Master-replica architecture in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the role of the master in a master-replica architecture?

The master handles all write operations and propagates data changes to replicas to keep them updated.

beginner

What is a replica in the master-replica architecture?

A replica is a copy of the master that receives data updates and handles read requests to reduce load on the master.

intermediate

How does data consistency work between master and replicas?

Replicas asynchronously receive updates from the master, so there might be a slight delay before replicas reflect the latest data.

beginner

Why use a master-replica architecture in Redis?

It improves read scalability by distributing read requests to replicas and increases availability if the master fails.

intermediate

What happens if the master fails in a master-replica setup?

One of the replicas can be promoted to master to continue serving writes, ensuring minimal downtime.

In Redis master-replica architecture, which node handles write operations?

A Neither

B Replica

C Both master and replica

D Master

Only the master node handles write operations; replicas handle read requests.

What is the main benefit of having replicas in Redis?

A To handle write requests

B To improve read scalability

C To store backups only

D To replace the master permanently

Replicas help by handling read requests, reducing load on the master.

How do replicas get updated data from the master?

A Asynchronously with some delay

B Manually by the user

C Synchronously in real-time

D They do not get updated

Replicas receive updates asynchronously, so there can be a small delay.

If the master fails, what is a common solution in Redis master-replica architecture?

A Stop all operations

B Wait for the master to recover

C Promote a replica to master

D Delete all data

A replica is promoted to master to maintain availability.

Which of these is NOT a feature of master-replica architecture?

A Automatic write load balancing across replicas

B Improved read performance

C Data redundancy

D Failover support

Write operations are only handled by the master, not balanced across replicas.

Explain how master and replicas work together in Redis master-replica architecture.

Think about who writes data and who reads data.

Describe what happens when the master node fails in a Redis master-replica setup.

Consider how the system keeps working without the master.

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