Setting Up Replication

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REPLICAOF Command(DOCUMENTATION)

Introduction

Configuring Redis replication involves pointing a replica server at a master. Redis supports both configuration-file and runtime approaches, giving you flexibility in how you manage your topology.

Key Concepts

  • REPLICAOF: The modern command to configure a server as a replica (replaces deprecated SLAVEOF)
  • Chained Replication: Replicas can replicate from other replicas to reduce master load
  • Diskless Replication: Sync without writing RDB to disk for faster initial synchronization
  • masterauth: Password the replica uses to authenticate with the master

Real World Context

Every production Redis setup begins with replication. Whether you are running a simple primary-standby pair or a large fan-out topology, understanding the configuration options ensures reliable data copies across your infrastructure.

Deep Dive

Method 1: Configuration File

bash
# On the replica server (redis.conf)
replicaof 192.168.1.10 6379
masterauth your_master_password
replica-read-only yes

Start the replica:

bash
redis-server /path/to/redis.conf

Method 2: Runtime Command

redis
REPLICAOF 192.168.1.10 6379

# To stop replication and become standalone master:
REPLICAOF NO ONE

Complete Setup Example

Master (192.168.1.10:6379)

bash
# redis-master.conf
bind 0.0.0.0
port 6379
requirepass master_password
save 900 1
appendonly yes

Replica 1 (192.168.1.11:6379)

bash
# redis-replica1.conf
bind 0.0.0.0
port 6379
requirepass replica_password
replicaof 192.168.1.10 6379
masterauth master_password
replica-read-only yes

Replica 2 (192.168.1.12:6379)

bash
# redis-replica2.conf
bind 0.0.0.0
port 6379
requirepass replica_password
replicaof 192.168.1.10 6379
masterauth master_password
replica-read-only yes

Verifying Replication

On Master:

redis
127.0.0.1:6379> INFO replication
# Replication
role:master
connected_slaves:2
slave0:ip=192.168.1.11,port=6379,state=online,offset=1234,lag=0
slave1:ip=192.168.1.12,port=6379,state=online,offset=1234,lag=0
master_replid:abc123...
master_repl_offset:1234

On Replica:

redis
127.0.0.1:6379> INFO replication
# Replication
role:slave
master_host:192.168.1.10
master_port:6379
master_link_status:up
master_last_io_seconds_ago:1
master_sync_in_progress:0
slave_read_only:1

Testing Replication

bash
# On master
redis-cli -h 192.168.1.10 SET mykey "Hello"

# On replica (should see the value)
redis-cli -h 192.168.1.11 GET mykey
# "Hello"

# Writing to replica should fail
redis-cli -h 192.168.1.11 SET mykey "World"
# (error) READONLY You can't write against a read only replica.

Chained Replication

Replicas can replicate from other replicas:

Master --> Replica1 --> Replica2

bash
# Replica 2 config
replicaof 192.168.1.11 6379  # Points to Replica 1

Benefits: Reduces load on master Drawback: Additional replication lag

Diskless Replication

For faster sync without writing RDB to disk:

bash
# On master
repl-diskless-sync yes
repl-diskless-sync-delay 5  # Wait for more replicas before starting

Common Pitfalls

  1. Forgetting masterauth -- If the master requires a password, the replica will fail to connect silently. Always configure masterauth when requirepass is set on the master.
  2. Running replicas without persistence -- If both master and replica have persistence disabled and the master restarts empty, the replica will also wipe its data during resync.

Best Practices

  1. Use configuration files for production -- Runtime REPLICAOF commands are lost on restart. Always persist replication settings in redis.conf.
  2. Enable diskless replication for large datasets -- Diskless sync avoids the disk I/O bottleneck when datasets exceed several GB.

Summary

  • Use REPLICAOF (config file or runtime) to set up replication
  • Verify with INFO replication on both master and replica
  • Chained replication reduces master load at the cost of extra lag
  • Diskless replication speeds up initial sync for large datasets
  • Always test write rejection on replicas to confirm read-only mode

Code Examples

bash
# Configure replica via redis.conf
replicaof 192.168.1.10 6379
masterauth your_master_password
replica-read-only yes

# Or at runtime
redis-cli REPLICAOF 192.168.1.10 6379
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