: b'name: server2\ndata: mariadb://172.16.0.112:3306' Stat: ZnodeStat(czxid=4294973707, mzxid=4294973707, ctime=1623718407178, mtime=1623718407178, version=0, cversion=0, aversion=0, ephemeralOwner=72057819866923297, dataLength=47, numChildren=0, pzxid=4294973707) ^C
$ python3 leader_example.py --server server3 --data "mariadb://172.16.0.113:3306" --chroot /application --zookeeper 172.16.0.111:2181,172.16.0.112:2181,172.16.0.113:2181
Datetime: June 15, 2021 23:38:59 Server name: server3 Nodes: - server3 with sequence 0000000002 - server2 with sequence 0000000001 - server1 with sequence 0000000000 Role: follower Data change detected on June 15, 2021 23:38:59: Data: b'name: server1\ndata: mariadb://172.16.0.111:3306' Stat: ZnodeStat(czxid=4294973701, mzxid=4294973701, ctime=1623718347592, mtime=1623718347592, version=0, cversion=0, aversion=0, ephemeralOwner=72057819866923296, dataLength=47, numChildren=0, pzxid=4294973701) Datetime: June 15, 2021 23:39:33 Server name: server3 Nodes: - server3 with sequence 0000000002 - server2 with sequence 0000000001 Role: follower Data change detected on June 15, 2021 23:39:33: Data: None Stat: None Data change detected on June 15, 2021 23:39:33: Data: b'name: server2\ndata: mariadb://172.16.0.112:3306' Stat: ZnodeStat(czxid=4294973707, mzxid=4294973707, ctime=1623718407178, mtime=1623718407178, version=0, cversion=0, aversion=0, ephemeralOwner=72057819866923297, dataLength=47, numChildren=0, pzxid=4294973707) Datetime: June 15, 2021 23:39:53 Server name: server3 Nodes: - server3 with sequence 0000000002 Role: leader Data change detected on June 15, 2021 23:39:53: Data: b'name: server3\ndata: mariadb://172.16.0.113:3306' Stat: ZnodeStat(czxid=4294973709, mzxid=4294973709, ctime=1623718429178, mtime=1623718429178, version=0, cversion=0, aversion=0, ephemeralOwner=216173555704463436, dataLength=47, numChildren=0, pzxid=4294973709) ^C
Start and stop each application to see how everything changes.
Beware, the error handling is missing to keep this example more concise.