Read replicas in GCP - Commands & Configuration

Introduction

Sometimes, many people want to read data from a database at the same time. This can slow down the main database. Read replicas are copies of the main database that only let you read data. They help by sharing the reading work so the main database stays fast.

When your website gets slow because many users read data at once.

When you want to keep your main database safe from heavy reading tasks.

When you want to spread out reading tasks across different servers to avoid overload.

When you want to back up your data in real-time without stopping your main database.

When you want to run reports or analytics without slowing down your main database.

Config File - read-replica-config.yaml

read-replica-config.yaml

apiVersion: sql.cnrm.cloud.google.com/v1beta1
kind: SQLInstance
metadata:
  name: my-primary-instance
spec:
  databaseVersion: POSTGRES_14
  region: us-central1
  settings:
    tier: db-f1-micro
---
apiVersion: sql.cnrm.cloud.google.com/v1beta1
kind: SQLInstance
metadata:
  name: my-read-replica
spec:
  region: us-central1
  replicaConfiguration:
    sourceInstanceName: my-primary-instance
  settings:
    tier: db-f1-micro

This file creates two Cloud SQL instances on Google Cloud Platform.

my-primary-instance is the main database where you can read and write data.

my-read-replica is the read replica that copies data from the main instance and only allows reading.

The replicaConfiguration.sourceInstanceName tells the replica which main database to copy from.

Both use a small machine type db-f1-micro for simplicity.

Commands

This command creates the main Cloud SQL instance where you can read and write data.

Terminal

gcloud sql instances create my-primary-instance --database-version=POSTGRES_14 --tier=db-f1-micro --region=us-central1

Expected Output

Created [https://sqladmin.googleapis.com/sql/v1beta4/projects/my-project/instances/my-primary-instance].

→

--database-version - Sets the database engine and version.

→

--tier - Defines the machine type for the instance.

→

--region - Specifies the location of the instance.

This command creates a read replica instance that copies data from the main instance for read-only use.

Terminal

gcloud sql instances create my-read-replica --region=us-central1 --tier=db-f1-micro --master-instance-name=my-primary-instance

Expected Output

Created [https://sqladmin.googleapis.com/sql/v1beta4/projects/my-project/instances/my-read-replica].

→

--master-instance-name - Specifies that this instance is a read replica of the main instance.

→

--region - Specifies the location of the replica.

→

--tier - Defines the machine type for the replica.

This command checks the details of the read replica to confirm it is set up correctly.

Terminal

gcloud sql instances describe my-read-replica

Expected Output

name: my-read-replica

region: us-central1

replicaConfiguration:

  sourceInstanceName: my-primary-instance

state: RUNNABLE

Key Concept

If you remember nothing else from this pattern, remember: read replicas copy data from the main database to share reading work and keep your app fast.

Common Mistakes

Creating a read replica without linking it to the main instance.

The replica won't know which database to copy data from and will not work.

Always set the main instance as the source using the --master-instance-name flag or replicaConfiguration.

Trying to write data directly to the read replica.

Read replicas are read-only and do not accept writes, causing errors.

Send all write operations to the main database instance only.

Using different regions for the main instance and replica without planning.

This can cause higher latency and extra costs for data transfer.

Keep replicas in the same region or plan for cross-region replication costs.

Summary

Create a main Cloud SQL instance to handle all read and write operations.

Create a read replica instance linked to the main instance to share read traffic.

Verify the read replica is connected and running to ensure it copies data correctly.

Practice

1.

What is the main purpose of a read replica in Google Cloud SQL?

easy

A. To copy the main database and handle read requests to reduce load

B. To store backups of the database for disaster recovery

C. To serve as a primary database for write operations

D. To encrypt data stored in the database

Solution

1. Step 1: Understand the role of read replicas

   Read replicas copy data from the main database and serve read requests to reduce load on the primary instance.

2. Step 2: Differentiate from other database roles

   Backups and encryption are different features; read replicas do not handle writes.

3. Final Answer:

   To copy the main database and handle read requests to reduce load -> Option A

4. Quick Check:

   Read replica purpose = reduce load by handling reads [OK]

Hint: Read replicas handle reads to reduce main database load [OK]

Common Mistakes:

• Confusing read replicas with backups
• Thinking read replicas handle writes
• Assuming read replicas encrypt data

2.

