Read-only replicas in Redis

Introduction

Read-only replicas help you get data faster and keep your main database safe by sharing the work of answering read requests.

When many users want to read data at the same time without changing it.

To reduce the load on your main Redis server so it can focus on writing data.

When you want to have backup copies of your data that can be used if the main server fails.

To improve the speed of reading data by placing replicas closer to users in different locations.

Syntax

Redis

replicaof <master_host> <master_port>

Use this command on a Redis server to make it a read-only replica of the master server.

Once set, the replica will copy data from the master and only allow read commands.

Examples

This makes the current Redis server a read-only replica of the master running on localhost at port 6379.

Redis

replicaof 127.0.0.1 6379

This command stops replication and makes the Redis server a master again.

Redis

replicaof no one

Sample Program

This command sets the current Redis server as a read-only replica of the master server at IP 192.168.1.100 on port 6379.

Redis

# On the replica Redis server CLI
replicaof 192.168.1.100 6379

# Now this server will copy data from the master at 192.168.1.100:6379 and only allow read commands.

Important Notes

Read-only replicas cannot accept commands that change data, like SET or DEL.

Replicas automatically sync data from the master and stay updated.

You can have many replicas for one master to handle lots of read requests.

Summary

Read-only replicas copy data from a master Redis server to share read load.

They improve performance and provide data safety by offloading reads from the master.

Use the replicaof command to set or remove replication.

Practice

1. What is the main purpose of a read-only replica in Redis?

easy

A. To handle write requests and update the master

B. To copy data from the master and handle read requests

C. To delete data from the master server

D. To act as a backup that does not sync data

Solution

1. Step 1: Understand the role of read-only replicas

   Read-only replicas copy data from the master Redis server and serve read requests to reduce load on the master.

2. Step 2: Identify the correct purpose

   They do not handle writes or delete data; their main job is to share read load.

3. Final Answer:

   To copy data from the master and handle read requests -> Option B

4. Quick Check:

   Read-only replicas = copy data + read requests [OK]

Hint: Replicas copy data and serve reads, not writes [OK]

Common Mistakes:

• Thinking replicas handle writes
• Believing replicas delete data
• Assuming replicas do not sync data

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

easy

A. SETREPLICA <port>

B. COPYFROM <port>

C. SYNCREPLICA <port>

D. REPLICAOF <port>

Solution

1. Step 1: Recall the correct Redis command for replication

   The command to set a Redis server as a replica is REPLICAOF followed by the master's IP and port.

2. Step 2: Verify other options

   Other commands like SETREPLICA, COPYFROM, SYNCREPLICA do not exist in Redis.

3. Final Answer:

   REPLICAOF <master-ip> <port> -> Option D

4. Quick Check:

   Replica setup command = REPLICAOF [OK]

Hint: Use REPLICAOF to set or remove replication [OK]

Common Mistakes:

• Using non-existent commands
• Confusing REPLICAOF with SYNC commands
• Misspelling the command name

3. Given a Redis replica configured with REPLICAOF 192.168.1.10 6379, what happens if you try to run SET key value on the replica?

medium

A. An error occurs because replicas are read-only

B. The key is set successfully on the replica

C. The command is forwarded to the master automatically

D. The replica disconnects from the master

Solution

1. Step 1: Understand replica behavior on write commands

   Redis replicas are read-only and reject write commands like SET.

2. Step 2: Identify the outcome of running SET on a replica

   Running SET on a replica causes an error because it cannot accept writes.

3. Final Answer:

   An error occurs because replicas are read-only -> Option A

4. Quick Check:

   Write commands on replicas = error [OK]

Hint: Replicas reject writes, only master accepts them [OK]

Common Mistakes:

• Assuming writes succeed on replicas
• Thinking commands auto-forward to master
• Believing replica disconnects on write

4. You configured a Redis replica with REPLICAOF 192.168.1.10 6379 but it is not syncing data. What is the most likely cause?

medium

A. The replica command should be REPLICA instead of REPLICAOF

B. The replica server is running an older Redis version

C. The master IP or port is incorrect

D. The master server is set as a replica itself

Solution

1. Step 1: Check configuration details

   If the replica is not syncing, the most common cause is incorrect master IP or port in the REPLICAOF command.

2. Step 2: Evaluate other options

   Older Redis versions may cause issues but less likely; REPLICAOF is the correct command; master being a replica can cause sync issues but less common.

3. Final Answer:

   The master IP or port is incorrect -> Option C

4. Quick Check:

   Incorrect master address = no sync [OK]

Hint: Verify master IP and port carefully [OK]

Common Mistakes:

• Using wrong command name
• Ignoring IP/port typos
• Assuming version mismatch is always cause

5. You want to temporarily stop replication on a Redis replica without shutting it down. Which command should you use?

hard

A. REPLICAOF NO ONE

B. REPLICAOF NONE

C. STOPREPLICA

D. DISABLEREPLICA

Solution

1. Step 1: Recall how to remove replication

   To stop replication temporarily, use REPLICAOF NO ONE which makes the replica a standalone server.

2. Step 2: Check other options

   Commands like REPLICAOF NONE, STOPREPLICA, DISABLEREPLICA do not exist in Redis.

3. Final Answer:

   REPLICAOF NO ONE -> Option A

4. Quick Check:

   Stop replication = REPLICAOF NO ONE [OK]

Hint: Use REPLICAOF NO ONE to stop replication temporarily [OK]

Common Mistakes:

• Using incorrect command variants
• Trying to shut down replica to stop replication
• Confusing NO ONE with NONE