Read-only replicas in Redis - Step-by-Step Execution

Concept Flow - Read-only replicas

Primary Redis Server

↓

Replicate Data to Read-only Replica

↓

Client Sends Read Request

↓

Read-only Replica Serves Data

↓

Client Receives Data

Data is copied from the primary Redis server to read-only replicas. Clients send read requests to replicas to reduce load on primary.

Execution Sample

Redis

1. PRIMARY SET key "value"
2. REPLICA syncs data
3. CLIENT GET key (read from replica)
4. REPLICA returns "value"

Shows how a write to primary is replicated and then read from a read-only replica.

Execution Table

Step	Action	Source	Target	Result
1	Write key 'key' with 'value'	Client	Primary	Primary stores key='value'
2	Replicate data	Primary	Replica	Replica copies key='value'
3	Client requests GET key	Client	Replica	Replica receives GET request
4	Replica returns value	Replica	Client	Client receives 'value'
5	Client tries to write on replica	Client	Replica	Replica rejects write (read-only)

💡 Replica rejects writes; only reads allowed on replicas

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Primary key	null	"value"	"value"	"value"	"value"
Replica key	null	null	"value"	"value"	"value"
Client read result	null	null	null	"value"	"value"

Key Moments - 2 Insights

Why can't the client write data directly to the read-only replica?

Because replicas are set to reject write commands to keep data consistent with the primary, as shown in step 5 of the execution_table.

How does the replica get updated data from the primary?

The primary server continuously replicates data changes to the replica, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what does the replica return to the client at step 4?

A null

B "value"

C Error

D "key"

💡 Hint

Check the 'Result' column in step 4 of the execution_table.

At which step does the replica copy data from the primary?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the replication action in the execution_table.

If the client tries to write to the replica, what happens according to the execution_table?

A Replica rejects write (read-only)

B Replica forwards write to primary

C Replica accepts and stores data

D Client write is ignored silently

💡 Hint

See step 5 in the execution_table for write attempts on replica.

Concept Snapshot

Read-only replicas in Redis copy data from the primary server.
Clients send read requests to replicas to reduce load.
Replicas reject write commands to keep data consistent.
Writes go only to the primary server.
Replicas serve fast reads and improve scalability.

Full Transcript

In Redis, a primary server handles all writes. It copies data to read-only replicas. Clients send read requests to replicas to reduce load on the primary. Replicas reject write commands to keep data consistent. This setup improves read scalability and availability.

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