Configuring read replicas in RDS for load balancing read traffic

Question

Pulumi · Accepted Answer

When setting up read replicas for an Amazon RDS (Relational Database Service) instance, we are essentially creating one or more copies of our primary database that can handle read traffic. This helps distribute the load and improve the performance of read-heavy database workloads.

In Pulumi, to configure read replicas for an RDS instance, we would use the `aws.rds.Instance` resource with the `replicateSourceDb` property set to the identifier of the existing RDS instance that you wish to replicate. However, it’s important to note that the primary RDS instance must have backups enabled, as the replication is done using the backup.

Let’s walk through setting up a read replica in TypeScript with Pulumi.

### Step 1: Set up the primary RDS instance
Firstly, we need to set up the primary database instance that we will be replicating. Note that this must have backups enabled to allow for replication.

### Step 2: Create the read replica
Once the primary database is up and running, we will create a new RDS instance to serve as the read replica. For this example, we'll assume that the primary DB instance has already been created – the `replicateSourceDb` parameter will take the ARN or DB instance identifier of this instance.

Let's put this into Pulumi code:

```typescript
import * as pulumi from "@pulumi/pulumi";
import * as aws from "@pulumi/aws";

// Assume primary RDS instance is already created and is identified by 'primaryDbInstanceIdentifier'.
const primaryDbInstanceIdentifier = "primary-rds-instance-identifier";

// Create an RDS read replica
const readReplica = new aws.rds.Instance("readReplica", {
    instanceClass: "db.t3.large", // The instance type of the replica
    replicateSourceDb: primaryDbInstanceIdentifier, // Identifier of the existing RDS instance to replicate
    availabilityZone: "us-west-2b", // Specify the availability zone if necessary
    tags: {
        // Optionally add tags here
        Name: "My Read Replica"
    },
    // Include any other necessary configuration options
    // ...
});

// Export the read replica endpoint
export const readReplicaEndpoint = readReplica.endpoint;
```

### Explanation:

- `aws.rds.Instance`: This is the Pulumi resource which manages RDS instances in AWS.
- `instanceClass`: Specifies the compute and memory capacity of the DB instance, such as `db.t3.large`.

The `replicateSourceDb` parameter is what tells AWS to create this instance as a read replica of an existing RDS DB instance.

And finally, we export the read replica's endpoint so it can be accessed or used in other parts of the Pulumi stack or in applications.

When you run this Pulumi program, it will provision a new RDS read replica that is a copy of your primary RDS instance.

Remember to replace the placeholder values with ones that match your specific AWS setup. This would typically be part of a larger Pulumi program that also handles networking, security groups, and other necessary cloud configuration. Also, be mindful that some properties (like `availabilityZone`, `instanceClass`, etc.) depend on your specific use case and will need to be adjusted accordingly.