Master-replica architecture in Redis

Introduction

Master-replica architecture helps keep data safe and available by copying data from one main server (master) to others (replicas).

You want to make sure your data is backed up automatically.

You need to handle many users reading data at the same time without slowing down.

You want to keep your app running even if one server stops working.

You want to spread the load so the main server doesn't get too busy.

Syntax

Redis

To set a Redis server as a replica:

SLAVEOF <master-ip> <master-port>

To stop being a replica and become a master again:

SLAVEOF NO ONE

The SLAVEOF command tells a Redis server to copy data from the master server.

Use SLAVEOF NO ONE to make a replica stop copying and become a master.

Examples

This command makes the current Redis server a replica of the master at IP 192.168.1.100 on port 6379.

Redis

SLAVEOF 192.168.1.100 6379

This command makes the current Redis server stop being a replica and become a master.

Redis

SLAVEOF NO ONE

Sample Program

This example sets the current Redis server as a replica of the master on localhost. Then it checks the server role.

Redis

# On replica server terminal
SLAVEOF 127.0.0.1 6379

# Now the replica copies data from the master running on localhost port 6379

# To check role:
ROLE

Important Notes

Replicas only copy data from the master; they do not accept writes directly.

When the master changes data, replicas update automatically to match.

Using replicas can improve read speed by spreading requests across servers.

Summary

Master-replica architecture copies data from one main server to others for safety and speed.

Use SLAVEOF to set a replica and SLAVEOF NO ONE to make it a master again.

Replicas help keep your app working even if the master has problems.

Practice

1. What is the main purpose of the master-replica architecture in Redis?

easy

A. To copy data from one main server to others for safety and speed

B. To delete data from all servers simultaneously

C. To run multiple unrelated databases on one server

D. To encrypt data on the master server only

Solution

1. Step 1: Understand the role of master and replicas

   The master server holds the original data, and replicas copy this data to keep backups and improve read speed.

2. Step 2: Identify the main goal of this setup

   This setup is mainly for safety (backup) and speed (load distribution).

3. Final Answer:

   To copy data from one main server to others for safety and speed -> Option A

4. Quick Check:

   Master-replica = data copy for safety and speed [OK]

Hint: Master copies data to replicas for safety and speed [OK]

Common Mistakes:

• Thinking replicas delete data
• Confusing master-replica with encryption
• Believing replicas run unrelated databases

2. Which Redis command correctly sets a server as a replica of another?

easy

A. REPLICAOF <port>

B. SETREPLICA <port>

C. SLAVEOF <port>

D. COPYTO <port>

Solution

1. Step 1: Recall the Redis command for setting a replica

   The correct command is SLAVEOF followed by the master's IP and port.

2. Step 2: Check the options for exact command syntax

   Only SLAVEOF <port> uses the correct command SLAVEOF.

3. Final Answer:

   SLAVEOF <master-ip> <port> -> Option C

4. Quick Check:

   Set replica with SLAVEOF command [OK]

Hint: Use SLAVEOF to set replica, not REPLICAOF or others [OK]

Common Mistakes:

• Using REPLICAOF instead of SLAVEOF
• Trying non-existent commands like SETREPLICA
• Confusing COPYTO with replication commands

3. Given this Redis command sequence on a replica server:

SLAVEOF 192.168.1.10 6379
GET user:100

What will happen if the master at 192.168.1.10 is down?

medium

A. The replica will return an error because it can't reach the master

B. The replica will return the last synced value for user:100

C. The replica will become master automatically

D. The replica will delete the user:100 key

Solution

1. Step 1: Understand replica behavior when master is down

   The replica keeps the last synced data even if the master is offline.

2. Step 2: Predict the GET command result on replica

   GET will return the last known value for the key from the replica's local data.

3. Final Answer:

   The replica will return the last synced value for user:100 -> Option B

4. Quick Check:

   Replica serves last synced data if master is down [OK]

Hint: Replica serves cached data if master is unreachable [OK]

Common Mistakes:

• Assuming replica errors out when master is down
• Thinking replica auto-promotes to master
• Believing replica deletes keys on master failure

4. You run SLAVEOF NO ONE on a Redis replica server but it still behaves like a replica. What is the likely problem?

medium

A. The server is still connected to the master and did not switch roles

B. The server needs a restart to apply the change

C. The command was typed incorrectly; it should be SLAVE OF NO ONE

D. SLAVEOF NO ONE only works on master servers

Solution

1. Step 1: Understand what SLAVEOF NO ONE does

   This command tells the server to stop replicating and become a master.

2. Step 2: Identify why the server still acts as replica

   If the server is still connected or replication not stopped properly, it remains a replica.

3. Final Answer:

   The server is still connected to the master and did not switch roles -> Option A

4. Quick Check:

   Server must disconnect from master to become master [OK]

Hint: Ensure replica disconnects from master to become master [OK]

Common Mistakes:

• Typing SLAVE OF instead of SLAVEOF
• Thinking restart is always needed
• Believing SLAVEOF NO ONE only works on masters

5. You want to ensure your Redis setup continues working if the master fails. Which approach best uses master-replica architecture?

hard

A. Use only one replica and manually copy data to it daily

B. Disable replication and rely on backups only

C. Run multiple masters without replicas to balance load

D. Set multiple replicas and configure automatic failover to promote a replica to master

Solution

1. Step 1: Identify the goal of continuous operation

   To keep Redis working if master fails, replicas must be ready to take over automatically.

2. Step 2: Evaluate options for failover and replication

   Set multiple replicas and configure automatic failover to promote a replica to master uses multiple replicas and automatic failover, which is the best practice.

3. Final Answer:

   Set multiple replicas and configure automatic failover to promote a replica to master -> Option D

4. Quick Check:

   Multiple replicas + auto failover = high availability [OK]

Hint: Use multiple replicas with auto failover for master failure [OK]

Common Mistakes:

• Relying on manual data copy only
• Running multiple masters without replicas
• Disabling replication and trusting backups alone