Read replicas in GCP - Step-by-Step Execution

Process Flow - Read replicas

Primary Database

↓

Create Read Replica

↓

Replica Syncs Data

↓

Read Traffic Directed to Replica

↓

Replica Handles Read Queries

↓

Primary Handles Writes

The primary database creates a read replica that syncs data continuously. Read queries go to the replica, while writes stay on the primary.

Execution Sample

GCP

gcloud sql instances create my-replica \
  --master-instance-name=my-primary \
  --region=us-central1

This command creates a read replica named 'my-replica' for the primary instance 'my-primary' in the specified region.

Process Table

Step	Action	Result	State Change
1	Create primary database instance	Primary instance 'my-primary' created	Primary database ready
2	Run command to create read replica	Read replica 'my-replica' creation started	Replica instance initializing
3	Replica syncs data from primary	Data replication in progress	Replica data catching up
4	Replica sync completes	Replica data up-to-date	Replica ready to serve reads
5	Application sends read queries	Queries routed to replica	Replica handles read traffic
6	Application sends write queries	Writes sent to primary	Primary handles writes
7	Replica continues syncing changes	Replica stays updated	Continuous replication ongoing

💡 Read replica is fully operational and serving read traffic while primary handles writes.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 7
Primary Instance	Not created	Created and running	Running	Running
Read Replica	Not created	Initializing	Ready and synced	Ready and continuously syncing
Data Sync Status	None	Starting sync	Synced	Continuous sync ongoing
Read Traffic	None	None	Starts routing to replica	Reads served by replica
Write Traffic	None	None	Handled by primary	Handled by primary

Key Moments - 3 Insights

Why does the read replica not handle write queries?

Because the read replica is designed only to serve read queries to reduce load on the primary. Writes must go to the primary to keep data consistent, as shown in execution_table steps 5 and 6.

What happens if the replica is not fully synced?

If the replica is not synced (step 3), it cannot serve accurate read queries. The system waits until sync completes (step 4) before routing reads to the replica.

Does the replica operate independently from the primary?

No, the replica continuously syncs data from the primary (step 7). It depends on the primary for updates and cannot accept writes.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the replica become ready to serve read queries?

A Step 2

B Step 4

C Step 5

D Step 7

💡 Hint

Check the 'Result' column for when the replica data is up-to-date and ready.

According to variable_tracker, what is the status of 'Data Sync Status' after step 7?

A None

B Synced

C Continuous sync ongoing

D Starting sync

💡 Hint

Look at the last column for 'Data Sync Status' in variable_tracker.

If the application sends a write query, where does it go according to execution_table?

A Primary database

B Both primary and replica

C Read replica

D Neither

💡 Hint

Refer to step 6 in execution_table under 'Action' and 'Result'.

Concept Snapshot

Read replicas copy data from a primary database to handle read queries.
Writes always go to the primary to keep data consistent.
Replica syncs continuously to stay updated.
This reduces load on the primary and improves read performance.
In GCP, create replicas with 'gcloud sql instances create' using --master-instance-name.

Full Transcript

Read replicas are copies of a primary database that handle read queries to reduce load on the primary. The primary database handles all write queries to maintain data consistency. When you create a read replica in GCP, it initializes and syncs data from the primary. Once synced, the replica serves read traffic while continuously syncing updates. This setup improves performance by distributing read load. The replica cannot accept writes and depends on the primary for data updates.

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