ZooKeeper

asab.zookeeper is an Apache ZooKeeper asynchronous client based on Kazoo library.

Apache ZooKeeper is a distributed coordination service that provides a hierarchical key-value data store, called a znode tree, to store and manage configuration, coordination, and synchronization data. The znode tree is similar to a file system tree, where each znode is like a file or a directory.

Apache ZooKeeper can be used to design microservices in a stateless manner by managing and coordinating the state information between microservices. In this design, each microservice does not store any state information, but instead relies on ZooKeeper for state management.

ZooKeeper container

A ZooKeeper container represents a connectivity to Apache ZooKeeper server(s). The application can operate multiple instances of ZooKeeper container.

This code illustrates the typical way of creating a ZooKeeper container:

import asab.zookeeper

class MyApplication(asab.Application):

    async def initialize(self):
        # Load the ASAB Zookeeper module
        self.add_module(asab.zookeeper.Module)

        # Initialize ZooKeeper Service
        self.ZooKeeperService = self.get_service("asab.ZooKeeperService")

        # Create the Zookeeper container
        self.ZooKeeperContainer = asab.zookeeper.ZooKeeperContainer(self.ZooKeeperService)

        # Subscribe to Zookeeper container ready event
        self.PubSub.subscribe("ZooKeeperContainer.state/CONNECTED!", self._on_zk_connected)

    async def _on_zk_connected(self, event_name, zookeeper):
        if zookeeper != self.ZooKeeperContainer:
            return
        print("Connected to Zookeeper!")

Specifications are obtained from:

1. z_path argument, which is Zookeeper URL (see below)
2. The configuration section specified by config_section_name argument
3. $ASAB_ZOOKEEPER_SERVERS environment variable

The z_path argument has precedence over config but the implementation will look at configuration if z_path URL is missing completely or partially. Also, if configuration section doesn't provide information about servers, ASAB_ZOOKEEPER_SERVERS environment variable is used.

Example of configuration section

[zookeeper]
servers=zookeeper-1:2181,zookeeper-2:2181,zookeeper-3:2181
path=myfolder

Example of $ASAB_ZOOKEEPERS_SERVERS environment variable

ASAB_ZOOKEEPER_SERVERS=zookeeper-1:2181,zookeeper-2:2181

Supported types of z_path URLs

1. Absolute URL:

   zookeeper://zookeeper-1:2181,zookeeper-2:2181,zookeeper-3:2181/etc/configs/file1
   

   The URL contains all information for a connectivity.

2. URL without servers with absolute path:

   zookeepers:///etc/configs/file1
   

   In this case the relative url is expanded as follows:

   zookeeper://{zookeeper_servers}/etc/configs/file1
   

   Where {zookeeper_servers} is substituted with the servers entry of the [zookeeper] configuration file section.

3. URL without servers with relative path

   zookeeper:./etc/configs/file1
   

   In this case, the relative URL is expanded as follows:

   zookeper://{zookeeper_servers}/{zookeeper_path}/etc/configs/file1
   

   Where {zookeeper_servers} is substituted with the servers entry of the [zookeeper] configuration file section a nd {zookeeper_path} is substituted with the "path" entry of the [zookeeper] configuration file section.

Reading from ZooKeeeper

TODO

PubSub messages

ZooKeeperContainer.state/CONNECTED!

ZooKeeperContainer.state/LOST!

ZooKeeperContainer.state/SUSPENDED!

When a Zookeeper connection is first created, it is in the LOST state. After a connection is established, it transitions to the CONNECTED state. If any connection issues come up, or if it needs to connect to a different Zookeeper cluster node, it will transition to SUSPENDED to let you know that commands cannot currently be run. The connection will also be lost if the Zookeeper node is no longer part of the quorum, resulting in a SUSPENDED state.

Upon re-establishing a connection, the client could transition to LOST if the session has expired, or CONNECTED if the session is still valid.

For more info, visit (https://kazoo.readthedocs.io/en/latest/basic_usage.html#understanding-kazoo-states)

Kazoo

Kazoo is the synchronous Python library for Apache Zookeeper.

It can be used directly for more complicated tasks. Kazoo client is accessible at ZooKeeperContainer.ZooKeeper.Client. Synchronous methods of Kazoo client must be executed using ProactorService.

Example

def write_to_zk():
self.ZooKeeperContainer.ZooKeeper.Client.create(path, data, sequence=True, ephemeral=True, makepath=True)

await self.ZooKeeperContainer.ZooKeeper.ProactorService.execute(write_to_zk)