Read replicas for performance in AWS - Commands & Configuration

Introduction

When your database gets too busy handling many read requests, it can slow down. Read replicas let you copy your database to other servers so they can share the reading work and keep things fast.

When your website has many visitors reading data but fewer writing changes.

When you want to reduce the load on your main database to keep it responsive.

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

When you want to improve read speed by spreading requests across multiple servers.

When you want a backup copy of your data that can also serve read requests.

Commands

This command creates a read replica named 'mydb-replica' from the primary database 'mydb-primary' in the us-east-1 region. It sets up a copy that will handle read requests.

Terminal

aws rds create-db-instance-read-replica --db-instance-identifier mydb-replica --source-db-instance-identifier mydb-primary --region us-east-1

Expected Output

{

    "DBInstance": {

        "DBInstanceIdentifier": "mydb-replica",

        "DBInstanceStatus": "creating",

        "Engine": "mysql",

        "DBInstanceClass": "db.t3.medium",

        "AvailabilityZone": "us-east-1a"

    }

}

→

--db-instance-identifier - Names the new read replica instance.

→

--source-db-instance-identifier - Specifies the primary database to copy from.

→

--region - Sets the AWS region where the replica will be created.

This command checks the status and details of the read replica to confirm it is ready to use.

Terminal

aws rds describe-db-instances --db-instance-identifier mydb-replica --region us-east-1

Expected Output

{

    "DBInstances": [

        {

            "DBInstanceIdentifier": "mydb-replica",

            "DBInstanceStatus": "available",

            "Engine": "mysql",

            "Endpoint": {

                "Address": "mydb-replica.us-east-1.rds.amazonaws.com",

                "Port": 3306

            }

        }

    ]

}

→

--db-instance-identifier - Specifies which database instance to describe.

→

--region - Sets the AWS region to query.

This command connects to the read replica database using the MySQL client to run read queries and verify it works.

Terminal

mysql -h mydb-replica.us-east-1.rds.amazonaws.com -u admin -p

Expected Output

Enter password: 

Welcome to the MySQL monitor.  Commands end with ; or \g.

Your MySQL connection id is 123

Server version: 8.0.28 MySQL Community Server - GPL



mysql> SELECT NOW();

+---------------------+

| NOW()               |

+---------------------+

| 2024-06-01 12:00:00 |

+---------------------+

1 row in set (0.00 sec)

Key Concept

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

Common Mistakes

Trying to write data to the read replica.

Read replicas are read-only and do not accept write operations, so writes will fail or cause errors.

Always send write requests to the primary database and only send read requests to the replicas.

Not checking the replica status before using it.

If the replica is still creating or not available, connections will fail or be unreliable.

Use the describe command to confirm the replica status is 'available' before connecting.

Creating replicas in a different region without considering latency.

Cross-region replicas can have higher latency and slower replication, affecting performance.

Create replicas in the same region as your application or understand the trade-offs of cross-region replication.

Summary

Create a read replica from your primary database to share read traffic.

Check the replica status to ensure it is ready before use.

Connect to the read replica for read-only queries to improve performance.