Read-only replicas in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a read-only replica in Redis?

A read-only replica is a copy of the main Redis database that can only be used to read data, not to write or change it. It helps spread the load and improve performance.

beginner

Why use read-only replicas in Redis?

Read-only replicas help by sharing the reading work, so the main database doesn't get too busy. This makes the system faster and more reliable.

intermediate

How does data stay updated on a Redis read-only replica?

The main Redis server sends updates to the read-only replicas automatically, so they always have the latest data to read.

beginner

Can you write data directly to a Redis read-only replica?

No, you cannot write data to a read-only replica. All writes must go to the main Redis server, which then updates the replicas.

intermediate

What happens if the main Redis server fails but you have read-only replicas?

Read-only replicas can still provide data for reading, but since they can't accept writes, the system needs a plan to promote a replica to main or fix the main server.

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

A To handle read requests and reduce load on the main server

B To accept write requests from clients

C To delete old data automatically

D To backup data to a different database

Read-only replicas are designed to handle read requests, helping the main server by reducing its workload.

Can you write new data directly to a Redis read-only replica?

A Only during maintenance

B No, writes must go to the main server

C Yes, anytime

D Only if the replica is promoted

Read-only replicas do not accept writes; all writes must be done on the main Redis server.

How do read-only replicas get updated data from the main Redis server?

A They pull data manually every hour

B Users must sync them manually

C They never update after creation

D The main server pushes updates automatically

The main Redis server automatically pushes updates to its read-only replicas to keep data in sync.

What is a common use case for Redis read-only replicas?

A Encrypting data at rest

B Storing backups offline

C Improving read performance by spreading load

D Running complex transactions

Read-only replicas help improve read performance by handling read requests, reducing load on the main server.

If the main Redis server fails, what is a limitation of read-only replicas?

A They cannot accept writes until promoted

B They can still accept writes

C They lose all data immediately

D They automatically become main servers

Read-only replicas cannot accept writes unless they are promoted to be the main server.

Explain what a Redis read-only replica is and why it is useful.

Think about how sharing work helps a busy kitchen.

Describe how data synchronization works between the main Redis server and its read-only replicas.

Imagine a teacher sending notes to students regularly.

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