Read replicas for performance in AWS - Mini Project: Build & Apply

Read replicas for performance

📖 Scenario: You manage a database for a popular online store. The main database handles all the writes like adding new orders. But many users also read product details and reviews. To keep the website fast, you want to add read replicas that copy data from the main database. This way, read requests can go to replicas, reducing load on the main database.

🎯 Goal: Create an AWS RDS instance with one read replica to improve read performance. You will first define the main database, then configure a read replica, and finally complete the setup to allow read traffic to the replica.

📋 What You'll Learn

Create a main RDS instance named main-db with MySQL engine version 8.0.28

Create a read replica named read-replica-1 of main-db

Enable public accessibility for both instances

Set the instance class to db.t3.micro for both

Configure the read replica to allow read traffic

💡 Why This Matters

🌍 Real World

Many web applications use read replicas to handle large volumes of read requests without slowing down the main database that handles writes.

💼 Career

Cloud architects and DevOps engineers often configure read replicas to optimize database performance and scalability.

1

Create the main RDS instance

Write AWS CloudFormation YAML code to create an RDS DB instance named MainDB with the identifier main-db. Use the MySQL engine version 8.0.28, instance class db.t3.micro, and set PubliclyAccessible to true.

AWS

# Define the main RDS instance resource here
# Your code here

Hint

Use AWS::RDS::DBInstance resource type and set the properties exactly as specified.

2

Add the read replica configuration

Add a new resource named ReadReplica1 to create a read replica of MainDB. Set the SourceDBInstanceIdentifier to main-db, use the instance class db.t3.micro, and set PubliclyAccessible to true.

AWS

Resources:
  MainDB:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceIdentifier: main-db
      Engine: mysql
      EngineVersion: 8.0.28
      DBInstanceClass: db.t3.micro
      PubliclyAccessible: true

# Add the read replica resource below
# Your code here

Hint

Use SourceDBInstanceIdentifier property to link the replica to the main DB.

3

Configure the read replica for read traffic

Add the property DBSubnetGroupName with value default to both MainDB and ReadReplica1 resources to allow network access for read traffic.

AWS

Resources:
  MainDB:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceIdentifier: main-db
      Engine: mysql
      EngineVersion: 8.0.28
      DBInstanceClass: db.t3.micro
      PubliclyAccessible: true
      # Add DBSubnetGroupName here
  ReadReplica1:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceIdentifier: read-replica-1
      SourceDBInstanceIdentifier: main-db
      DBInstanceClass: db.t3.micro
      PubliclyAccessible: true
      # Add DBSubnetGroupName here
# Your code here

Hint

Adding DBSubnetGroupName helps the instances be reachable in the network.

4

Enable read traffic by setting the appropriate tags

Add a tag with Key set to Purpose and Value set to ReadReplica to the ReadReplica1 resource to indicate its role for read traffic.

AWS

Resources:
  MainDB:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceIdentifier: main-db
      Engine: mysql
      EngineVersion: 8.0.28
      DBInstanceClass: db.t3.micro
      PubliclyAccessible: true
      DBSubnetGroupName: default
  ReadReplica1:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceIdentifier: read-replica-1
      SourceDBInstanceIdentifier: main-db
      DBInstanceClass: db.t3.micro
      PubliclyAccessible: true
      DBSubnetGroupName: default
      # Add Tags here
# Your code here

Hint

Tags help identify the role of the resource in AWS.

Practice

1. What is the main purpose of using read replicas in AWS databases?

easy

A. To store backups of the database

B. To handle more read requests and improve performance

C. To increase write speed on the main database

D. To encrypt data at rest automatically

Solution

1. Step 1: Understand the role of read replicas

   Read replicas are copies of the main database used to handle read traffic separately.

2. Step 2: Identify the performance benefit

   By offloading read requests to replicas, the main database is less busy, improving overall speed.

