Apache ZooKeeper is a centralized service for maintaining configuration information, naming, providing distributed synchronization, and providing group services.

Features

Configuration Management: Centralized configuration management
Naming Service: Distributed naming service
Synchronization: Distributed synchronization primitives
Group Services: Group membership and leader election
High Availability: Built-in replication and failover

Quick Start

Start ZooKeeper:

docker compose up -d

Configuration

Environment Variables

ZOOKEEPER_VERSION: ZooKeeper version (default: 3.9.3)
ZOOKEEPER_CLIENT_PORT_OVERRIDE: Client port (default: 2181)
ZOOKEEPER_ADMIN_PORT_OVERRIDE: Admin server port (default: 8080)
ZOO_TICK_TIME: Tick time in milliseconds (default: 2000)
ZOO_INIT_LIMIT: Init limit (default: 10)
ZOO_SYNC_LIMIT: Sync limit (default: 5)
ZOO_MAX_CLIENT_CNXNS: Maximum client connections (default: 60)
ZOO_4LW_COMMANDS_WHITELIST: Four letter words commands whitelist (default: srvr,mntr,ruok)

Access

Client Port: localhost:2181
Admin Server: http://localhost:8080

Usage

Connect to ZooKeeper

docker compose exec zookeeper zkCli.sh

Check Status

echo ruok | nc localhost 2181

Response should be imok if ZooKeeper is running properly.

Get Server Stats

echo stat | nc localhost 2181

Four Letter Words Commands

ZooKeeper supports a set of commands that are exactly four letters:

ruok: Tests if server is running
stat: Lists server statistics and clients
srvr: Lists server information
mntr: Lists server metrics
conf: Shows server configuration

Data Storage

ZooKeeper data is stored in three volumes:

zookeeper_data: Main data directory
zookeeper_datalog: Transaction logs
zookeeper_logs: Application logs

Resources

Resource Limits: 1 CPU, 1G RAM
Resource Reservations: 0.25 CPU, 512M RAM

Production Considerations

For production deployments:

Use an ensemble (cluster) of at least 3 nodes
Configure proper disk I/O for transaction logs
Monitor heap memory usage
Set up proper backup strategies
Configure firewall rules for all required ports