Which gcloud command correctly creates a read replica named replica-instance of a master instance called master-instance?

easy

A. gcloud sql instances create replica-instance --master-instance-name=master-instance

B. gcloud sql instances create replica-instance --replica-of=master-instance

C. gcloud sql replicas create replica-instance --master=master-instance

D. gcloud sql create instance replica-instance --master=master-instance

Solution

1. Step 1: Recall the correct command syntax

   The correct command to create a read replica uses gcloud sql instances create with the flag --master-instance-name.

2. Step 2: Verify option correctness

   gcloud sql instances create replica-instance --master-instance-name=master-instance matches the correct syntax exactly; other options use incorrect flags or commands.

3. Final Answer:

   gcloud sql instances create replica-instance --master-instance-name=master-instance -> Option A

4. Quick Check:

   Correct flag for read replica creation = --master-instance-name [OK]

Hint: Use --master-instance-name flag with gcloud sql instances create [OK]

Common Mistakes:

• Using incorrect flags like --replica-of
• Using wrong gcloud subcommands
• Mixing up command order or syntax

3.

Consider this command output snippet after creating a read replica:

NAME              REGION       MASTER_INSTANCE
replica-instance  us-central1  master-instance

What does this output indicate?

medium

A. The master-instance is a read replica of replica-instance

B. The replica-instance is the primary database in us-central1 region

C. The replica-instance is a read replica of master-instance in us-central1 region

D. The replica-instance is a backup stored in us-central1

Solution

1. Step 1: Analyze the output columns

   The output shows the replica name, its region, and the master instance it replicates.

2. Step 2: Interpret the relationship

   Since replica-instance lists master-instance as its master, it is a read replica of that master in the specified region.

3. Final Answer:

   The replica-instance is a read replica of master-instance in us-central1 region -> Option C

4. Quick Check:

   Replica points to master-instance = read replica relationship [OK]

Hint: Master instance listed means replica points to it for reads [OK]

Common Mistakes:

• Confusing replica as primary
• Thinking master is replica
• Assuming backup instead of replica

4.

You tried to create a read replica with this command:

gcloud sql instances create replica1 --master=master1

But it failed. What is the likely cause?

medium

A. You must create the replica before the master instance

B. The replica name cannot contain numbers

C. The master instance must be in a different region

D. The flag --master is incorrect; should use --master-instance-name

Solution

1. Step 1: Check the command flags

   The correct flag to specify the master instance is --master-instance-name, not --master.

2. Step 2: Validate other options

   Replica names can contain numbers; master and replica can be in same region; master must exist before replica.

3. Final Answer:

   The flag --master is incorrect; should use --master-instance-name -> Option D

4. Quick Check:

   Correct flag for master instance = --master-instance-name [OK]

Hint: Use exact flag --master-instance-name to link replica to master [OK]

Common Mistakes:

• Using wrong flag --master
• Thinking replica name rules are stricter
• Assuming region mismatch causes failure

5.

You have a master instance handling both reads and writes. To improve read performance globally, you want to create two read replicas in different regions. Which approach is best?

1. Create two read replicas using gcloud sql instances create with --master-instance-name for each region.
2. Configure your application to send read queries to replicas and write queries to the master.
3. Ensure replicas are in regions close to your users for low latency.

What is the correct reasoning behind this approach?

hard

A. Read replicas automatically handle writes to reduce master load

B. Read replicas reduce load on master and improve read latency by being regionally closer to users

C. Creating replicas in multiple regions duplicates writes to all replicas instantly

D. Replicas replace the master for all database operations

Solution

1. Step 1: Understand read replica benefits

   Read replicas copy data from the master and serve read requests, reducing load on the master.

2. Step 2: Consider regional placement and query routing

   Placing replicas near users lowers latency; routing reads to replicas and writes to master optimizes performance.

3. Step 3: Clarify write behavior

   Replicas do not handle writes; writes go only to the master and replicate asynchronously.

4. Final Answer:

   Read replicas reduce load on master and improve read latency by being regionally closer to users -> Option B

5. Quick Check:

   Replicas improve read speed and reduce master load [OK]

Hint: Replicas serve reads near users; writes stay on master [OK]

Common Mistakes:

• Assuming replicas handle writes
• Thinking replicas instantly sync writes synchronously
• Believing replicas replace the master