3. Final Answer:

   To handle more read requests and improve performance -> Option B

4. Quick Check:

   Read replicas improve read performance [OK]

Hint: Read replicas help with reads, not writes [OK]

Common Mistakes:

• Thinking read replicas speed up writes
• Confusing read replicas with backups
• Assuming read replicas encrypt data automatically

2. Which AWS CLI command correctly creates a read replica for an existing RDS instance named mydb?

easy

A. aws rds create-db-instance-read-replica --db-instance-identifier mydb-replica --source-db-instance-identifier mydb

B. aws rds create-db-instance --db-instance-identifier mydb-replica --source-db-instance-identifier mydb

C. aws rds create-read-replica --db-instance-identifier mydb-replica --source-db-instance mydb

D. aws rds replicate-db-instance --source mydb --target mydb-replica

Solution

1. Step 1: Identify the correct AWS CLI command for read replicas

   The correct command is create-db-instance-read-replica to create a read replica.

2. Step 2: Check the parameters

   The parameters --db-instance-identifier and --source-db-instance-identifier are required to specify the replica and source.

3. Final Answer:

   aws rds create-db-instance-read-replica --db-instance-identifier mydb-replica --source-db-instance-identifier mydb -> Option A

4. Quick Check:

   Correct AWS CLI syntax for read replica creation [OK]

Hint: Look for 'create-db-instance-read-replica' command [OK]

Common Mistakes:

• Using 'create-db-instance' which creates a new DB, not a replica
• Wrong command names like 'create-read-replica'
• Mixing source and target parameters

3. Given this AWS CLI command output snippet after creating a read replica:

{
  "DBInstanceIdentifier": "mydb-replica",
  "DBInstanceStatus": "creating",
  "ReadReplicaSourceDBInstanceIdentifier": "mydb"
}

What does the DBInstanceStatus value indicate?

medium

A. The read replica is ready and available for use

B. The read replica is being deleted

C. The read replica is currently being created and not ready yet

D. The read replica creation failed

Solution

1. Step 1: Understand the meaning of 'DBInstanceStatus'

   This field shows the current state of the database instance.

2. Step 2: Interpret the status 'creating'

   'creating' means the instance is in progress of being set up and is not yet ready.

3. Final Answer:

   The read replica is currently being created and not ready yet -> Option C

4. Quick Check:

   Status 'creating' means setup in progress [OK]

Hint: Status 'creating' means not ready yet [OK]

Common Mistakes:

• Assuming 'creating' means ready
• Confusing 'creating' with 'failed'
• Thinking 'creating' means deletion

4. You created a read replica but notice it is not receiving updates from the source database. What is a likely cause?

medium

A. The source database has no write traffic

B. The read replica was created in a different AWS region

C. The read replica is set to multi-AZ deployment

D. The source database is not configured for automated backups

Solution

1. Step 1: Understand replication requirements

   Read replicas require the source database to have automated backups enabled to support replication.

2. Step 2: Analyze the cause of no updates

   If automated backups are off, replication cannot start, so the replica won't receive updates.

3. Final Answer:

   The source database is not configured for automated backups -> Option D

4. Quick Check:

   Automated backups must be enabled for replication [OK]

Hint: Check if automated backups are enabled on source [OK]

Common Mistakes:

• Assuming region mismatch always blocks replication
• Confusing multi-AZ with read replica settings
• Thinking no write traffic stops replication

5. You have a high-traffic web app using an AWS RDS MySQL database. You want to improve read performance without affecting writes. Which setup is best?

hard

A. Create multiple read replicas and direct read queries to them

B. Increase the instance size of the main database only

C. Enable multi-AZ deployment without read replicas

D. Use a single read replica and send all traffic to it

Solution

1. Step 1: Identify the goal

   The goal is to improve read performance without slowing down writes.

2. Step 2: Evaluate options

   Creating multiple read replicas allows distributing read traffic, reducing load on the main DB and improving read speed.

3. Step 3: Compare other options

   Increasing instance size helps but may be costly and less scalable; multi-AZ improves availability, not read performance; a single replica may become a bottleneck.

4. Final Answer:

   Create multiple read replicas and direct read queries to them -> Option A

5. Quick Check:

   Multiple read replicas scale read performance best [OK]

Hint: Use many read replicas to spread read load [OK]

Common Mistakes:

• Relying only on bigger main DB instance
• Confusing multi-AZ with read scaling
• Using only one read replica for heavy reads