Replication configuration in Redis

Introduction

Replication helps copy data from one Redis server to another. This keeps data safe and allows sharing work between servers.

You want a backup server that always has the latest data.

You want to spread read requests to multiple servers to improve speed.

You want to keep data safe if one server stops working.

You want to update data on one server and have others copy it automatically.

Syntax

Redis

slaveof <masterip> <masterport>

This command sets the current Redis server as a replica (slave) of the master server.

Use replicaof instead of slaveof in Redis 5.0 and later for clearer naming.

Examples

Make this Redis server a replica of the master at IP 192.168.1.100 on port 6379.

Redis

replicaof 192.168.1.100 6379

Stop replication and make this server a master again.

Redis

replicaof no one

Sample Program

This command configures the current Redis server to replicate data from the master server running on the same machine at port 6379.

Redis

# Connect to Redis server and run:
replicaof 127.0.0.1 6379

# This makes the current server copy data from the master at localhost port 6379.

Important Notes

Replication is asynchronous, so replicas may be slightly behind the master.

Use replicaof no one to stop replication and promote the replica to master.

Summary

Replication copies data from a master Redis server to replicas.

Use replicaof <ip> <port> to set replication.

Use replicaof no one to stop replication.

Practice

1. What is the primary purpose of Redis replication?

easy

A. To compress data stored on disk

B. To delete old data from the database

C. To copy data from a master server to replicas automatically

D. To encrypt data during transmission

Solution

1. Step 1: Understand replication concept

   Replication means copying data from one server (master) to others (replicas) automatically.

2. Step 2: Identify the main goal

   This copying helps keep data safe and allows multiple servers to serve read requests.

3. Final Answer:

   To copy data from a master server to replicas automatically -> Option C

4. Quick Check:

   Replication = Data copying [OK]

Hint: Replication means copying data from master to replicas [OK]

Common Mistakes:

• Confusing replication with data deletion
• Thinking replication encrypts data
• Assuming replication compresses data

2. Which Redis command is used to configure a server as a replica of another?

easy

A. REPLICAOF <master_ip> <master_port>

B. CONNECT <master_ip> <master_port>

C. SYNC <master_ip> <master_port>

D. COPYFROM <master_ip> <master_port>

Solution

1. Step 1: Recall Redis replication commands

   Redis uses the command REPLICAOF (formerly SLAVEOF) to set a server as a replica.

2. Step 2: Check command correctness

   Only REPLICAOF accepts master IP and port to configure replication.

3. Final Answer:

   REPLICAOF <master_ip> <master_port> -> Option A

4. Quick Check:

   Replica config command = REPLICAOF [OK]

Hint: Use REPLICAOF to set replication target server [OK]

Common Mistakes:

• Using CONNECT which is not a Redis replication command
• Confusing SYNC with configuration command
• Inventing COPYFROM which does not exist

3. Given this Redis config snippet:

replicaof 192.168.1.10 6379
replica-read-only yes

What is the effect of replica-read-only yes?

medium

A. The replica rejects write commands and only allows reads

B. The replica can accept write commands from clients

C. The replica disables replication from the master

D. The replica automatically becomes a master

Solution

1. Step 1: Understand replica-read-only setting

   Setting replica-read-only to yes means the replica will not accept writes from clients.

2. Step 2: Identify behavior of replica

   The replica only allows read commands and rejects any write commands to keep data consistent with master.

3. Final Answer:

   The replica rejects write commands and only allows reads -> Option A

4. Quick Check:

   replica-read-only yes = read-only replica [OK]

Hint: replica-read-only yes means no writes allowed on replica [OK]

Common Mistakes:

• Thinking replica can write data with this setting
• Assuming replication stops with this option
• Believing replica becomes master automatically

4. You run replicaof 192.168.1.20 6379 on a Redis server but replication does not start. What is a likely cause?

medium

A. The command should be replicaof start instead

B. The master IP or port is incorrect or unreachable

C. The replica server has no data to replicate

D. Redis does not support replication

Solution

1. Step 1: Check network and master details

   If replication does not start, often the master IP or port is wrong or the replica cannot reach the master.

2. Step 2: Verify command correctness

   The command replicaof with IP and port is correct; no 'start' keyword is needed.

3. Final Answer:

   The master IP or port is incorrect or unreachable -> Option B

4. Quick Check:

   Replication fails if master unreachable [OK]

Hint: Check master IP and port if replication fails [OK]

Common Mistakes:

• Using wrong command syntax like replicaof start
• Assuming replication needs data on replica first
• Thinking Redis does not support replication

5. You want to set up a Redis replica that can serve read requests but also allow some writes during maintenance. Which configuration is best?

hard

A. Use replicaof <master_ip> <master_port> and set replica-read-only yes

B. Set the replica as master with no replication

C. Do not use replication; instead manually copy data

D. Use replicaof <master_ip> <master_port> and set replica-read-only no

Solution

1. Step 1: Understand replica-read-only option

   Setting replica-read-only to no allows writes on the replica, useful during maintenance.

2. Step 2: Combine with replication setup

   Using replicaof configures replication; setting replica-read-only no allows writes while still replicating.

3. Final Answer:

   Use replicaof <master_ip> <master_port> and set replica-read-only no -> Option D

4. Quick Check:

   replica-read-only no allows writes on replica [OK]

Hint: Set replica-read-only no to allow writes on replica [OK]

Common Mistakes:

• Assuming replica-read-only yes allows writes
• Thinking manual copy replaces replication
• Confusing replica with master roles